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Role of Particle Size and Forage Quality in Digestion and Passage by Cattle and Sheep1, 2
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW Role of Particle Size and Forage Quality in Digestion and Passage by Cattle and Sheep1,2 F. A. M A R T Z Missouri Cluster US Dairy-Forage Center USDA-ARS Animal Physiology Research Unit University of Missouri Columbia 65211 R. L. B E L Y E A Dairy Science Department University of Missouri Columbia 65211 ABSTRACT
Other body functions, such as lactation, appear to influence chewing patterns and rumination. These relationships are poorly understood at best and need additional intensive examination.
Diminution of forage particles includes mastication, chewing, and digestion. In rumen of cattle and sheep fed all forage diets, particle size can range from 200 to over 1200 /am. Particle size reduction to about <1200 gtm must occur before passage. Dietary particle size may influence rumen particle size, but mastication and rumination minimizes differences among diets. Rumnants expend considerable effort to move digesta. Density, cell wall percentage, osmotic pressure, and pH may affect propulsion. Dense particles may sink to the bottom and resist escape. Cell wall may reduce digestion and passage. Osmotic pressure or pH may affect digestive efficiency and rhythm of intestinal tract muscles. Chewing, exercise, physiological functions, and body size may also affect the reduction of forage particle size. More effort is necessary to chew high than low fiber diets. Young cattle (<225 kg) lack rumination capability and body size to process forage particles efficiently. Exercised sheep (26,400 kg-m/d) ate less forage and ruminated less than controls.
INTRODUCTION
The usefulness of forage in the diet of the dairy animal is a function of forage intake and digestion. To develop an understanding of forage intake and utilization, attention has been drawn to the importance of particulate comminution before and during the ruminant digestive process (20, 29). Thus, there is a critical need to pull together the current understanding of this subject to give direction for the future. It is beyond the scope of this paper to review the techniques and principles used to measure particle size. PROPULSION AND MIXING OF S T O M A C H C O N T E N T S
Received May 17, 1985. t Contribution from US Department of Agriculture, Agricultural Research Service in cooperation with the University of Missouri Agricultural Experiment Station. Journal Series Number 9865. 2Mention of a trade name or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture or the University of Missouri Agriculture Experiment Station and does not imply its approval to the exclusion of other products that may also be suitable. 1986 J Dairy Sci 69:1996-2008
Researchers have been unaware of the significance of the physiological and muscular action of the cow and the role it plays in propelling digesta through the digestive tract. Extensive radiographic studies have recently been reported from 16 sheep consuming either dried grass or chaffed lucerne ad libitum (41). This technique, which used x-ray filming of animals dosed with barium sulfate, allowed direct observation of the digestive tract in a normal animal trained to stand with the instrumentation in place. This report confirms many findings already in the literature and adds some new details. A bolus of particulate matter, upon entering the reticulorumen area, may take one of two pathways. If it is dense enough to sink in the region of the cardia, the particles stay cranial to
1996
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW the cranial pillar. If it floats, the particles stay close to the cardia until the next reticular contraction pushes them into the dorsal rumen, caudal to the cranial pillar. A second pathway may be followed, especially in the fasted animal. If the bolus enters the cardia region at the time of a reticulorumen contraction, it is swept into the midrumen stream and carried either cranial or caudal to the cranial pillar. The midrumen stream is a circulating interface passing between the cranial and the caudal pillar. The direction that the bolus material takes, caudally or cranially, depends on its density. The first pathway was most common in sheep fed the ad libitum and the second pathway was most common in the fasted sheep (41) (Figure 1). The movements of the reticulum and rumen apparently create forces in the digesta that cause it to stream or flow in one of two directions. One stream appears to circulate in the dorsal rumen and the other in the ventral rumen with a midrumen stream extending approximately from the top of the cranial pillar to the top of the caudal pillar equal to the positions of these pillars at the resting stage. Less dense particulate digesta floats into the cranial sac and is moved into the dorsal rumen; as the digesta becomes more dense from digestion and mastication, it is later transferred to the ventral rumen and eventually into the cranial sac and reticulum where it flows out through the reticuloorifice. Dense digesta, which sinks immediately into the cranial sac, appears to bypass completely the dorsal and ventral rumen. The time required for digesta to complete the circuit of the dorsal and ventral rumen depends on the consistency of the digesta, which in turn depends on the diet. F o r sheep (41) consuming chopped lucerne hay, the time for digesta to travel from the caudal pillar across the dorsal sac was 30 rain and across the ventral sac was 31 rain. When the sheep consumed dried pelleted grass, the respective times were shortened to 13 and 20 min. Contractions of the reticulum and tureen appear to cause a flow of digesta into the cardia region periodically, which starts rumination. At the height of the reticular contraction, negative pressure changes occur in the esophagus, and particulate matter ascends into the mouth. Fluid and probably small particles are squeezed from the bolus while in the mouth and are
1997
reswallowed. The bolus is then masticated and swallowed. Thus, mastication may provide accessory particulate sorting (41). In view of the processes described, a major m e t h o d for sorting particulates in the rumen appears to be a flotation system. Such a system depends heavily on changes in specific gravity of particles that cause them to sink to facilitate passage. As witl be seen later, sorting by particle. size has also been proposed, which suggests that particles larger than 1200 /~m will not usually pass from the reticulorumen. This proposal infers that some mechanism in the rumen acts like a sieve that resists large particles from passing. Particle sizing may be significant even in a flotation separation system because of the influence of particle size on changes in density of the particle and physical consistency of the digesta. The passage of particulate matter is not a function of motility of the stomach compartments (36) but rather a function of flow associated with each contraction of the compartment involved. Therefore, the flow is greatly dependent on consistency of the digesta and pressure exerted as a result of ruminal contractions. The abomasum has been shown to increase flow per contraction when it is distended, probably because of increased pressure associated with the distention, and this change was not associated with an increase in frequency of contractions. When the abomasum was distended, reticulorumen activity appeared to decrease (41). This combination of events would probably decrease passage of contents from the reticulum through the retiuloomasaI
DORSAL SAC
~R~.~A/ ~
OMASUM //
CAUDAL
....
\
RETiCULUM CRANIAL SAC HORIZONTAL VIEW
VENTRAL SAC VERTICAL VIEW
Figure 1. Summary of digesta movement in reticulorumen (41). Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELY'EA
orifice. Such a system would mean that the abomasum would act as mediator of digesta flow from the reticulorumen, and the consistency and possibly the chemical composition of digesta entering and leaving the abomasum and digesta in the small intestine would have a significant effect of rate of flow. Physical form of forage and particle size may affect both the frequency and amplitude of rumen contraction (23) (Figure 2). Pharr et al. (23) fed sheep 908 g of oat hay that had been ground through a .6t+, .24, or .10-cm screen. The control feed was long hay. The smaller particle size resulted in rumen contraction less frequent and of less amplitude. In another trial (7), cattle were fed either alfalfa tops, oat hay, or a combination of both. Frequency of ruminal contraction was highest for alfalfa tops and least for oat hay. Differences in contraction frequency were not accompanied by differences in ruminal pressure. Waghorn and Reid (36) observed similar results for 24-h periods, but differences between chopped Lucerne hay and fresh rye grass were not significant. The data suggest that rumen contractions may be more frequent with higher quality immature forage and less with lower quality more mature forage, because rumen contractions do not depend on rumination. If motility is increased with high quality forage and the flow per contraction is increased because of changes in physical consistency, this would partially explain the rapid passage observed on succulent pasture grasses as compared with the slower passage of lower quality forages.
EFFECT OF PHYSICAL FORM OF FEED ON D I G E S T I B I L I T Y
The effect of particle size and pelleting of forage on digestibility has been reviewed (37). Depending on the quality of forage, particle size reduction usually reduces digestibility and increases intake. This relationship may be less pronounced for low quality than for high quality forages, because the digestibility of low quality forages may be discounted less by pelleting while intake is increased relatively more. In a comparison of 26 studies, digestibility was depressed an average of 3.3 percentage units by some form of grinding or fine chopping compared with longer forms of the same forage (22) (Table 1). Early and medium cut ryegrass have been compared as chopped versus pelleted. Particle size was about 1000 /~m for chopped and 250 /2m for pelleted samples (6, 37). Digestion of cell wall of pellets was depressed in the rumen but digestion increased in the lower intestine, so total digestibility was not depressed as much as was digestion in rumen. Pelleting appeared to depress total digestion more in the early cut than in the medium cut forage. Alfalfa and red clover appeared to have greater depres-
--10 I
--6
I
p SIZE OF R U M E N PARTICLES A N D PARTICLES L E A V I N G THE RUMEN
The size of the majority of rumen particles ranges from about 200 to >1200/~m (25, 29, 39). Particles that leave the rumen are thought to be < 1200/~m, which is similar to the particle size of digesta found in the duodenum and feces (25). There is probably a difference in size of rumen particles between species. Sheep have fewer large ruminal particles (> 1200/2m) than cattle (25). Sheep also have a chewing pattern different from cattle. Size of particles in the rumen may also be different due to time of sampling; digesta sampled at a greater time postfeeding would have smaller particle size because the animal has had more time to chew. Journal of Dairy Science Vol. 69, No. 7, 1986
I
32B
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52B 6-16 -65 IOcm OAT HAY Figure 2. Influence of long versus finely ground (.10 cm) oat hay on amplitude and frequency of rumen contractions: A) atmospheric pressure; B) resting rumen pressure; C) large or primary rumen contraction; D) small or eructation contraction. Scale graduated in centimeters water.
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW
1999
TABLE 1. Influence of particle size on apparent digestibility (34). Cell wall digestibility Rye grass
Form
Particle size (tam)
Dry matter intake
Total tract
Rumen
Lower tract
(g)
(%)
-
Early cut
Chopped Pellet
1030 260
1400 1400
87 80
93 73
2 7
5 19
Early cut
Chopped Pellet
1030 260
900 900
76 76
88 86
-1 4
13 10
Medium cut
Chopped Pellet
960 220
900 900
86 75
89 70
4 -1
6 31
sion in energy digestibility in the rumen but also greater compensation in the small intestine than grasses (3, 6, 33). Digestibility is thought to be depressed for pelleted forages because of their fast passage through the rumen. Apparently the particle size in pelleted forages is nearly equal to that of particles leaving the rumen. Thus, these particles escape the rumen rapidly, w h i c h does not allow enough time for complete microbial digestion (35). Rate of feed passage is increased for pelleted forage so that intake of digestible matter is increased, especially with low quality forage. R E L A T I O N OF D I E T A R Y P A R T I C L E SIZE OF F O R A G E TO P A R T I C L E SIZE OF R U M E N D I G E S T A
It is expected that size of particles in the rumen arc a function of feed particle size, amount of mastication, and amount of rumination. In our laboratory we measured rumen particles from a fistulated cow that was consuming alfalfa in long, chopped-cubed, or pelleted form (38) (Table 2). Particle size appeared to affect size of particles found in the rumen. Alfalfa and orchard grass had been ground through a 13-mm screen had particle sizes of 996 and 857/am, respectively (29). Ruminal particle size from sheep fed these forages was 826 and 558 gm respectively, which was a decrease of 17 and 35% from feed to rumen. Because these feed particles were nearly the size of rumen digesta, less particle size reduction might be expected than for coarser materials. The particle size of masticate
-
Duodenum and ileum (% of total tract)
from cows fed long Bermuda grass hay has been studied (24). Pond et al. (24) did not report a mean particle size, but 82% of the particles in the masticate were larger than 1600/am. It has been suggested that while eating the ruminant chews the ingested particles only sufficiently to form a bolus acceptable for deglutition. Heifers spent less time eating chopped alfalfa hay, which had a mean particle size of 1440/am, than did heifers eating long alfalfa hay (17). Jaster and Murphy (17) theorized that the chopped hay was near the appropriate size for deglutition when ingested and had to be masticated less than long hay. Others have observed that in cows consuming herbage, mean particle size of particulate matter in the boli ranged from 1244 to 1602 /am (12). It appears that the particle size of forage affects the size of rumen particles but not to the extent expected, because the particles must be masticated to a size acceptable for deglutition before ever entering the rumen.
TABLE 2. Effect of physical form of alfalfa on particle size in cattle (38).
Alfalfa
Geometric mean diameter of rumen particles (urn) SD
Long Cubed Pelleted
706 623 485
6.2 4.9 5.3
Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
Poppi et al. (26) reported the particle size of leaves and stems of chopped Pangola and Rhodes grasses that had been harvested as regrowth at 6 and 12 wk (Table 3). Large particles were defined as greater than 1180/~m. The chopped leaves and stems, which were fed to cattle, contained the same amount of large particles-about 85% (26). Mastication reduced large particles to 58 and 76% for leaves and stems; rumen contents contained 27 and 34% l a n e particles, respectively (27). Compared with cattle, sheep had fewer large particles in their rumen 23 vs. 58% and 24 vs. 76% for leaves and stems (26, 28). P A R T I C L E SIZE R E D U C T I O N IN THE R U M E N A N D PASSAGE F R O M THE R U M E N
Because rumen capacity appears to limit feed intake with diets exceeding 35% cell wall (19), factors associated with rate of turnover of rumen particulate matter are of prime importance. Indigestible matter has been suggested as a primary limitation in rumen digesta turnover; digestion rate is less significant (20). There appears to be little information about the relative quantitative importance of these two factors. Critical particle size has been used as a concept in the development of models of digesta flow. Particles larger than a certain size rarely pass into the abomasum from the reticulorumen (13, 22, 34). The concept of critical particle size divides rumen particles into two pools: a large particle pool, which cannot pass out of the rumen, and a small particle pool, which can leave the rumen (28). Size of particles is reduced by rumination and digestion. In most early studies, particle size of ingesta was measured in slaughtered animals. Later reports examined the critical size concept using sheep and cattle equipped with ruminal, abomasal, and ileal cannulas (28) (Figure 3). The concept, as presented, does not appear to include changes in density of particles as digestion progresses. Poppi et al. (26) suggest that the particle size of digesta does not change significantly after it leaves the rumen-reticulum and that fecal particle size is a good estimate of particle sizes leaving the rumen. This report challenges the critical size theory, because some particles larger than 1180 /am escape the rumen. However, the percentage of large particles escaping Journal of Dairy Science Vol. 69, No. 7, 1986
the rumen was small and may be insignificant in practice. Therefore, Poppi et al. (26) concluded that particles greater than 1180 /~m strongly resist escape from the rumen and seldom do escape. In a later report, this same group reported the rate of particle size reduction did not have a significant relation to rate of flow of dry matter from the rumen. (27). Thus, small particles were trapped in the ruminal ingesta mat and impeded from passing from the rumen until the large particles were reduced and began to flow. Polypropylene ribbon has been used to study the passage of rumen particles (39). Significant amounts of particles as large as 5000 to 10,000 /am were recovered in the feces in these studies. It would appear that pliability may be a factor in the passage of such large particles. Apparently, pliable particles bend and pass whole, whereas particles more fibrous and rigid than polypropylene may be more resistant to bending and to passage. Cows in our laboratory passed a small number of particles of tall fescue 5 to 10 cm long, but feces from cows consuming alfalfa contained no such particles. CHEWING TIME A N D P A R T I C L E SIZE
Chewing time may be used as a measure of the roughage index of feeds (2). This index was proposed to be related to the extent of physical
TABLE 3. Proportion of large particles in chopped and masticated grass from two cattle (25). Grass species
Pangola Rhodes
Pangola Rhodes
Regrowth
Large * particles Leaf Stem
(wk)
Chopped grass (g/g)
6 12 6 12 Mean
.87 .87 .90 .89 .81 .84 .80 .82 .85 .86 Masticated grass (g/g)
6 12 6 12 Mean
Greater than 1180 ~m.
.62 .54 .59 .56 .58
.79 .77 .77 .71 .76
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW
I00
80
A
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40
i,I O
Z
20
CO CO
Ld
Off
0'
I
I
I
I
2 4 SIEVE PORE DIAMETER (mm)
Figure 3. Relative resistance to flow from the rumen of different size particles in ten forages (25).
fibrousness of roughages and should be related to the ability of the roughage to support the metabolic well-being of cattle. Santini (31) reported that chewing rate in cattle varied with
2001
the coarseness of feeds and that chewing time was related to the roughage index (Table 4). Chewing time (min/kg) may be used as a roughage value index for forages in dairy cow rations; this index would serve to formulate rations that support high lactation performance (32). Sudweeks et al. (32) found that the optimum chewing time for dairy cows was 31.4 min/kg feed. Earlier work (18) showed that cows consuming alfalfa silage cut at three theoretical lengths (.62, 1.27, and 1.91 cm) chewed longer per unit feed on the coarser cut alfalfa than on the shorter cut alfalfa. Cows in this study needed to chew about 30 min/kg feed for optimum performance. More recently (31), particle length and chewing time have been investigated as a basis for roughage index. In these studies, mean particle lengths did not appear related to intake within the range of their study, but particle length was related to chewing time. As particle length increased chewing time increased, but cows chewed less time per kilogram-centimeter of adjusted intake as particle length increased. This indicated that as cows consumed longer length alfalfa hay and silage, they were more efficient chewers (31). Another report (17) included similar indirect evidence (Table 5). Heifers were fed long, medium, and finely chopped alfalfa hay. Heifers fed long hay had a fecal mean particle size of 227/am compared with 297/am for the finely chopped hay. The heifers consuming long hay had similar chewing times but had significantly more boluses per minute of rumination with less time (not significant) chewing each
TABLE 4. Total chewing time (minutes per kilogram-centimeters of adjusted intake) of cows fed diets with different ratios of forage to contentrate (27). Ratio of forage to concentrate 2 .31
33:67 .43
.51
.31
50:50 .43
.51
222 503 609 3434
177 372 460 2440
171 341 431 2248
162 357 430 2339
138 291 360 1880
134 277 346 1886
MPL1 (cm) Forage Neutral detergent fiber Acid detergent fiber Acid detergent liguin
Mean forage particle length. 2Dry matter basis. Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
TABLE 5. Chewing and ruminatingbehavior of Holstein heifers fed alfalfa hay (17). Particle size of alfalfa Parameter
Long
Dry matter intake, kg Total chewing time, h Chewing time, min/kg Boluses per minute of ruminationI Seconds per bolus Boluses per day Fecal particle size, ttm
2000/~m
8.0 16.9 127 1.27 47 606 227
1500 ~m
8.4 16.6 118 i. 18 51 566
8.5 16.6 117 1.15 52 584 297
290
t Means different (P<.10).
bolus than heifers consuming finely chopped hay. Rumen particles from long hay may have cycled through the rumen at a slower rate than the fine chopped particles; thus, they became more hydrated, softer, and needed less mastication during rumination. RUMINATION
One obvious purpose of rumination is to reduce the size of particles of rumen contents. Rumen motility may be associated with but is not dependent on rumination and these activities most probably have impact on the passage of particles from the rumen. Abomasal motility has been reviewed recently (4). A number of physiological factors affect motility of the reticulorumen and abomasum such as ingesta pH, osmotic pressure, and hormone feedback through action on the duodenum and abomasum. The effect of rumen pH on rumination has been measured (39). Four amounts of molasses or sucrose were administered to sheep (Table 6); amounts of 200 g or above reduced rumination for 6 h following administration, and this inactivity was associated with depressed ruminal pH (39). The pH of the rumen was increased by administration of sodium bicarbonate, causing rumination to cease for more than 5 h. The conclusion of these and other studies with varied osmotic pressure was that osmotic pressure was most related to rumination activity and that rumination did not occur at osmotic pressures greater than .350 OsM. A recent report (30), which investigated rate of passage of digesta in dairy cows fed long and chopped alfalfa hay, showed that cows fed sodium Journal of Dairy Science Vol. 69, No. 7, 1986
bicarbonate with chopped hay had slower rates of digesta passage than cows fed long hay. Feeding grain has variable effects on rumination (39). Small amounts of grain dilute out ration cell wall concentration and reduce rumination in proportion to cell wall intake. Larger amounts of grain disrupt rumination. This disruption is probably related to rumen pH as well as osmolarity and other physical characteristics of the grain particles. Research with particle length of alfalfa (31) suggested that particle length affected total chewing time, and chewing time in turn affected rumen volatile fatty acids, pH, and milk fat percentage. D E N S I T Y OF RUMEN PARTICLES
Dense particles, such as grains, migrate to the reticulum and caudal sac of the rumen upon ingestion. Density of forage particles is of interest. Rumen ingesta, a mixture of water
TABLE 6. Effect of molasses on rumination time (39).
Molasses
Posttreatment rumination time during first 6 h postfeeding
(g)
(min)
o lOO 200 300 400
127 128 48 90 0
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW colloids and feed particles, has a density of .9 to .98 g/ml because feed and particularly forage particles have a density of less than 1.0. Density of forage particles is influenced by several factors: density of fiber is less than water, spaces surrounding plant cell may contain air, and particles absorb varying amounts of water (hydration) resulting in varying densities. Also, smaller particles have increased surface area and pack more densely than larger particles. Feed or forage particle size is also related to internal cellular space of plant cell walls-as particle size is decreased, internal cellular space is broken down and decreases. Rumination destroys this internal cellular space, which decreases hydration (absorbtion of water), probably due to the low affinity for water for cellulose and lignin, but increases density of particles due to changes in physical structure (35) (Figure 4). Small wheat straw particles (70 /am) hydrate about 30% less than larger particles (35). The ability of forage particles to absorb water and swell is related to their chemical composition (35). Cellulose, for example, resists hydration. High quality forages containing less structural material may be more susceptible to hydration than forage of low quality. Rumination, chemical composition, and hydration all interact to increase the density of particles upon entering the rumen, but apparently these relationships are not well understood for forage particles. As comminution progresses, specific gravity of the rumen mass increases (11). Passage of inert particles was maximized with a specific gravity of 1.1 to 1.2 (5). The interaction of particle length and specific gravity of nylon particles in the rumen has been studied (9). The specific gravity of these particles was 1.17 and 1.77. Long light and short heavy particles were retained in the rumen longer than long heavy and short light particles, which demonstrated the interaction between length and specific gravity of particles. The length and specific gravity of particles may not have been representative of feed particles in the rumen, however. Alfalfa particles of 2.07 mm, which had been mordanted with chromium to produce particles with densities from 1.12 to 1.70, were fed to cows (10) (Table 7). Rumen turnover rate was positively related to particle density, but there was a quadratic effect; as density increased, turnover rate increased less. In this experiment, denser particles, due to the chro-
2003
mium mordant, were less digestible, which confounds the findings. It is probable that rumen particles can be made so dense that the tendency to settle to the bottom of the reticulum and cranial sac is great enough to resist passage from the reticulorumen. The functional specific gravity of ground hay samples in ionic solutions has been studied (15). Functional specific gravity is defined as the specific gravity of particles with air or gas pockets included. As a particle hydrates and is digested, voids or pockets are eliminated and functional specific gravity is increased. Ionic composition, pH, and osmolarity increase forage functional specific gravity in vitro. In another report, the same researcher (16) studied the effects of particle size and forage composition on functional specific gravity. Rate of change of functional specific gravity was higher in legumes than in grasses. Specific gravity of samples with small particles was higher at all times measured than that of samples with large particles. Particle size influenced rate of change of specific gravity. Forage particles were placed in nylon bags and suspended in the rumen (16). Functional specific gravity of ground forage changed rapidly (within 1 h) after introduction into the rumen. Physical consistency could interact with particle size and density to promote or retard passage of particles from the rumen. Welch (39)
,o
8 6 E
3
2 ~
C [..@710
I
130
I
250
~
d
480 600
MEAN SIZE (microns) .25
I
2
4
6
SCREEN OPENING (ram) Figure 4. Effect of grinding on dry bulk volume (A) and hydration capacity (e) of wheat straw (35). Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
TABLE 7. Influence of density and chromium on rumen turnover rates of alfalfa fiber (10).
Density
Chromium concentration m cell wall
Turnover rate
(g/ml)
(mg~)
(h)
1.126 1.165 1.242 1.396 1.703
24.2 28,2 31.9 38.9 41.1
.0107 .0072 .0190 .0228 .0194
measured the transit time of particles placed in the dorsal sac, central rumen, and ventral sac of animals fed different amounts of feed. Particles passed the reticuloomasal orifice quickest from the bottom of the tureen and slowest from the dorsal sac, which is consistent with the report of Wyburn (41). However, Welch (39) concluded that consistency (amount of grain vs. long hay) did not relate to the transit time of particles, and thus, particle entrapment must be due to other mechanisms, because there was no effect on turnover due to feeding. Specific gravity of the particles was not reported. This finding is not in agreement with that of Wyburn (41), who showed that consistency of rumen contents appeared to affect ruminal flow. SOME PRACTICAL I M P L I C A T I O N S
Chewing activity in ruminants probably is the cumulative outward manifestation of a number of more fundamental factors. Chewing is related to diet form and composition, rurnen motility, stress, metabolic drive, and metabolic size. Under normal conditions, cattle tend to chew and ruminate a constant amount of time each day, apparently even with increasing fiber (8) (Figure 5). However, because cattle tend to eat less dry matter and spend less time eating high than low fiber diets, rumination and chewing efficiency as grams per hour is decreased markedly for high fiber diets. These relationships appear to be a result of greater difficulty in reducing the particulate size of the more fibrous materials in the ration. Not all research findings are in agreement. Balch (2) proposed that cows ruminate a constant amount of time no matter what amount of fiber is consumed. Others (32) believe that cows attempt to adjust Journal of Dairy Science Vol. 69, No. 7, 1986
by increasing chewing time per day when they consume more fiber. It appears that anything the producer can do to aid the cow to become a more efficient chewer is a step in the right direction. The cow must handle as much dry matter and useable nutrients per unit chewing time as possible, because the cow will probably only chew a constant amount of time each day on a given diet. Prebloom, earlybloom, mid bloom, and full bloom alfalfa hay has been compared for total chewing time in lactating cows IN. A. Jorgenson, 1984, personal communication (Figure 6)]. Cows consuming early cut, high quality alfalfa spent less time chewing, ate more dry matter, and produced more milk than cows consuming the lower quality full bloom alfalfa. Cows consuming the early cut alfalfa appeared to have greater chewing efficiency (nutrients chewed per unit time).Other researchers have shown a relationship between chewing time and rumen pH and volatile fatty acid concentration (31, 32). These reports help explain the importance of keeping concentrate below 55 to 60% of the ration and making up the remainder with quality forage of proper particle size for optimum milk production and animal health. Chewing and ruminating require energy and may result in greater energy cost to the animal than once thought. The effect of exercise on hay intake was studied in sheep (40). Sheep exerting 26,400 kg-m/d exercise ate less hay and ruminated less time during exercise days than nonexercise days. Minutes of rumination
g DM/h
mn/d Chewing time
750-
300-
Ruminating time 500-
200-
Rumination Chewing
efficiency 250-
100-
Crude fibre content 0 20
/ 25
I 30
I ~ 35
20
Figure 5. Influence of fiber content on feeding and ruminating of sheep given fresh grasses [110 samples (8)]. DM = Dry matter.
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW 18 ---%
,
Body size relates to rumination and chewing efficiency (1) (Table 8). Bae et al. found that large older cattle are more efficient chewers than smaller young cattle. Size of fecal particles was similar for all sizes of cattle, indicating that smaller cattle pass the same size particles as large cattle. These findings appear to explain why young cattle (less than 225 kg) do not do well on forage alone even when it is relatively good quality. Young small cattle are not efficient enough chewers to handle a relatively high cell wall ration.
16
rr 14 "IW --~ F- 12 (.9 Z hi
.l-
2005
10 CONCLUSIONS
d
°o
2b
T'l
CONCENTRATE (% DMI) Figure 6. Total chewing time (h/d) as related to change in maturity of alfalfa and concentrate amount (N. A. Jorgensen, 1984, personal communication). DMI = Dry matter intake.
per gram cell wall and fecal particle size did not change with exercise. It may be that additional exercise, disruption, or stress reduces the animals' ability or drive to reduce rumen particulate size. Physiolgical functions, such as milk production, appear to influence other body functions and enable cows to process particulate matter better. Numerous reports indicate that lactation increases ration dry matter intake. Apparently, the lactating cow increases rumen turnover rate of both grain and forage with increasing production (14). Hay, liquid, and grain were marked differentially and fed to dry and lactating cows. Individual cow variation was large and hay to grain ratio was confounded with lactation stage. The data indicate that early lactation is associated with higher ruminal turnover than in late lactation and during the dry period. Lactating cows appear to be more efficient chewers than dry cows (32). It may be that lactation increases the drive toward more chewing, rumination, and rumen motility or more thorough chewing per boll to increase chewing efficiency.
The ruminant animal works to grind ingested particulate matter and propel it through the digestive tract. Figure 7 illustrates a scheme for the passage of forage particles through the ruminant digestive tract. Some forage plants may be consumed whole and are broken down via mastication whereas others may be chopped or ground mechanically before ingestion. As forage particle size is reduced and becomes more nearly equal to rumen particulate size, less reduction of particle size via mastication rumination is necessary. Animals consuming pelleted forage seldom ruminate. There are suggestions that forage is masticated to a particle size of 1000 to 2000 /am before being swallowed. However, the size of particle required for forming a bolus and subsequent swallowing is not well established. Particulate size must be about 1 2 0 0 / l m or less to escape the rumen. It is not entirely clear if this size varies with species of forage and the type of feed. Size of particles in the feces appears to be similar to the size of particles
TABLE 8. Ad libitum intake of forage cell wall and rumination capability for cattle of different size (1).
Animal
Calves Heifers 1 Heifers 2 Heifers 3
Body weight (BW)
Cell wall
Load per day
(kg)
(g/kg"7s )
(g/kg BW"Ts)
119 213 342 456
36 56 80 98
17 21 31 30
Journal of Dairy Science Vol. 69, No. 7, 1986
2006
MARTZ AND BELYEA
Whole Plant
!
Mechanical Reduction of Particle Size I
astication I
~
$ Swallow Particles I000- 2000 um or less
$
Rumination of Particles >1200 um
~ e ~ e g
Rumen Fermentation I and Digestion
Wet Sifting and Mixing, Particles <1200 um Escape
I
Fecal Output Particles <1200 um
ulators of Passage:1 i. 2. 3. 4. 5. 6. 7. 8. 9.
Size of Particles Density of Particles? Cell Wall Content? Rate of Particle Size Reduction? pH and Osmotic Pressure? Distention of Rumen and Abomasum? Strength and Frequency of Ruminal Contractions? Strength and Frequency of Abomasal Contractions? Consistency of Ingesta?
lltems followed by a "?" are not as well understood as other items. Figure 7. Scheme of passage of forage particles in the ruminant.
escaping the rumen. Therefore, measurement of fecal particle size should indicate the size of particles escaping the rumen. Little is known about factors that regulate rate of propulsion of ingesta through the rumen and abomasum. It is reasonably clear that size Journal of Dairy Science Vol. 69, No. 7, 1986
of ruminal particles alone does not regulate rate of passage. Density of particles appears also to play a role, but that role is not well understood. Cell wall content of the forage appears to affect particle passage, but the mechanism is not understood. For example, do high cell
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW wall forages pass f r o m t h e r u m e n slowly because t h e y require m o r e c h e w i n g per se, or is it because t h e y digest m o r e slowly t h a n low cell wall forages and are t h u s p r e s e n t longer? O s m o t i c pressure and p H o f t h e r u m e n and a b o m a s u m a p p e a r t o i n f l u e n c e m o t i l i t y o f t h e s e organs, b u t h o w passage o f ingesta is i n f l u e n c e d is n o t clear. The role t h a t d i s t e n t i o n and m u s c u l a r c o n t r a c t i o n s o f t h e r u m e n , r e t i c u l u m , and a b o m a s u m play in passage o f ingesta has n o t b e e n s t u d i e d extensively. T h e r e are i n d i c a t i o n s that the frequency of contractions of the a b o m a s u m d o e s n o t change m a r k e d l y in n o r m a l r u m i n a n t s . H o w , t h e n , can s o m e diets be passed m o r e rapidly t h a n others? It m a y be t h a t a d d i t i o n a l pressure ( d i s t e n t i o n ) for s o m e diets or m e t a b o l i c states cause a g r e a t e r f l o w rate p e r c o n t r a c t i o n and t h u s a greater t o t a l passage o f p a r t i c u l a t e m a t t e r . However, this c o n c e p t is n o t i n d i c a t e d in t h e literature cited. C o n s i s t e n c y o f t h e ingesta m a y also play a role in passage. Even t h o u g h t h e r e have b e e n a f e w studies to s h o w t h a t c o n s i s t e n c y is o f little i m p o r t a n c e , this line o f research has n o t b e e n p u r s u e d very diligently.
REFERENCES
1 Bae, D. H., J. G. Welch, and B. E. Gilman. 1983. Mastication and rumination in relation to body size of cattle. J. Dairy Sci. 66:2137. 2 Balch, C. C. 1971. Proposal to use time spent chewing as an index to the extent to which diets for ruminants possess the physical property of fibrousness characteristics of roughages. Br. J. Nutr. 36:383. 3 Beever, D. E., D. F. Osbourn, S. B. Cammell, and R. A. Terry. 1981. The effect of grinding and pelleting on the digestion of Italian ryegtass and timothy by sheep. Br. J. Nutr. 46:357. 4 Bell, F. R., 1980. The mechanisms controlling abomasal emptying and secretion. Page 81 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc. Westport, CT. 5 Campling, R. C., and M. Freer. 1962. The effect of specific gravity and size on the mean time of retention of inert particles in the alimentary tract of the cow. Br. J. Nutr. 16:507. 6 Coelho da Silva, J. F. 1972. The effect in sheep of physical form on the sites of digestion of rye grass. Br. J. Nutr. 28:357. 7 Colvin, H. W., Jr., P. T. Cupps, and H. H. Cole. 1958. Dietary infuences on eructation and related ruminal phenomina in cattle. J. Dairy Sci. 41 : 1565. 8 Dulphy, J. P., B. Remond, and M. Theriez. 1980. Ingestive behavior and related activities in ruminants. Page 103 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc.,
2007
Westport, CT. 9 Durkwa, L., and J. G. Welch. 1982. Particle length and specific gravity on rumination and rate of passage. J. Dairy Sci. 65(Suppl. 1):145. (Abstr.) 10 Ehle, F. R. 1984. Influence of feed particle density on particulate passage from rumen of holstein cow. J. Dairy Sci. 67:693. 11 Evans, E. W., G. R. Pearce, J. Burnett, and S. L. Pillinger. 1973. Changes in some physical characteristics of the digesta in the reticulo-rumen of cows fed once daily. Br. J. Nutr. 29:357. 12 Gill, J., R. C. Campling, and D. R. Wentgarth. 1966. A study of chewing during eating in the cow. Br. J. Nutr. 20:13. 13 Greenhalgh, J.F.D., and G. W. Reid. 1973. The effects of pelleting various diets on intake and digestibility in sheep and cattle. Anim. Prod. 16:223. 14 Harmell, G. F., and L. D. Satter. 1979. Determination of rumen fill retention time and ruminal turnover rates of ingesta at different stages of lactation in dairy cows. J. Anita. Sci. 48:381. 15 Hooper, A. P., and J. G. Welch. 1985. Functional specific gravity of ground hay samples in ionic solutions. J. Dairy Sci. 68:848. 16 Hooper, A. P., and J. G. Welch. 1985. Effects of particle size and forage composition on functional specific gravity. J. Dairy Sci. 68:1181. 17 Jaster, E. H., and M. R. Murphy. 1983. Effects of varying particle size of forage and digestion and chewing behavior of dairy heifers. J. Dairy Sci. 66:802. 18 J orgensen, N. A., M. E. Finner, and J. P. Marquardt. 1978. Effect of forage particle size on animal performance. Page 1048 in Proc. Am. Soc. Agric. Eng. 19 Mertens, D. R. 1982. Using neutral detergent fiber to formulate dairy rations. Page 116 in Proc. Georgia Nutr. Conf. Univ. Georgia. 20 Mertens, D. R., and L. O. Ely. 1979. A dynamic model of fiber digestion and passage in the ruminant for evaluating forage quality. J. Anita. Sci. 49:1085. 21 Minson, D. J. 1963. The effect of pelleting and wafering on the feeding value of roughages-a review. J. Br. Grass. Soc. 18:39. 22 Pearce, G. R. 1967. Changes in particle size in the reticulorumen of sheep. Aust. J. Agric. Res. 18:119. 23 Pharr, L. D., H. W. Colvin, Jr., and P. R. Noland. 1968. Rumen motility of sheep as affected by the physical form of oat hay. J. Anim. Sci. 27:414. 24 Pond, K. R., D. E. Akin, M. Molloji, and W. C. Ellis. 1982. Effects of ingestive mastication on the physical and chemical degradation of Coastal bermudagtass. Proc. Annu. Mtg. Am. Forage Grassl. Counc. 25 Poppi, D. P., B. W. Norton, D. J. Minson, and R. E. Hendricksen. 1980. The validity of the critical size theory for particles leaving the rumen. J. Agric. Sci. 94:275. 26 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1981. Studies of cattle and sheep eating leaf and stem fractions of grass. III. The retention time in the rumen of large feed particles. Aust. J. Agric. Journal of Dairy Science Vol. 69, No. 7, 1986
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Res. 32:123. 27 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1981. Studies of cattle and sheep eating leaf and stem fractions of grasses. II. Factors controlling the retention of feed in the reticulo-rumen. Aust. J. Agric. Res. 32:109. 28 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1980. Studies of cattle and sheep eating leaf and stem fractions of grasses. I. The voluntary intake digestibility and retention time in the reticulorumen. Aust. J. Agric. Res. 32:99. 29 Robles, A. Y., R. L. Belyea, F. A. Martz, and M. F. Weiss. 1980. Effect of particles size upon digestible cell wall and rate of in vitro digestion of alfalfa and orchardgrass forage. J. Anim. Sci. 51:783. 30 Rogers, J. A., L. D. Muller, T. J. Snyder, and T. L. Maddox. 1985. Milk production, nutrient digestion, and rate of digesta passage in dairy cows fed long or chopped hay supplemented with sodium bicarbonate. J. Dairy Sci. 68:868. 31 Santini, F. J., A. R. Hardi, and N. A. Jorgensen. 1983. Proposed use of adjusted intake based on forage particle length for calculation of rouhgage indexes. J. Dairy Sci. 66:811. 32 Sudweeks, E. M., L. O. Ely, and L. R. Sisk. 1980. Using a roughage value index in formulating dairy rations. Page 80 in Proc. Georgia Nutr. Conf. Univ. Georgia.
Journal o f Dairy Science Vol. 69, No. 7, 1986
33 Thompson, D. J., P. E. Beever, J. F. Coelho Da Silva, and D. G. Armstrong. 1972. The effect in sheep of physical form on the sites of digestion of a dried lucern diet. Br. J. Nutr. 28:31. 34 Troelsen, J. E., and J. B. Campbell. 1968. Voluntary consumption of forage by sheep and its relation to the size and shape of particles in the digestive tract. Anita. Prod. 10:289. 35 Van Soest, P. J. 1982. Nutritional ecology of the ruminant. O & B Books, Inc. Corvallis, OR. 36 Waghorn, G. V., and C.S.W. Reid. 1983. Rumen motility in sheep and cattle given different diets. N . Z . J . Agric. Res. 26:289. 37 Waldo, D. R. 1973. Effects of physical form of feed on digestibility. Page 6 in Maryland Nutr. Conf., Univ. Maryland. 38 Warren, W. P. 1976. Dietary effects on ruminant digestion and intake. Ph.D. Thesis, Univ. Missouri, Columbia. 39 Welch, J. G. 1982. Rumination, particle size and passage from the rumen. J. Anim. Sci. 54:885. 40 Welch, J. G., R. H. Palmer, B. E. Gilman, and L. S. Bull. 1983. Hay intake and rumination in exercised sheep. J. Dairy Sci. 66(Suppl. 1): 147. (Abstr.) 41 Wyburn, R. S. 1980. The mixing and propulsion of the stomach contents of ruminants. Page 35 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc., Westport, CT.
Other body functions, such as lactation, appear to influence chewing patterns and rumination. These relationships are poorly understood at best and need additional intensive examination.
Diminution of forage particles includes mastication, chewing, and digestion. In rumen of cattle and sheep fed all forage diets, particle size can range from 200 to over 1200 /am. Particle size reduction to about <1200 gtm must occur before passage. Dietary particle size may influence rumen particle size, but mastication and rumination minimizes differences among diets. Rumnants expend considerable effort to move digesta. Density, cell wall percentage, osmotic pressure, and pH may affect propulsion. Dense particles may sink to the bottom and resist escape. Cell wall may reduce digestion and passage. Osmotic pressure or pH may affect digestive efficiency and rhythm of intestinal tract muscles. Chewing, exercise, physiological functions, and body size may also affect the reduction of forage particle size. More effort is necessary to chew high than low fiber diets. Young cattle (<225 kg) lack rumination capability and body size to process forage particles efficiently. Exercised sheep (26,400 kg-m/d) ate less forage and ruminated less than controls.
INTRODUCTION
The usefulness of forage in the diet of the dairy animal is a function of forage intake and digestion. To develop an understanding of forage intake and utilization, attention has been drawn to the importance of particulate comminution before and during the ruminant digestive process (20, 29). Thus, there is a critical need to pull together the current understanding of this subject to give direction for the future. It is beyond the scope of this paper to review the techniques and principles used to measure particle size. PROPULSION AND MIXING OF S T O M A C H C O N T E N T S
Received May 17, 1985. t Contribution from US Department of Agriculture, Agricultural Research Service in cooperation with the University of Missouri Agricultural Experiment Station. Journal Series Number 9865. 2Mention of a trade name or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture or the University of Missouri Agriculture Experiment Station and does not imply its approval to the exclusion of other products that may also be suitable. 1986 J Dairy Sci 69:1996-2008
Researchers have been unaware of the significance of the physiological and muscular action of the cow and the role it plays in propelling digesta through the digestive tract. Extensive radiographic studies have recently been reported from 16 sheep consuming either dried grass or chaffed lucerne ad libitum (41). This technique, which used x-ray filming of animals dosed with barium sulfate, allowed direct observation of the digestive tract in a normal animal trained to stand with the instrumentation in place. This report confirms many findings already in the literature and adds some new details. A bolus of particulate matter, upon entering the reticulorumen area, may take one of two pathways. If it is dense enough to sink in the region of the cardia, the particles stay cranial to
1996
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW the cranial pillar. If it floats, the particles stay close to the cardia until the next reticular contraction pushes them into the dorsal rumen, caudal to the cranial pillar. A second pathway may be followed, especially in the fasted animal. If the bolus enters the cardia region at the time of a reticulorumen contraction, it is swept into the midrumen stream and carried either cranial or caudal to the cranial pillar. The midrumen stream is a circulating interface passing between the cranial and the caudal pillar. The direction that the bolus material takes, caudally or cranially, depends on its density. The first pathway was most common in sheep fed the ad libitum and the second pathway was most common in the fasted sheep (41) (Figure 1). The movements of the reticulum and rumen apparently create forces in the digesta that cause it to stream or flow in one of two directions. One stream appears to circulate in the dorsal rumen and the other in the ventral rumen with a midrumen stream extending approximately from the top of the cranial pillar to the top of the caudal pillar equal to the positions of these pillars at the resting stage. Less dense particulate digesta floats into the cranial sac and is moved into the dorsal rumen; as the digesta becomes more dense from digestion and mastication, it is later transferred to the ventral rumen and eventually into the cranial sac and reticulum where it flows out through the reticuloorifice. Dense digesta, which sinks immediately into the cranial sac, appears to bypass completely the dorsal and ventral rumen. The time required for digesta to complete the circuit of the dorsal and ventral rumen depends on the consistency of the digesta, which in turn depends on the diet. F o r sheep (41) consuming chopped lucerne hay, the time for digesta to travel from the caudal pillar across the dorsal sac was 30 rain and across the ventral sac was 31 rain. When the sheep consumed dried pelleted grass, the respective times were shortened to 13 and 20 min. Contractions of the reticulum and tureen appear to cause a flow of digesta into the cardia region periodically, which starts rumination. At the height of the reticular contraction, negative pressure changes occur in the esophagus, and particulate matter ascends into the mouth. Fluid and probably small particles are squeezed from the bolus while in the mouth and are
1997
reswallowed. The bolus is then masticated and swallowed. Thus, mastication may provide accessory particulate sorting (41). In view of the processes described, a major m e t h o d for sorting particulates in the rumen appears to be a flotation system. Such a system depends heavily on changes in specific gravity of particles that cause them to sink to facilitate passage. As witl be seen later, sorting by particle. size has also been proposed, which suggests that particles larger than 1200 /~m will not usually pass from the reticulorumen. This proposal infers that some mechanism in the rumen acts like a sieve that resists large particles from passing. Particle sizing may be significant even in a flotation separation system because of the influence of particle size on changes in density of the particle and physical consistency of the digesta. The passage of particulate matter is not a function of motility of the stomach compartments (36) but rather a function of flow associated with each contraction of the compartment involved. Therefore, the flow is greatly dependent on consistency of the digesta and pressure exerted as a result of ruminal contractions. The abomasum has been shown to increase flow per contraction when it is distended, probably because of increased pressure associated with the distention, and this change was not associated with an increase in frequency of contractions. When the abomasum was distended, reticulorumen activity appeared to decrease (41). This combination of events would probably decrease passage of contents from the reticulum through the retiuloomasaI
DORSAL SAC
~R~.~A/ ~
OMASUM //
CAUDAL
....
\
RETiCULUM CRANIAL SAC HORIZONTAL VIEW
VENTRAL SAC VERTICAL VIEW
Figure 1. Summary of digesta movement in reticulorumen (41). Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELY'EA
orifice. Such a system would mean that the abomasum would act as mediator of digesta flow from the reticulorumen, and the consistency and possibly the chemical composition of digesta entering and leaving the abomasum and digesta in the small intestine would have a significant effect of rate of flow. Physical form of forage and particle size may affect both the frequency and amplitude of rumen contraction (23) (Figure 2). Pharr et al. (23) fed sheep 908 g of oat hay that had been ground through a .6t+, .24, or .10-cm screen. The control feed was long hay. The smaller particle size resulted in rumen contraction less frequent and of less amplitude. In another trial (7), cattle were fed either alfalfa tops, oat hay, or a combination of both. Frequency of ruminal contraction was highest for alfalfa tops and least for oat hay. Differences in contraction frequency were not accompanied by differences in ruminal pressure. Waghorn and Reid (36) observed similar results for 24-h periods, but differences between chopped Lucerne hay and fresh rye grass were not significant. The data suggest that rumen contractions may be more frequent with higher quality immature forage and less with lower quality more mature forage, because rumen contractions do not depend on rumination. If motility is increased with high quality forage and the flow per contraction is increased because of changes in physical consistency, this would partially explain the rapid passage observed on succulent pasture grasses as compared with the slower passage of lower quality forages.
EFFECT OF PHYSICAL FORM OF FEED ON D I G E S T I B I L I T Y
The effect of particle size and pelleting of forage on digestibility has been reviewed (37). Depending on the quality of forage, particle size reduction usually reduces digestibility and increases intake. This relationship may be less pronounced for low quality than for high quality forages, because the digestibility of low quality forages may be discounted less by pelleting while intake is increased relatively more. In a comparison of 26 studies, digestibility was depressed an average of 3.3 percentage units by some form of grinding or fine chopping compared with longer forms of the same forage (22) (Table 1). Early and medium cut ryegrass have been compared as chopped versus pelleted. Particle size was about 1000 /~m for chopped and 250 /2m for pelleted samples (6, 37). Digestion of cell wall of pellets was depressed in the rumen but digestion increased in the lower intestine, so total digestibility was not depressed as much as was digestion in rumen. Pelleting appeared to depress total digestion more in the early cut than in the medium cut forage. Alfalfa and red clover appeared to have greater depres-
--10 I
--6
I
p SIZE OF R U M E N PARTICLES A N D PARTICLES L E A V I N G THE RUMEN
The size of the majority of rumen particles ranges from about 200 to >1200/~m (25, 29, 39). Particles that leave the rumen are thought to be < 1200/~m, which is similar to the particle size of digesta found in the duodenum and feces (25). There is probably a difference in size of rumen particles between species. Sheep have fewer large ruminal particles (> 1200/2m) than cattle (25). Sheep also have a chewing pattern different from cattle. Size of particles in the rumen may also be different due to time of sampling; digesta sampled at a greater time postfeeding would have smaller particle size because the animal has had more time to chew. Journal of Dairy Science Vol. 69, No. 7, 1986
I
32B
12-24-64
--2 0
A
---6
I MINUTE LONG OAT HA'
I
-
ic//
2A. o ._j I \ -
rE6
DA
I--2
._./ \
,M,NUTE
I~_
j
j
-2
-2
52B 6-16 -65 IOcm OAT HAY Figure 2. Influence of long versus finely ground (.10 cm) oat hay on amplitude and frequency of rumen contractions: A) atmospheric pressure; B) resting rumen pressure; C) large or primary rumen contraction; D) small or eructation contraction. Scale graduated in centimeters water.
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW
1999
TABLE 1. Influence of particle size on apparent digestibility (34). Cell wall digestibility Rye grass
Form
Particle size (tam)
Dry matter intake
Total tract
Rumen
Lower tract
(g)
(%)
-
Early cut
Chopped Pellet
1030 260
1400 1400
87 80
93 73
2 7
5 19
Early cut
Chopped Pellet
1030 260
900 900
76 76
88 86
-1 4
13 10
Medium cut
Chopped Pellet
960 220
900 900
86 75
89 70
4 -1
6 31
sion in energy digestibility in the rumen but also greater compensation in the small intestine than grasses (3, 6, 33). Digestibility is thought to be depressed for pelleted forages because of their fast passage through the rumen. Apparently the particle size in pelleted forages is nearly equal to that of particles leaving the rumen. Thus, these particles escape the rumen rapidly, w h i c h does not allow enough time for complete microbial digestion (35). Rate of feed passage is increased for pelleted forage so that intake of digestible matter is increased, especially with low quality forage. R E L A T I O N OF D I E T A R Y P A R T I C L E SIZE OF F O R A G E TO P A R T I C L E SIZE OF R U M E N D I G E S T A
It is expected that size of particles in the rumen arc a function of feed particle size, amount of mastication, and amount of rumination. In our laboratory we measured rumen particles from a fistulated cow that was consuming alfalfa in long, chopped-cubed, or pelleted form (38) (Table 2). Particle size appeared to affect size of particles found in the rumen. Alfalfa and orchard grass had been ground through a 13-mm screen had particle sizes of 996 and 857/am, respectively (29). Ruminal particle size from sheep fed these forages was 826 and 558 gm respectively, which was a decrease of 17 and 35% from feed to rumen. Because these feed particles were nearly the size of rumen digesta, less particle size reduction might be expected than for coarser materials. The particle size of masticate
-
Duodenum and ileum (% of total tract)
from cows fed long Bermuda grass hay has been studied (24). Pond et al. (24) did not report a mean particle size, but 82% of the particles in the masticate were larger than 1600/am. It has been suggested that while eating the ruminant chews the ingested particles only sufficiently to form a bolus acceptable for deglutition. Heifers spent less time eating chopped alfalfa hay, which had a mean particle size of 1440/am, than did heifers eating long alfalfa hay (17). Jaster and Murphy (17) theorized that the chopped hay was near the appropriate size for deglutition when ingested and had to be masticated less than long hay. Others have observed that in cows consuming herbage, mean particle size of particulate matter in the boli ranged from 1244 to 1602 /am (12). It appears that the particle size of forage affects the size of rumen particles but not to the extent expected, because the particles must be masticated to a size acceptable for deglutition before ever entering the rumen.
TABLE 2. Effect of physical form of alfalfa on particle size in cattle (38).
Alfalfa
Geometric mean diameter of rumen particles (urn) SD
Long Cubed Pelleted
706 623 485
6.2 4.9 5.3
Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
Poppi et al. (26) reported the particle size of leaves and stems of chopped Pangola and Rhodes grasses that had been harvested as regrowth at 6 and 12 wk (Table 3). Large particles were defined as greater than 1180/~m. The chopped leaves and stems, which were fed to cattle, contained the same amount of large particles-about 85% (26). Mastication reduced large particles to 58 and 76% for leaves and stems; rumen contents contained 27 and 34% l a n e particles, respectively (27). Compared with cattle, sheep had fewer large particles in their rumen 23 vs. 58% and 24 vs. 76% for leaves and stems (26, 28). P A R T I C L E SIZE R E D U C T I O N IN THE R U M E N A N D PASSAGE F R O M THE R U M E N
Because rumen capacity appears to limit feed intake with diets exceeding 35% cell wall (19), factors associated with rate of turnover of rumen particulate matter are of prime importance. Indigestible matter has been suggested as a primary limitation in rumen digesta turnover; digestion rate is less significant (20). There appears to be little information about the relative quantitative importance of these two factors. Critical particle size has been used as a concept in the development of models of digesta flow. Particles larger than a certain size rarely pass into the abomasum from the reticulorumen (13, 22, 34). The concept of critical particle size divides rumen particles into two pools: a large particle pool, which cannot pass out of the rumen, and a small particle pool, which can leave the rumen (28). Size of particles is reduced by rumination and digestion. In most early studies, particle size of ingesta was measured in slaughtered animals. Later reports examined the critical size concept using sheep and cattle equipped with ruminal, abomasal, and ileal cannulas (28) (Figure 3). The concept, as presented, does not appear to include changes in density of particles as digestion progresses. Poppi et al. (26) suggest that the particle size of digesta does not change significantly after it leaves the rumen-reticulum and that fecal particle size is a good estimate of particle sizes leaving the rumen. This report challenges the critical size theory, because some particles larger than 1180 /am escape the rumen. However, the percentage of large particles escaping Journal of Dairy Science Vol. 69, No. 7, 1986
the rumen was small and may be insignificant in practice. Therefore, Poppi et al. (26) concluded that particles greater than 1180 /~m strongly resist escape from the rumen and seldom do escape. In a later report, this same group reported the rate of particle size reduction did not have a significant relation to rate of flow of dry matter from the rumen. (27). Thus, small particles were trapped in the ruminal ingesta mat and impeded from passing from the rumen until the large particles were reduced and began to flow. Polypropylene ribbon has been used to study the passage of rumen particles (39). Significant amounts of particles as large as 5000 to 10,000 /am were recovered in the feces in these studies. It would appear that pliability may be a factor in the passage of such large particles. Apparently, pliable particles bend and pass whole, whereas particles more fibrous and rigid than polypropylene may be more resistant to bending and to passage. Cows in our laboratory passed a small number of particles of tall fescue 5 to 10 cm long, but feces from cows consuming alfalfa contained no such particles. CHEWING TIME A N D P A R T I C L E SIZE
Chewing time may be used as a measure of the roughage index of feeds (2). This index was proposed to be related to the extent of physical
TABLE 3. Proportion of large particles in chopped and masticated grass from two cattle (25). Grass species
Pangola Rhodes
Pangola Rhodes
Regrowth
Large * particles Leaf Stem
(wk)
Chopped grass (g/g)
6 12 6 12 Mean
.87 .87 .90 .89 .81 .84 .80 .82 .85 .86 Masticated grass (g/g)
6 12 6 12 Mean
Greater than 1180 ~m.
.62 .54 .59 .56 .58
.79 .77 .77 .71 .76
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW
I00
80
A
°d 60 i! O
40
i,I O
Z
20
CO CO
Ld
Off
0'
I
I
I
I
2 4 SIEVE PORE DIAMETER (mm)
Figure 3. Relative resistance to flow from the rumen of different size particles in ten forages (25).
fibrousness of roughages and should be related to the ability of the roughage to support the metabolic well-being of cattle. Santini (31) reported that chewing rate in cattle varied with
2001
the coarseness of feeds and that chewing time was related to the roughage index (Table 4). Chewing time (min/kg) may be used as a roughage value index for forages in dairy cow rations; this index would serve to formulate rations that support high lactation performance (32). Sudweeks et al. (32) found that the optimum chewing time for dairy cows was 31.4 min/kg feed. Earlier work (18) showed that cows consuming alfalfa silage cut at three theoretical lengths (.62, 1.27, and 1.91 cm) chewed longer per unit feed on the coarser cut alfalfa than on the shorter cut alfalfa. Cows in this study needed to chew about 30 min/kg feed for optimum performance. More recently (31), particle length and chewing time have been investigated as a basis for roughage index. In these studies, mean particle lengths did not appear related to intake within the range of their study, but particle length was related to chewing time. As particle length increased chewing time increased, but cows chewed less time per kilogram-centimeter of adjusted intake as particle length increased. This indicated that as cows consumed longer length alfalfa hay and silage, they were more efficient chewers (31). Another report (17) included similar indirect evidence (Table 5). Heifers were fed long, medium, and finely chopped alfalfa hay. Heifers fed long hay had a fecal mean particle size of 227/am compared with 297/am for the finely chopped hay. The heifers consuming long hay had similar chewing times but had significantly more boluses per minute of rumination with less time (not significant) chewing each
TABLE 4. Total chewing time (minutes per kilogram-centimeters of adjusted intake) of cows fed diets with different ratios of forage to contentrate (27). Ratio of forage to concentrate 2 .31
33:67 .43
.51
.31
50:50 .43
.51
222 503 609 3434
177 372 460 2440
171 341 431 2248
162 357 430 2339
138 291 360 1880
134 277 346 1886
MPL1 (cm) Forage Neutral detergent fiber Acid detergent fiber Acid detergent liguin
Mean forage particle length. 2Dry matter basis. Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
TABLE 5. Chewing and ruminatingbehavior of Holstein heifers fed alfalfa hay (17). Particle size of alfalfa Parameter
Long
Dry matter intake, kg Total chewing time, h Chewing time, min/kg Boluses per minute of ruminationI Seconds per bolus Boluses per day Fecal particle size, ttm
2000/~m
8.0 16.9 127 1.27 47 606 227
1500 ~m
8.4 16.6 118 i. 18 51 566
8.5 16.6 117 1.15 52 584 297
290
t Means different (P<.10).
bolus than heifers consuming finely chopped hay. Rumen particles from long hay may have cycled through the rumen at a slower rate than the fine chopped particles; thus, they became more hydrated, softer, and needed less mastication during rumination. RUMINATION
One obvious purpose of rumination is to reduce the size of particles of rumen contents. Rumen motility may be associated with but is not dependent on rumination and these activities most probably have impact on the passage of particles from the rumen. Abomasal motility has been reviewed recently (4). A number of physiological factors affect motility of the reticulorumen and abomasum such as ingesta pH, osmotic pressure, and hormone feedback through action on the duodenum and abomasum. The effect of rumen pH on rumination has been measured (39). Four amounts of molasses or sucrose were administered to sheep (Table 6); amounts of 200 g or above reduced rumination for 6 h following administration, and this inactivity was associated with depressed ruminal pH (39). The pH of the rumen was increased by administration of sodium bicarbonate, causing rumination to cease for more than 5 h. The conclusion of these and other studies with varied osmotic pressure was that osmotic pressure was most related to rumination activity and that rumination did not occur at osmotic pressures greater than .350 OsM. A recent report (30), which investigated rate of passage of digesta in dairy cows fed long and chopped alfalfa hay, showed that cows fed sodium Journal of Dairy Science Vol. 69, No. 7, 1986
bicarbonate with chopped hay had slower rates of digesta passage than cows fed long hay. Feeding grain has variable effects on rumination (39). Small amounts of grain dilute out ration cell wall concentration and reduce rumination in proportion to cell wall intake. Larger amounts of grain disrupt rumination. This disruption is probably related to rumen pH as well as osmolarity and other physical characteristics of the grain particles. Research with particle length of alfalfa (31) suggested that particle length affected total chewing time, and chewing time in turn affected rumen volatile fatty acids, pH, and milk fat percentage. D E N S I T Y OF RUMEN PARTICLES
Dense particles, such as grains, migrate to the reticulum and caudal sac of the rumen upon ingestion. Density of forage particles is of interest. Rumen ingesta, a mixture of water
TABLE 6. Effect of molasses on rumination time (39).
Molasses
Posttreatment rumination time during first 6 h postfeeding
(g)
(min)
o lOO 200 300 400
127 128 48 90 0
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW colloids and feed particles, has a density of .9 to .98 g/ml because feed and particularly forage particles have a density of less than 1.0. Density of forage particles is influenced by several factors: density of fiber is less than water, spaces surrounding plant cell may contain air, and particles absorb varying amounts of water (hydration) resulting in varying densities. Also, smaller particles have increased surface area and pack more densely than larger particles. Feed or forage particle size is also related to internal cellular space of plant cell walls-as particle size is decreased, internal cellular space is broken down and decreases. Rumination destroys this internal cellular space, which decreases hydration (absorbtion of water), probably due to the low affinity for water for cellulose and lignin, but increases density of particles due to changes in physical structure (35) (Figure 4). Small wheat straw particles (70 /am) hydrate about 30% less than larger particles (35). The ability of forage particles to absorb water and swell is related to their chemical composition (35). Cellulose, for example, resists hydration. High quality forages containing less structural material may be more susceptible to hydration than forage of low quality. Rumination, chemical composition, and hydration all interact to increase the density of particles upon entering the rumen, but apparently these relationships are not well understood for forage particles. As comminution progresses, specific gravity of the rumen mass increases (11). Passage of inert particles was maximized with a specific gravity of 1.1 to 1.2 (5). The interaction of particle length and specific gravity of nylon particles in the rumen has been studied (9). The specific gravity of these particles was 1.17 and 1.77. Long light and short heavy particles were retained in the rumen longer than long heavy and short light particles, which demonstrated the interaction between length and specific gravity of particles. The length and specific gravity of particles may not have been representative of feed particles in the rumen, however. Alfalfa particles of 2.07 mm, which had been mordanted with chromium to produce particles with densities from 1.12 to 1.70, were fed to cows (10) (Table 7). Rumen turnover rate was positively related to particle density, but there was a quadratic effect; as density increased, turnover rate increased less. In this experiment, denser particles, due to the chro-
2003
mium mordant, were less digestible, which confounds the findings. It is probable that rumen particles can be made so dense that the tendency to settle to the bottom of the reticulum and cranial sac is great enough to resist passage from the reticulorumen. The functional specific gravity of ground hay samples in ionic solutions has been studied (15). Functional specific gravity is defined as the specific gravity of particles with air or gas pockets included. As a particle hydrates and is digested, voids or pockets are eliminated and functional specific gravity is increased. Ionic composition, pH, and osmolarity increase forage functional specific gravity in vitro. In another report, the same researcher (16) studied the effects of particle size and forage composition on functional specific gravity. Rate of change of functional specific gravity was higher in legumes than in grasses. Specific gravity of samples with small particles was higher at all times measured than that of samples with large particles. Particle size influenced rate of change of specific gravity. Forage particles were placed in nylon bags and suspended in the rumen (16). Functional specific gravity of ground forage changed rapidly (within 1 h) after introduction into the rumen. Physical consistency could interact with particle size and density to promote or retard passage of particles from the rumen. Welch (39)
,o
8 6 E
3
2 ~
C [..@710
I
130
I
250
~
d
480 600
MEAN SIZE (microns) .25
I
2
4
6
SCREEN OPENING (ram) Figure 4. Effect of grinding on dry bulk volume (A) and hydration capacity (e) of wheat straw (35). Journal of Dairy Science Vol. 69, No. 7, 1986
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MARTZ AND BELYEA
TABLE 7. Influence of density and chromium on rumen turnover rates of alfalfa fiber (10).
Density
Chromium concentration m cell wall
Turnover rate
(g/ml)
(mg~)
(h)
1.126 1.165 1.242 1.396 1.703
24.2 28,2 31.9 38.9 41.1
.0107 .0072 .0190 .0228 .0194
measured the transit time of particles placed in the dorsal sac, central rumen, and ventral sac of animals fed different amounts of feed. Particles passed the reticuloomasal orifice quickest from the bottom of the tureen and slowest from the dorsal sac, which is consistent with the report of Wyburn (41). However, Welch (39) concluded that consistency (amount of grain vs. long hay) did not relate to the transit time of particles, and thus, particle entrapment must be due to other mechanisms, because there was no effect on turnover due to feeding. Specific gravity of the particles was not reported. This finding is not in agreement with that of Wyburn (41), who showed that consistency of rumen contents appeared to affect ruminal flow. SOME PRACTICAL I M P L I C A T I O N S
Chewing activity in ruminants probably is the cumulative outward manifestation of a number of more fundamental factors. Chewing is related to diet form and composition, rurnen motility, stress, metabolic drive, and metabolic size. Under normal conditions, cattle tend to chew and ruminate a constant amount of time each day, apparently even with increasing fiber (8) (Figure 5). However, because cattle tend to eat less dry matter and spend less time eating high than low fiber diets, rumination and chewing efficiency as grams per hour is decreased markedly for high fiber diets. These relationships appear to be a result of greater difficulty in reducing the particulate size of the more fibrous materials in the ration. Not all research findings are in agreement. Balch (2) proposed that cows ruminate a constant amount of time no matter what amount of fiber is consumed. Others (32) believe that cows attempt to adjust Journal of Dairy Science Vol. 69, No. 7, 1986
by increasing chewing time per day when they consume more fiber. It appears that anything the producer can do to aid the cow to become a more efficient chewer is a step in the right direction. The cow must handle as much dry matter and useable nutrients per unit chewing time as possible, because the cow will probably only chew a constant amount of time each day on a given diet. Prebloom, earlybloom, mid bloom, and full bloom alfalfa hay has been compared for total chewing time in lactating cows IN. A. Jorgenson, 1984, personal communication (Figure 6)]. Cows consuming early cut, high quality alfalfa spent less time chewing, ate more dry matter, and produced more milk than cows consuming the lower quality full bloom alfalfa. Cows consuming the early cut alfalfa appeared to have greater chewing efficiency (nutrients chewed per unit time).Other researchers have shown a relationship between chewing time and rumen pH and volatile fatty acid concentration (31, 32). These reports help explain the importance of keeping concentrate below 55 to 60% of the ration and making up the remainder with quality forage of proper particle size for optimum milk production and animal health. Chewing and ruminating require energy and may result in greater energy cost to the animal than once thought. The effect of exercise on hay intake was studied in sheep (40). Sheep exerting 26,400 kg-m/d exercise ate less hay and ruminated less time during exercise days than nonexercise days. Minutes of rumination
g DM/h
mn/d Chewing time
750-
300-
Ruminating time 500-
200-
Rumination Chewing
efficiency 250-
100-
Crude fibre content 0 20
/ 25
I 30
I ~ 35
20
Figure 5. Influence of fiber content on feeding and ruminating of sheep given fresh grasses [110 samples (8)]. DM = Dry matter.
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW 18 ---%
,
Body size relates to rumination and chewing efficiency (1) (Table 8). Bae et al. found that large older cattle are more efficient chewers than smaller young cattle. Size of fecal particles was similar for all sizes of cattle, indicating that smaller cattle pass the same size particles as large cattle. These findings appear to explain why young cattle (less than 225 kg) do not do well on forage alone even when it is relatively good quality. Young small cattle are not efficient enough chewers to handle a relatively high cell wall ration.
16
rr 14 "IW --~ F- 12 (.9 Z hi
.l-
2005
10 CONCLUSIONS
d
°o
2b
T'l
CONCENTRATE (% DMI) Figure 6. Total chewing time (h/d) as related to change in maturity of alfalfa and concentrate amount (N. A. Jorgensen, 1984, personal communication). DMI = Dry matter intake.
per gram cell wall and fecal particle size did not change with exercise. It may be that additional exercise, disruption, or stress reduces the animals' ability or drive to reduce rumen particulate size. Physiolgical functions, such as milk production, appear to influence other body functions and enable cows to process particulate matter better. Numerous reports indicate that lactation increases ration dry matter intake. Apparently, the lactating cow increases rumen turnover rate of both grain and forage with increasing production (14). Hay, liquid, and grain were marked differentially and fed to dry and lactating cows. Individual cow variation was large and hay to grain ratio was confounded with lactation stage. The data indicate that early lactation is associated with higher ruminal turnover than in late lactation and during the dry period. Lactating cows appear to be more efficient chewers than dry cows (32). It may be that lactation increases the drive toward more chewing, rumination, and rumen motility or more thorough chewing per boll to increase chewing efficiency.
The ruminant animal works to grind ingested particulate matter and propel it through the digestive tract. Figure 7 illustrates a scheme for the passage of forage particles through the ruminant digestive tract. Some forage plants may be consumed whole and are broken down via mastication whereas others may be chopped or ground mechanically before ingestion. As forage particle size is reduced and becomes more nearly equal to rumen particulate size, less reduction of particle size via mastication rumination is necessary. Animals consuming pelleted forage seldom ruminate. There are suggestions that forage is masticated to a particle size of 1000 to 2000 /am before being swallowed. However, the size of particle required for forming a bolus and subsequent swallowing is not well established. Particulate size must be about 1 2 0 0 / l m or less to escape the rumen. It is not entirely clear if this size varies with species of forage and the type of feed. Size of particles in the feces appears to be similar to the size of particles
TABLE 8. Ad libitum intake of forage cell wall and rumination capability for cattle of different size (1).
Animal
Calves Heifers 1 Heifers 2 Heifers 3
Body weight (BW)
Cell wall
Load per day
(kg)
(g/kg"7s )
(g/kg BW"Ts)
119 213 342 456
36 56 80 98
17 21 31 30
Journal of Dairy Science Vol. 69, No. 7, 1986
2006
MARTZ AND BELYEA
Whole Plant
!
Mechanical Reduction of Particle Size I
astication I
~
$ Swallow Particles I000- 2000 um or less
$
Rumination of Particles >1200 um
~ e ~ e g
Rumen Fermentation I and Digestion
Wet Sifting and Mixing, Particles <1200 um Escape
I
Fecal Output Particles <1200 um
ulators of Passage:1 i. 2. 3. 4. 5. 6. 7. 8. 9.
Size of Particles Density of Particles? Cell Wall Content? Rate of Particle Size Reduction? pH and Osmotic Pressure? Distention of Rumen and Abomasum? Strength and Frequency of Ruminal Contractions? Strength and Frequency of Abomasal Contractions? Consistency of Ingesta?
lltems followed by a "?" are not as well understood as other items. Figure 7. Scheme of passage of forage particles in the ruminant.
escaping the rumen. Therefore, measurement of fecal particle size should indicate the size of particles escaping the rumen. Little is known about factors that regulate rate of propulsion of ingesta through the rumen and abomasum. It is reasonably clear that size Journal of Dairy Science Vol. 69, No. 7, 1986
of ruminal particles alone does not regulate rate of passage. Density of particles appears also to play a role, but that role is not well understood. Cell wall content of the forage appears to affect particle passage, but the mechanism is not understood. For example, do high cell
SYMPOSIUM: FORAGE UTILIZATION BY THE LACTATING COW wall forages pass f r o m t h e r u m e n slowly because t h e y require m o r e c h e w i n g per se, or is it because t h e y digest m o r e slowly t h a n low cell wall forages and are t h u s p r e s e n t longer? O s m o t i c pressure and p H o f t h e r u m e n and a b o m a s u m a p p e a r t o i n f l u e n c e m o t i l i t y o f t h e s e organs, b u t h o w passage o f ingesta is i n f l u e n c e d is n o t clear. The role t h a t d i s t e n t i o n and m u s c u l a r c o n t r a c t i o n s o f t h e r u m e n , r e t i c u l u m , and a b o m a s u m play in passage o f ingesta has n o t b e e n s t u d i e d extensively. T h e r e are i n d i c a t i o n s that the frequency of contractions of the a b o m a s u m d o e s n o t change m a r k e d l y in n o r m a l r u m i n a n t s . H o w , t h e n , can s o m e diets be passed m o r e rapidly t h a n others? It m a y be t h a t a d d i t i o n a l pressure ( d i s t e n t i o n ) for s o m e diets or m e t a b o l i c states cause a g r e a t e r f l o w rate p e r c o n t r a c t i o n and t h u s a greater t o t a l passage o f p a r t i c u l a t e m a t t e r . However, this c o n c e p t is n o t i n d i c a t e d in t h e literature cited. C o n s i s t e n c y o f t h e ingesta m a y also play a role in passage. Even t h o u g h t h e r e have b e e n a f e w studies to s h o w t h a t c o n s i s t e n c y is o f little i m p o r t a n c e , this line o f research has n o t b e e n p u r s u e d very diligently.
REFERENCES
1 Bae, D. H., J. G. Welch, and B. E. Gilman. 1983. Mastication and rumination in relation to body size of cattle. J. Dairy Sci. 66:2137. 2 Balch, C. C. 1971. Proposal to use time spent chewing as an index to the extent to which diets for ruminants possess the physical property of fibrousness characteristics of roughages. Br. J. Nutr. 36:383. 3 Beever, D. E., D. F. Osbourn, S. B. Cammell, and R. A. Terry. 1981. The effect of grinding and pelleting on the digestion of Italian ryegtass and timothy by sheep. Br. J. Nutr. 46:357. 4 Bell, F. R., 1980. The mechanisms controlling abomasal emptying and secretion. Page 81 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc. Westport, CT. 5 Campling, R. C., and M. Freer. 1962. The effect of specific gravity and size on the mean time of retention of inert particles in the alimentary tract of the cow. Br. J. Nutr. 16:507. 6 Coelho da Silva, J. F. 1972. The effect in sheep of physical form on the sites of digestion of rye grass. Br. J. Nutr. 28:357. 7 Colvin, H. W., Jr., P. T. Cupps, and H. H. Cole. 1958. Dietary infuences on eructation and related ruminal phenomina in cattle. J. Dairy Sci. 41 : 1565. 8 Dulphy, J. P., B. Remond, and M. Theriez. 1980. Ingestive behavior and related activities in ruminants. Page 103 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc.,
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Westport, CT. 9 Durkwa, L., and J. G. Welch. 1982. Particle length and specific gravity on rumination and rate of passage. J. Dairy Sci. 65(Suppl. 1):145. (Abstr.) 10 Ehle, F. R. 1984. Influence of feed particle density on particulate passage from rumen of holstein cow. J. Dairy Sci. 67:693. 11 Evans, E. W., G. R. Pearce, J. Burnett, and S. L. Pillinger. 1973. Changes in some physical characteristics of the digesta in the reticulo-rumen of cows fed once daily. Br. J. Nutr. 29:357. 12 Gill, J., R. C. Campling, and D. R. Wentgarth. 1966. A study of chewing during eating in the cow. Br. J. Nutr. 20:13. 13 Greenhalgh, J.F.D., and G. W. Reid. 1973. The effects of pelleting various diets on intake and digestibility in sheep and cattle. Anim. Prod. 16:223. 14 Harmell, G. F., and L. D. Satter. 1979. Determination of rumen fill retention time and ruminal turnover rates of ingesta at different stages of lactation in dairy cows. J. Anita. Sci. 48:381. 15 Hooper, A. P., and J. G. Welch. 1985. Functional specific gravity of ground hay samples in ionic solutions. J. Dairy Sci. 68:848. 16 Hooper, A. P., and J. G. Welch. 1985. Effects of particle size and forage composition on functional specific gravity. J. Dairy Sci. 68:1181. 17 Jaster, E. H., and M. R. Murphy. 1983. Effects of varying particle size of forage and digestion and chewing behavior of dairy heifers. J. Dairy Sci. 66:802. 18 J orgensen, N. A., M. E. Finner, and J. P. Marquardt. 1978. Effect of forage particle size on animal performance. Page 1048 in Proc. Am. Soc. Agric. Eng. 19 Mertens, D. R. 1982. Using neutral detergent fiber to formulate dairy rations. Page 116 in Proc. Georgia Nutr. Conf. Univ. Georgia. 20 Mertens, D. R., and L. O. Ely. 1979. A dynamic model of fiber digestion and passage in the ruminant for evaluating forage quality. J. Anita. Sci. 49:1085. 21 Minson, D. J. 1963. The effect of pelleting and wafering on the feeding value of roughages-a review. J. Br. Grass. Soc. 18:39. 22 Pearce, G. R. 1967. Changes in particle size in the reticulorumen of sheep. Aust. J. Agric. Res. 18:119. 23 Pharr, L. D., H. W. Colvin, Jr., and P. R. Noland. 1968. Rumen motility of sheep as affected by the physical form of oat hay. J. Anim. Sci. 27:414. 24 Pond, K. R., D. E. Akin, M. Molloji, and W. C. Ellis. 1982. Effects of ingestive mastication on the physical and chemical degradation of Coastal bermudagtass. Proc. Annu. Mtg. Am. Forage Grassl. Counc. 25 Poppi, D. P., B. W. Norton, D. J. Minson, and R. E. Hendricksen. 1980. The validity of the critical size theory for particles leaving the rumen. J. Agric. Sci. 94:275. 26 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1981. Studies of cattle and sheep eating leaf and stem fractions of grass. III. The retention time in the rumen of large feed particles. Aust. J. Agric. Journal of Dairy Science Vol. 69, No. 7, 1986
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Res. 32:123. 27 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1981. Studies of cattle and sheep eating leaf and stem fractions of grasses. II. Factors controlling the retention of feed in the reticulo-rumen. Aust. J. Agric. Res. 32:109. 28 Poppi, D. P., D. J. Minson, and J. H. Ternouth. 1980. Studies of cattle and sheep eating leaf and stem fractions of grasses. I. The voluntary intake digestibility and retention time in the reticulorumen. Aust. J. Agric. Res. 32:99. 29 Robles, A. Y., R. L. Belyea, F. A. Martz, and M. F. Weiss. 1980. Effect of particles size upon digestible cell wall and rate of in vitro digestion of alfalfa and orchardgrass forage. J. Anim. Sci. 51:783. 30 Rogers, J. A., L. D. Muller, T. J. Snyder, and T. L. Maddox. 1985. Milk production, nutrient digestion, and rate of digesta passage in dairy cows fed long or chopped hay supplemented with sodium bicarbonate. J. Dairy Sci. 68:868. 31 Santini, F. J., A. R. Hardi, and N. A. Jorgensen. 1983. Proposed use of adjusted intake based on forage particle length for calculation of rouhgage indexes. J. Dairy Sci. 66:811. 32 Sudweeks, E. M., L. O. Ely, and L. R. Sisk. 1980. Using a roughage value index in formulating dairy rations. Page 80 in Proc. Georgia Nutr. Conf. Univ. Georgia.
Journal o f Dairy Science Vol. 69, No. 7, 1986
33 Thompson, D. J., P. E. Beever, J. F. Coelho Da Silva, and D. G. Armstrong. 1972. The effect in sheep of physical form on the sites of digestion of a dried lucern diet. Br. J. Nutr. 28:31. 34 Troelsen, J. E., and J. B. Campbell. 1968. Voluntary consumption of forage by sheep and its relation to the size and shape of particles in the digestive tract. Anita. Prod. 10:289. 35 Van Soest, P. J. 1982. Nutritional ecology of the ruminant. O & B Books, Inc. Corvallis, OR. 36 Waghorn, G. V., and C.S.W. Reid. 1983. Rumen motility in sheep and cattle given different diets. N . Z . J . Agric. Res. 26:289. 37 Waldo, D. R. 1973. Effects of physical form of feed on digestibility. Page 6 in Maryland Nutr. Conf., Univ. Maryland. 38 Warren, W. P. 1976. Dietary effects on ruminant digestion and intake. Ph.D. Thesis, Univ. Missouri, Columbia. 39 Welch, J. G. 1982. Rumination, particle size and passage from the rumen. J. Anim. Sci. 54:885. 40 Welch, J. G., R. H. Palmer, B. E. Gilman, and L. S. Bull. 1983. Hay intake and rumination in exercised sheep. J. Dairy Sci. 66(Suppl. 1): 147. (Abstr.) 41 Wyburn, R. S. 1980. The mixing and propulsion of the stomach contents of ruminants. Page 35 in Digestive physiology and metabolism in ruminants. AVI Publ. Co., Inc., Westport, CT.