Degradability characteristics of dry matter and crude protein of forages in ruminants

EJSEVIER

Animal Feed Science Technology 57 ( 1996) 291-3 11

Degradability characteristics of dry matter and crude protein of forages in ruminants M.A.G. von Keyserlingk a**,M.L. Swift b, R. Puchala ‘, J.A. Shelford a a Department of Animal Science, 2357 Main Mall, University of British Columbia, Vancouver, B.C. V6T 124, Canada b Pro Form Feeds Inc., 46255 Chilliwack Central Road, P.O. Box 1000, Chilliwack, B.C. V2P 656. Canada ’ Department ofAnimal Physiology, Warsaw Agricultural Universiry, Nowoursynowska 166,02 766 Warsaw, Poland

Received 13 December 1994; accepted 3 1 August 1995

Abstract Twelve corn silages, 22 grass silages and 14 grass hays, obtained from various farms located in the lower Fraser Valley region of British Columbia, and 16 alfalfa hays, grown primarily in the Columbia basin of central Washington State, were evaluated using both the rumen and the mobile nylon bag in situ techniques. Nylon bags containing each forage were incubated in duplicate for 0, 2, 4, 8, 12, 24, 48, 72, or 96 h in two of six non-lactating Holstein cows fitted with rumen and duodenal cannulae. All forage types were evaluated in terms of the following dry matter (DM) and crude protein (CP) digestion characteristics: soluble fraction A, degradable fraction B, degradation rate, lag phase, and effective degradability. The mobile nylon bag technique was used to determine intestinal disappealance of DM and CP from the forages following pre-incubation in the rumen for 12 h. Significant (P < 0.05) differences in degradation characteristics occurred within all forages with regard to the soluble and potentially degradable DM and CP fractions. Soluble CP content in the rumen varied from 44.08 to 75.37% and from 18.74 to 65.38% in the corn and grass silages, respectively, and from 48.27 to 75.43% and from 30.13 to 65.95% in the alfalfa and grass hays, respectively. Significant differences within each forage type were also observed for the degradable CP in fraction B: 10.89 to 45.28% for corn silage, 20.72 to 82.77% for grass silage, 16.67 to 44.88% for grass hay and 25.44 to 62.93% for alfalfa hays. Significant differences (P > 0.05) were observed in fractional rates of ruminal DM degradation of the grass hays and corn silages. Significant differences did exist in the fractional rates of ruminal CP degradation within all forage types with the exception of alfalfa hays. Effective degradabilities of DM and CP

* Corresponding author. Tel.: (604) 822 3954; fax: (604) 822 4400; e-mail: [email protected]. 0377-8401/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0377-8401(95)00865-9

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were also significantly different between samples of a particular forage type. The mobile nylon bag data indicated that approximately 20% of the original CP in the grass silage, grass hay and alfalfa hay samples disappeared in the intestine and that there was significant variation between individual samples. On average, in the corn silage samples more than 10% of the original nitrogenous material disappeared in the intestine. The results presented in this study clearly demonstrate that the use of tabulated values for describing individual batches of forages in terms of their degradability characteristics is inaccurate since they may not reflect the particular forage being used in the ration and thus may lead to errors in diet formulation. Keywords:

Silage-corn; Silage-grass; Alfalfa hay; Grass hay; Digestibility-forages

1. Introduction Values for crude protein (CP) and ruminally undegradable CP content are now required in feed evaluation systems currently used in North America (NRC, 1989). The determination of forage CP content is a standard procedure for most laboratories as it is relatively easy to measure (Hoffman et al., 1993). In contrast, the estimation of rumen degradable and rumen undegradable fractions is considerably more complicated. Whereas conventional in vivo digestibility measurements are thought to accurately reflect the feeding value of total diets they are limited in that they are laborious and time consuming (Amrane and Michalet-Doreau, 1993) and are restricted to assessing one feed or a combination of feeds in the form of a diet. An alternative method, which has become increasingly popular as a means to estimate the rumen degradable and rnmen undegradable fraction, is the in situ technique (Nocek and English, 1986). This technique allows a number of feeds to be assessed concurrently and is now accepted as one of the basic methods required by the new protein evaluation systems proposed by NRC (1989) and other organizations (IZlrskov, 1992). Routine in situ analysis of forages is however difficult, since it is also time consuming and labor intensive (Van Straalen and Tamminga, 1990). The feed manufacturing industry, therefore, relies on values such as those published by NRC (1989). Unfortunately, the accuracy of ration formulation depends on the assumption that all forages are represented by these limited published values describing rnmen degradability. There is little known concerning the variability in values for rumen degradability within a given forage type (NRC, 1989). This is a serious concern since forages may contribute up to one-third of dietary rumen undegradable protein in a lactating dairy cow ration (Hoffman et al., 1993). Some of the problems encountered while feeding dairy cattle may be a result of incorrect mmen degradability parameters being assigned to the forages during ration formulation. In reviewing the literature it is evident that there has been limited work conducted with regard to assessing the possible range of values for degradability characteristics of dry matter (DM) and CP within a type of forage (Hoffman et al., 1993). The use of fermented feeds in dairy cattle rations has gained popularity because preservation of forage material in this manner minimizes the loss of nutrients during harvesting and throughout storage (McDonald, 1981). In the Fraser Valley region of British Columbia and the Pacific Northwest region of the United States the use of grass

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and corn silages in dairy cattle rations is increasing. The majority of the lactating dairy cattle rations in these areas consists of equal proportions of forage and concentrate. The proportion of corn silage to grass silage in the forage component ranges from 50:50 to 25:75 (corn silage: grass silage). This forage combination is ideal since corn silage, which provides more rumen fermentable non-fiber carbohydrate (starch) (Mahanna, 1994), complements grass silage, which tends to be much higher in CP. Although corn and grass silages make up the majority of the roughage component, approximately 10% of the ration is comprised of grass and/or alfalfa hay. The objectives of the present study were to assess in situ ruminal and intestinal degradability and fractional rates of digestion and the effective degradability in the rumen of the DM and CP fractions of corn silages, grass silages, grass hays and alfalfa hays selected randomly from dairy farms in the Fraser Valley region of British Columbia.

2. Materials and methods 2.1. Forage collection Twelve corn silages, 22 grass silages, 14 grass hays and 16 alfalfa hays (Medicago satiua) were evaluated using the in situ rumen and mobile nylon bag techniques. Silage samples were collected by the Feed Manufacturing Industry field staff from silos located on farms in the Fraser Valley of British Columbia. Samples were collected from each silo by sampling in at least seven different areas. All seven samples were thoroughly mixed together and a composite sample was taken. Silage samples were representative of typical silages used by dairy farmers and were not identified as to maturity, species or variety of the grasses, corns or alfalfas. Hay samples were collected from individual farms by representatives of the Feed Manufacturing Industry field staff by subsampling at least five to seven bales using a bale core sampling device prior to pooling, thorough mixing and obtaining a composite sample. All hay samples were representative of typical hays used by dairy farmers and were not separated by maturity, species or variety differences. All forage samples were frozen in a chest freezer at - 10°C immediately after returning to the laboratory until the onset of the in situ determinations. After thawing, dry matter was determined by drying the entire sample (approximately 1-2 kg) in a forced air oven at 55°C until constant weight was achieved. The forage samples were then ground through a 2 mm screen prior to incubation. Residual DM determinations were conducted on all feed samples by drying at 100°C in a forced air oven. 2.2. Animals and feeding Six non-lactating Holstein cows were fitted with a rumen fistula and a T-shaped duodenal cannula. They were fed a diet consisting of 5.5 kg of an alfalfa (60%) and grass hay (40%) mixture and 3.0 kg of 16% protein commercially prepared dairy concentrate per day. The feed was fed, in four equal portions, every 6 h in order to

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maintain a relatively stable rumen environment. Animals were cared for according to the guidelines provided by the Canadian Council on Animal Care (1993). 2.3. Rumen in situ incubation offeeds Nylon bags (5 cm X 10 cm; pore size 52 Frn; ANKOM, New York) were filled with 2-3 g of dry ground forage. Each forage sample was incubated in duplicate in each of two cows and a maximum of three different feeds were incubated in one cow per incubation period. Nylon bags were placed in the rumen 96,72,48,24, 12, 8,4, 2 or 0 h before removal at a common time in order to minimize the variation in time that the bags were exposed to air after incubation and to enable simultaneous washing of all bags. Nylon bags were sealed using rubber bands which were easily removed following rumen incubation. Nylon bags were suspended in the rumen in a polyester mesh bag (25 cm X 40 cm; pore size 3 mm). Following ruminal incubation, the polyester mesh bags containing all the nylon bags (including the zero bags) were rinsed with cold water to remove particulate material. The nylon bags were then removed from the mesh bags and placed in a conventional clothes washing machine. The machine was allowed to fill with water and to agitate for 5 min prior to draining. This procedure was repeated until the rinse water remained clear, normally four to five washes. Samples were dried in a forced air oven at 55°C prior to determination of dry matter disappearance. Replicates within cows were pooled and ground through a 0.5 mm screen prior to nitrogen (N) analyses. 2.4. Intestinal in situ incubation of feeds Measurement of nutrient disappearance during passage through the intestine was determined using the mobile nylon bag technique (De Boer et al., 1987). Duodenal nylon bags (3.5 cm X 5 cm; 52 pm; ANKOM, New York) were filled with 0.5 g of dry ground forage and sealed with a heat sealer (Audion Electra Sealmaster No. 230). Duodenal incubations were undertaken once weekly. Forages were pre-incubated in quadruplicate in the rumen of one cow for 12 h. Following rumen incubation, the bags were rinsed thoroughly in cold tap water and stored at 4°C until time of insertion into the duodenum. One bag was inserted into the duodenum every 30 min. Duodenal bags were collected from the feces and hand washed until the rinse water remained clear. Dry matter disappearance was determined for all replicates by drying them in a forced air oven at 55°C until constant weight was achieved. All replicates were ground through a 0.5 mm screen. Due to limited amounts of sample remaining after in situ incubation, replicates within cows were pooled prior to N analysis. Residual DM determinations were conducted on the pooled mobile bag incubation residues by drying at 100°C for 6 h. 2.5. Chemical analyses Nitrogen determination was conducted using the Kjeldahl method in an automatic Kjelfoss apparatus (Foes Electric, Copenhagen, Denmark). Acid detergent fiber (ADF),

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neutral detergent fiber (NDF) and acid detergent insoluble N (ADIN) were measured according to the methods of Goering and Van Soest (1970). 2.6. Estimation

of rumen degradation parameters

Incorporation of microbial protein into the nylon bag contents was not measured. It was assumed that microbial contamination was zero when the DM and CP disappearance curves were calculated. The percent disappearance of the DM and CP at each individual incubation time was calculated as the difference between the feed and the portion remaining after incubation in the rumen. These values were used to determine the portion in the soluble A fraction, degradable B fraction and fractional rate of disappearance of the degradable B fraction using non-linear regression. The following equation developed by 0rskov and McDonald (1979) and subsequently modified by Dhanoa (1988) and Denham et al. (1989) was used p =

a

+

b(

1 -

emc(leLf))

(1)

where p is percentage of material in the bag which disappeared at time t, fraction a is the y-intercept which is the amount of soluble material determined in the 0 h bags which had been subjected only to the washing procedure, fraction b is the amount which in time will degrade (degradable fraction) at a fractional rate c (% h- ’>, taking into account any lag time (Lt) prior to the onset of digestion. The Marquardt method, a linear iterative curve fitting procedure, of the PROC NLIN procedure (SAS Institute Inc., 1987) was used to reduce the residual sums of squares associated with the regression model (Nocek and English, 1986) thereby solving for the factors in the exponential equation (Eq. 1) mentioned above. In the case of the CP degradability determinations, the PROC NLIN procedure was unable to fit the linear iterative curve for 2 grass silage samples and therefore they were calculated as missing values. 2.7. Estimation

of effective degradability

in the rumen

Eq. 1 described above, fails to take into consideration the fractional rate of flow through the rumen which is known to have a major effect on rumen degradability (Mir et al., 1991). McDonald (1981) proposed an equation taking into consideration the fractional rate of passage which influenced the effective degradability (P) of a specific nutrient. Dhanoa (1988) and Denham et al. (1989), however, modified the equation of McDonald (1981) to take into consideration the time period (lag) prior to the onset of degradation of the potentially degradable but insoluble b fraction. The equation used to determine effective degradability in the present study was

where a, b, c, kp and Lt are as defined previously. Effective degradability of DM and CP was calculated using a hypothetical fractional rate of passage of the particulate matter in the rumen of 6.0% h-r (Broderick and Hristov, 1993).

2%

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2.8. Disappearance in the intestine

Disappearance in the intestinal tract was calculated by subtracting the amount which disappeared in the rumen following 12 h of incubation from that which disappeared in the total tract (which is the total amount of material disappearing following both ruminal and intestinal in situ incubation). 2.9. Statistical analyses Statistically, the data were analyzed using the General Linear Model (GLM) procedures of SAS Institute Inc. (1987). which uses least-square means for each parameter. Feeds were the only sources of variation considered, thus differences between feeds were partially confounded by animal and period effects. Analytical variability was included in the error variance. The data were analyzed using procedures of SAS Institute Inc. (1987) to determine ranges and standard deviation (SD) values. Coefficient of variation (CV) values (not stated explicitly) were also calculated using procedures of SAS Institute Inc. (1987). Pearson correlation coefficients were computed using procedures of SAS Institute Inc. (1987) between initial feed DM, N, ADIN, ADF and NDF and the soluble CP fraction A, degradable CP fraction B and effective CP degradability.

3. Results The nutrient composition of the forages is given in Table 1. The DM content of corn silages used in this experiment ranged from 220 to 320 g kg-’ and the CP content from 63.8 to 97.5 g kg-’ of DM. The ADIN content of the corn silages ranged from 0.5 to 1.10 g kg-’ of DM. The grass silage DM content ranged from 190 to 780 g kg-‘, the CP content ranged from 78.8 to 233.8 g kg-’ of DM, and ADIN ranged from 1.40 to 2.60 g kg- ‘. The grass silage ADF and NDF content ranged from 300.1 to 457.0 g kg- ’ and from 459.4 to 726.0 g kg- ’ of DM, respectively. Alfalfa hay samples averaged 208.8 g kg-’ CP with a range from 186.2 to 236.3 g kg- ’ of DM. Acid-detergent fiber and NDF content from alfalfa hays ranged from 245.9 to 372.4 g kg- ’ and 320.7 to 451.8 g kg-’ of DM, respectively. Grass hays had an average CP content of 160.0 g kg-’ of DM, with a range from 106.3 to 204.4 g kg-’ of DM. Acid detergent insoluble N ranged from 0.70 to 3.2 g kg- ’ of DM in the grass hays. The range found in the percentage DM present in soluble fraction A and in degradable fraction B, in the fractional rate of DM degradation and the lag phase associated with each forage type is given in Table 2. Corn silages showed less variation than the other three forages in the soluble DM fraction. Corn silage soluble DM Fraction A ranged from 43.76 to 56.93% with a standard deviation of 2.89. The grass silage soluble DM values had a SD of 6.58 with values ranging from 28.34 to 59.28%. Although alfalfa hay only had a standard deviation (SD) of 3.63 in soluble fraction A DM values there was still more than a 10 percentage unit difference between the minimum and maximum values. Grass hays exhibited a greater variation in DM soluble fraction A value than alfalfa hay with a SD of 7.55.

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Table 1 Nutrient composition

(g kg-

’ of DM) of preserved forages

Nutrient ’

Minimum

Maximum

Mean

Corn silage (n = 12) DM CP ADIN ADF NDF

220.0 63.8 0.5 274.5 458.2

320.0 97.5 1.1 342.0 586.3

270.0 80.0 0.9 294.8 500.0

30.0 9.4 0.1 18.4 33.3

Grass silage (n = 22) DM CP ADIN ADF NDF

190.0 78.8 1.4 300.1 459.4

780.0 233.8 2.6 457.0 726.0

380.0 130.6 1.7 353.2 561.9

160.0 42.5 0.4 38.5 71.4

Arfalfa hay (n = 16) DM CP ADIN ADF NDF

876.3 186.2 1.3 245.9 320.7

960.3 236.3 2.3 372.4 451.8

919.6 208.8 1.9 322.7 393.8

26.2 13.1 0.3 31.2 35.1

Grass hay (n = 14) DM CP ADIN ADF NDF

852.0 106.3 0.7 304.0 399.6

936.0 204.4 3.2 407.4 673.2

895.0 160.0 1.6 349.9 572.2

25.0 31.9 0.8 31.8 86.8

a DM, dry matter; CP, crude protein; NDF, neutral-detergent fiber. b Standard deviation.

ADIN, acid-detergent

insoluble

nitrogen;

SDb

ADF, acid-detergent

fiber,

Degradable fraction B DM values behaved in a similar fashion to the soluble fraction A values with the corn silages and alfalfa hays exhibiting less variation than the grass silages or the grass hays (Table 2). In the case of corn silages, there were no significant (P > 0.05) differences observed in the degradable B Fractions of the 12 samples tested. The range in the fractional rate of corn silage DM degradation in Fraction B (3.01 to 6.78% h-l) was fairly narrow in comparison with the grass silages (1.01 to 12.76% h-’ ). Alfalfa hays, however, did show significant differences (P < 0.05) in DM degradable fraction B values between individual samples tested, ranging from 32.22 to 41.76%. The CV value of 25.62% obtained for the DM degradable fraction B values between the individual grass hays indicated that there was very little homogeneity between samples. This was also reflected in the wide range of values obtained which ranged from 24.82 to 65.23%. Although the SD of the degradable fraction B DM of grass silage was 7.70, there were no significant (P > 0.05) differences observed between any of the grass silages tested. The length of the lag phases (Table 2) calculated from the in situ DM disappearance

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Table 2 Dry matter degradation characteristics: soluble fraction A, degradable fraction B, the fractional rate of degradation (c) and the length of the lag observed in forages incubated in the rumen Minimum

Maximum

Mean

43.76 30.70 3.01 0.00

56.93 42.05 6.78 6.41

48.22 38.17 4.70 2.42

2.89 2.76 1.05 2.42

Grass silage (n = 22) Fraction A (o/o) 28.34 Fraction B (%) 13.42 c(% h-‘) 1.01 Lag (h) O.%

59.28 58.28 12.76 11.41

41.07 43.16 4.84 6.17

6.58 7.70 4.83 2.57

**

Corn silage (n = 12) Fraction A (%) Fraction B (%) c(%h-‘) Lag (h)

SD’

Significance b **

**

Arfalfa hay (n = 16)

Fraction A (%) Fraction B (%) c(% h-i) Lag (h)

37.98 32.22 2.87 0.00

52.30 41.76 14.17 5.96

45.94 35.97 7.47 1.66

3.63 2.67 2.74 1.36

** **

Grass hay (n = 141 Fraction A (“lo) Fraction B (%) c(% h-‘) Lag (h)

35.62 24.82 1.43 0.18

61.81 65.23 8.32 9.96

46.22 40.81 4.62 4.74

7.55 10.45 1.70 2.84

** ** * *

’ Standard deviation. b Indicating that there were significant differences between at least two of the samples within a forage type: P < 0.05; * ’ P < 0.01. ??

data showed large SD values for all forages; however, only significant differences (P < 0.05) were observed between the individual corn silages and grass hays. Grass hays were the only forage type which exhibited significant (P < 0.05) differences in the fractional rate of DM degradation with individual values ranging from 1.43 to 8.32% h-l. The soluble CP fraction A values for the corn silages differed significantly (P < 0.05) ranging from 44.08 to 75.37% with a mean of 63.24 f 9.28% (Table 3). In grass silages there was also a significant difference (P < 0.05) in soluble CP fraction A values observed between the samples ranging from 18.74 to 65.38%. Grass hays exhibited significant (P < 0.05) differences in the soluble CP in fraction A with values ranging from 30.13 to 65.95% (Table 3). Although soluble CP in the alfalfa hays ranged from 48.27 to 75.43%, the SD (6.74) was much less than that for the grass hays (11.56). There were also significant differences (P < 0.05) within individual forage types in terms of the CP degradable fraction B. Crude protein potential degradability (fraction B) values for corn silage ranged from 10.89 to 45.28%. The CP degradable B fraction values for grass silages showed considerable variation with values ranging from 20.72% to 82.77% with an average of 45.33 + 13.67%. The grass hays had a mean value of

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Table 3 crude protein degradation characteristics: soluble fraction A, degradable fraction B, the fractional rate of degradation (c) and the length of the lag phase observed in forages incubated in the lumen Minimum

Maximum

Mean

SDa

Significance b

44.08 10.89 2.91 0.00

75.37 45.28 11.11 21.31

63.24 24.40 6.25 4.41

9.28 9.10 1.96 6.34

** **

Grass silage (n = 20) 18.74 Fraction A (o/o) 20.72 Fraction B (o/o) 0.88 c(%h-‘) 0.00 Lag (h)

65.38 82.77 17.50 11.44

48.21 45.33 5.11 3.49

11.60 13.67 2.97 3.21

** ** ** **

Arfaljb hay (n = 16) Fraction A (%o) Fraction B (o/o) c(%h-‘1 Lag (h)

48.27 16.67 4.13 0.00

75.43 44.88 17.07 3.15

58.99 33.16 8.24 1.15

6.74 6.68 2.89 1.07

** ** **

30.13 25.44 3.56 0.00

65.95 62.93 13.32 16.45

43.97 46.11 6.61 4.62

11.56 11.09 2.31 4.19

** **

Corn silage (n = 12)

Fraction A (%o) Fraction B (%) c(%h-‘1 Lag (h)

**

Grass hay (n = 14)

Fraction A (o/o) Fraction B (o/o) c(%h-‘) Lag (h)

**

a Standard deviation. b Indicating that there were significant differences between at least two of the samples within a forage type: * * P < 0.01.

46.11 &-11.09% for CP degradable fraction B, ranging from 25.44 to 62.93%. The fractional rate of CP degradation in grass silage and alfalfa hay varied considerably ranging from 0.88 to 17.50% h-’ (CV= 57.98) and 4.13 to 17.07% h-‘, respectively. There were no significant (P > 0.05) differences between individual samples in the fractional rate of CP degradation of grass hays. The length of the lag phase calculated from the rumen in situ CP data ranged from 0 to 21.31 h and from 0 to 11.44 h for corn silages and grass silages, respectively. The length of the lag phases calculated from the rumen in situ CP data ranged from 0 to 3.15 h for alfalfa hays and 0 to 16.45 h for grass hays. The effective degradability of DM in the four types of forages is given in Table 4. The data have been calculated using a fractional rate of passage (kp) of 6.0% h-‘. In all cases investigated there were significant differences (P < 0.05) between samples in effective DM or CP degradability within a forage type. Corn silages exhibited the least variation with a mean of 62.62 f 2.84% using the 6.0% h-’ kp value. Within the grass silage samples there was a difference of 36 percentage units between the minimum and maximum effective DM degradability values. Alfalfa hays had a minimum effective DM degradability at the 6.0% hh ’ kp of 54.47% and a maximum of 68.79%. The grass hays,

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Table 4 Effective degradability a (%) of dry matter (DM) and crude protein (CP) in corn and grass silages calculated using a fractional rate of passage (Q) of 6.0% h-’ Item

Minimum

Maximum

Mean

68.42 80.06

62.62 73.36

2.84 5.16

** **

13.89 77.81

53.56 62.93

8.55 11.18

** **

68.79 83.46

63.16 76.29

3.66 4.21

* *

74.99 77.51

58.50 62.04

7.82 9.18

** **

SDb

Significance ’

Corn silage fn = 12)

DM CP

56.84 63.41

Grass silage fn = 22) DM 37.58 CP 33.02 A&z&z hay (n = 16)

DM CP

54.47 68.43

Grass hay (n = 14)

DM CP

46.00 47.86

a Effective degradability estimates were calculated using the equation P = a +(bc/(c + kp))e(-“PL’) with the fractional rate of passage set at 6.0% h- ‘. b Standard deviation. ’ Indicating that there were significant differences behveen at least two of the samples within a forage type: * P < 0.05; * * P < 0.01.

however, were more variable. There was a 28 percentage unit spread between the minimum and maximum effective DM degradability values for grass hays. The range of values obtained for the effective degradability of CP are also given in Table 5. Corn silages ranged (P < 0.05) in effective CP degradability from 63.41 to 80.06% using the kp of 6.0% h - I. Grass silages also exhibited a significant difference between individual samples in CP effective degradability. They had calculated CP mean effective degradability values of 62.93 f 11.78% with a range of 33.02-77.78%. The effective CP degradability in alfalfa hay ranged from 68.43 to 83.46%. Grass hays exhibited a larger variation, ranging from 47.86 to 77.51%. The correlation coefficients between initial nutrient composition (DM, N, ADIN, ADF, and NDF) of the forages and the soluble CP A fraction, degradable CP B fraction as well as the effective degradability of CP are provided in Table 5. In the case of the corn silages there was no significant relationship (P > 0.05) between any of the parameters tested. Grass silages, however, exhibited a significant (P < 0.05) relationship between the NDF content of the initial forage and the degradable fraction B and the effective degradability of CP. Although the effective degradability of CP from grass hays was most closely related (P < 0.01) to initial feed NDF (r = -0.91) there was no significant (P > 0.05) relationship in the case of initial NDF content of alfalfa hay and effective CP degradability. On the other hand, the effective CP degradability of alfalfa exhibited a significant (P < 0.05) relationship with initial feed DM content (r = 0.52). Tables 6 and 7 illustrate the differences observed in the amount of DM and CP, respectively, which disappeared following 12 h rumen incubation in the intestines and in

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Table 5 Correlation coefficients (r) between nutrient composition (g kg-’ ) of the feeds and the soluble crude protein A fraction, degradable crude protein B fraction and effective crude protein degradability Item a

Fraction A

Fraction B

Effective CP degradability

Corn silage DM (n = 12) CP(n= 11) ADIN(n= 11) ADF(n= 11) NDF(n= 11)

-0.12 -0.31 0.00 0.21 0.12

0.16 0.38 - 0.02 -0.18 - 0.05

- 0.07 0.00 - 0.27 0.05 - 0.09

Grass silage DM(n= 14) CP(n= 20) ADlN(n=9) ADF (n = 13) NDF(n=9)

-0.38 0.03 -0.31 - 0.09 - 0.20

0.55 0.21 -0.51 - 0.33 -0.73 *

-0.21 0.21 - 0.52 -0.31 -0.76 *

Alfarfa hay DM (n = 15) CP(n= 15) ADIN(n= 15) ADF(n= 15) NDF(n= 15)

- 0.32 0.48 -0.38 - 0.04 -0.31

0.35 - 0.43 0.39 - 0.08 0.21

-0.52 * 0.42 - 0.39 0.10 -0.19

Grass hay DM(n= 14) CP(n= 14) ADIN (n = 14) ADF(n= 14) NDF (n = 14)

0.00 0.20 -0.11 - 0.57 -0.89

??

0.07 0.13 0.37 0.37 0.76 ’

- 0.43 0.43 - 0.49 - 0.49 -0.91 *

a DM, dry matter; CP, crude protein; ADIN, acid-detergent insoluble nitrogen; ADF, acid-detergent fiber; NDF, neutral-detergent fiber. P < 0.05. ??

the total digestive tract. The range in rumen 12 h disappearance values was significant (P < 0.05). In the case of the corn silages there was a minimum of 53.83% and a maximum of 77.07% DM disappearance after 12 h incubation. Grass silage 12 h DM disappearance values ranged from a low of 30.29% to a high of 77.20%. In both silage types the minimum percent DM disappearance (Table 6) observed in the intestine was essentially 0, whereas the maximum values amounted to 10.89 to 34.59% for corn silages and grass silages, respectively. Total tract CP disappearance (Table 7) for both types of silage tested ranged from 77.61 to 90.04% for the corn silages and 64.48 to 92.98% for the grass silages. Alfalfa hay and grass hay DM had mean 12 h disappearance values of 65.37 f 5.67 and 54.85 f 10.17%, respectively. The proportion of DM which disappeared in the intestinal tract within a hay type was highly variable. In both types of hays studied, the minimum percent of actual DM disappearance (Table 6) observed in the intestine was essentially 0; whereas the maximum values were 23.49% and 18.40% for grass and alfalfa hays, respectively. Crude protein disappearance in the

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Table 6 Dry matter disappearance (o/o)from corn and grass silages in nylon bags incubated in situ Minimum

Maximum

Mean

53.83 0 57.67

77.07 10.89 81.53

63.28 2.71 64.49

5.18 3.01 3.35

** ** **

30.29 0 42.19

77.20 34.59 86.87

48.37 14.49 63.00

10.21 8.57 10.59

** ** **

49.55 0.00 65.54

75.54 18.40 82.65

65.37 8.15 73.37

5.67 4.24 3.95

** ** **

40.53 0.18 49.54

77.13 23.49 86.58

54.85 10.10 65.68

10.17 6.05 9.36

** ** **

SD

Significance b

Corn silage (n = 12)

Rumen ’ Intestine * Total tract ’

Grass silage (n = 22)

Rumen Intestine Total tract Alfarfa hay (n = 16)

Rumen Intestine Total tract Grass hay (n = 14)

Rumen Intestine Total tract

a Standard deviation. b Indicating that there were significant differences between at least two of the samples within a forage type: P
intestines was also variable for both hay types. Alfalfa hays and grass hays exhibited mean intestinal CP disappearance values of 18.07 f 6.98 and 22.03 + 9.38%, respectively. Total tract CP disappearance for alfalfa hays ranged from 81.24 to 93.82% and for grass hays ranged from 71.70 to 93.43% (Table 7).

4. Discussion In view of the fact that it was not possible to fully identify the forage samples as to maturity, species or variety the following discussion will be based primarily on possible reasons for the observed variability within a forage type. It is however important to emphasize that the Feed Manufacturing Industry rarely has such information when called upon to formulate a ration for production oriented dairy herds. Currently, the Canadian Feed Manufacturing Industry formulates forage containing rations for lactating dairy cows based on rumen degradability factors for these forages from publications such as the US National Research Council (NRC, 1989). The accuracy of these values to predict feedstuff rumen degradability characteristics is important since accurate estimates of degradability characteristics are key to successful ration formula-

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Table I Crude protein disappearance (%) from forages in nylon bags incubated in situ Minimum

Maximum

Mean

SD a

57.38 3.36 77.61

83.90 23.78 90.04

74.0 1 10.56 84.82

6.69 5.01 2.95

** ** **

Grass silage (n = 20) Rumen 32.86 Intestine 6.56 Total tract 64.48

81.13 43.10 92.98

61.69 20.17 82.1 1

12.40 9.39 8.52

** ** **

42.12 10.74 81.24

82.02 36.64 93.82

70.5 1 18.07 88.48

8.26 6.98 2.74

** ** **

40.37 9.20 71.70

80.51 48.68 93.43

60.60 22.03 83.68

11.15 9.38 5.76

** ** **

Significance b

Corn silage (n = 12)

Rumen ’ Intestine d Total tract e

Aljalfa hay fn = 16)

Rumen Intestine Total tract Grass hay (n = 14)

Rumen Intestine Total tract

a Standard deviation. b Indicating that there were significant differences between at least two of the samples within a forage type: **P
tion. It is well known that the composition of forages depends on factors such as species, maturity, fertilization level, season, soil type and weather conditions as itemized in a review by Van Straalen and Tamminga (19901, which in turn may influence rumen degradability characteristics. The variation in nutrient composition (Table 1) of all forages may be attributed to differences in management practices such as level of fertilization, species (in the case of grass), variety, maturity, season, soil type and weather conditions (Van Straalen and Tamminga, 1990). The range of CP values (Table 1) in the alfalfa samples analyzed in this experiment are within the range given by Balde et al. (1993) and by NRC (1989). The grass hay samples which had high initial CP content (Table 3) may have been harvested at a younger stage of maturity and/or subjected to higher levels of N fertilization (Van Straalen et al., 1993) than the others. The mean value for the soluble CP fraction A of grass hays (43.97%) (Table 3) was considerably higher than the value of 22.0% reported by APRC (1993). The higher value obtained in the present study may be explained by the differences in initial CP content; APRC (1993) reported a mean value of 8.1% whereas the mean for initial CP content observed in the present experiment was 16.00% (Table 1).

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In the present study corn silages were found to exhibit less variation than the grass silages in terms of the degradable DM fraction and the fractional rate of degradation (c) of the degradable CP fraction in-the rumen (Tables 2 and 3). This was not unexpected since the literature indicates small differences in in vivo DM digestibilities between corn varieties. BarrEre et al. (1992) examined numerous published studies which assessed differences between corn silage varieties in terms of DM digestibility by sheep. They reported ranges from 64.6 to 69.4% when six varieties were tested (Andrieu and Demarquilly, 1974) and from 70.0 to 73.6% between eight varieties (Gallais et al., 1976). The average soluble DM value of 45.94% for alfalfa hay observed in this experiment (Table 2) was higher than that reported by Mir et al. (1991) (36.00%). Of interest, however, is that the soluble DM value for an alfalfa hay reported by Mir et al. (1991) was only slightly lower than the minimum value (37.98%) observed in the present experiment. The degradable DM fraction B value of 34% reported by Mir et al. (1991) was similar to the mean (35.97%) recorded in this study (Table 2). These discrepancies may also be attributed to differences in sample preparation prior to in situ incubations between the two studies. The mean values obtained for the corn silage soluble DM in fraction A (48.22%) and degradable DM B fraction (38.17%) in the present study were similar to those reported by De Visser et al. (1993). These authors reported that 45% and 38% of the organic matter found in maize silage was in the soluble A and the degradable B fractions, respectively. The range for soluble DM may be attributed to differences in non-structural carbohydrate content. Most non-structural carbohydrates in corn are water soluble and are thus fermented during the ensiling process (Russell et al., 1992). A number of factors may have influenced the differences in the CP degradability values measured in this study for all forage types. It should be noted that there was no correction made for microbial contamination of the nylon bag residues. Although several reports have stated that microbial contamination can affect the results (Mathers and Aitchison, 1981; Varvikko and Lindberg, 1985); it has also been established that it is most important to make such corrections when evaluating feedstuffs containing low levels of CP (Waters and Givens, 1992). Furthermore, Hoffman et al. (1993) reported that although correction for bacterial crude protein (CP> tended to cause a decrease in the undegradable CP fraction it was not significant (P > 0.05). Thus, the effect of microbial contamination should be minimal in the present study as all samples were treated in the same manner. The corn silages had a large portion of CP in the water soluble A fraction (mean 63.24%). This result concurred with the value (66.0%) reported by AFRC (1993). The values, however, ranged from 44.08 to 75.37%. The differences in CP solubility between individual samples may be attributed to differences in solubilization of the CP during the fermentation stages of ensiling. Overall there was a proportionally smaller amount of CP in the degradable B fraction than in the soluble A fraction. Whether the observed differences in both the DM and CP degradabilities were a result of differences in grain to stover ratio, maturity, or planting density (Russell et al., 1992) is unknown. The differences between the individual grass silages in the present study are difficult to explain since limited background data are available. Van Straalen and Tamminga

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(1990) examined 36 different grass silage samples and concluded that the main sources of variation contributing to differences in the amount of protein degraded were initial DM and CP content and date of harvest. The in situ results for grass silages showed a more rapid degradation of the CP fractions as compared to the DM fractions (Tables 2 and 3). This higher loss of CP in fraction A may be due to amino acid fermentation during the ensiling process which can result in increased concentrations of ammonia N (De Visser et al., 1993). The mean value obtained for the soluble CP in fraction A (48.21%) was considerably lower than that reported by the AFRC (1993) for grass silages (mean 63.00%). However, it is interesting to note that the AFRC (1993) value did fall within the range (18.74-65.38%), albeit at the high end, observed in this study. Correspondingly, the CP degradable fraction B value (31 .OO%)recorded by AFRC (1993) was lower than the observed mean (45.33%) in the present study. The fractional rate of CP degradation (6.61% h-’ ) of the alfalfa hay degradable B fraction was considerably lower than that reported in AFRC (1993) (29% h-l). Of interest is that the value reported by APRC (1993) did not fall within the range reported in the present study. This may be explained by the fact that alfalfa hay is not a common forage used in ruminant feeding systems in the United Kingdom, thus, limited sample numbers may have been incorporated into the AFRC (1993) mean. In addition, AFRC (1993) provides no explanation as to how samples were prepared prior to the in situ incubations; thus, differences in methodology may explain some of the observed differences between the two data sets. AFRC (1993) reported that in dried grass the soluble CP A fraction and the degradable CP fraction B accounted for 32% and 63% of the CP, respectively, and the fractional rate of CP degradation was 4% h- ’. Although the mean value obtained in the present study for the soluble CP fraction was slightly higher at 44%, the AFRC (1993) (32%) value fell within the range measured in this study. The values for the soluble CP obtained in this study, however, were considerably higher than those reported by Waters and Givens (1992) who investigated the in situ degradation characteristics of 19 commercially grown ryegrass based herbages. They reported a mean soluble CP A fraction from ryegrass regrowths of 21%. They also reported a range in fraction B degradable CP from 61.7 to 84.1%. This range was much higher than that observed for fraction B degradable CP in the present study for grass hay. The differences may be attributed to the fact that the latter authors incubated oven dried hays which had been previously frozen fresh whereas the present study dealt exclusively with field dried grass samples. AFRC (1993) reported a degradable CP value for grass hay of 65%. Although this is only slightly higher than the maximum 62.93% (Table 3) value observed in the present study; it is evident that this value fails to represent the majority of the samples analyzed in the present study. The differences in both soluble and degradable CP values may be a result of differences in growing conditions and practices on individual farms. Van Straalen and Tamminga (1990) stated that forage protein degradability can be influenced. by manipulating such parameters as level of N fertilization and maturity. The proportion of undegradable protein, determined after an extended period of rumen incubation (10 d), has been shown to decrease when silages are prepared from grass provided with high

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levels of N fertilization (Tamminga et al., 1990). The latter authors also reported that a later stage of maturity at the time of harvest and progression of the season had the opposite effect, namely causing an increase in undegradable protein. However, Beever et al. (1976) showed no effect of season of harvest on protein degradation in vivo with ryegrass. The description of feedstuffs in terms of soluble and degradable fractions fails to take into consideration the fractional rate of flow through the rumen which is known to have a major effect on rumen degradability (Uden et al., 1980). Thus the term effective degradability of a nutrient is commonly used to estimate the proportion of nutrients contained within a feed which could be degraded in the rumen (0rskov and McDonald, 1979; Vik-Mo, 1989; Mir et al., 1991). This is particularly true in the case of protein since it is well established that increased fractional rates of passage decrease fractional rates of rumen protein degradability. This in turn allows for increased availability of feed protein in the intestine (Mir et al., 1991). The calculation of effective degradability (Eq. 2) was done in an attempt to incorporate the fractional rate of passage into the estimates of degradability, thereby, giving a more realistic description of the feedstuff. Fractional rates of passage will vary depending on feed intake, which in turn will influence production. The 6% h- ’ fractional rate of passage was chosen as an example of the average producing dairy cow consuming 2-2.5 times that at maintenance. Although the range in effective DM and CP degradability values exhibited by the corn silages (Table 4) was significant (P < 0.05), the CV values were much less than those observed for grass silages. This was not unexpected since corn silage tends to be a fairly uniform crop, subjected to a single harvest, although it may differ in maturity and ratio of grain to stover (Fisher and Fairey, 19821, fertilization practice and ensiling system. The grass silages had a high CV value between samples for the initial CP content (Table 1). The samples which had high CP content may have been harvested at an early stage of maturity and have been subjected to high levels of N fertilization (Van Straalen et al., 1993). Grass silages also exhibited a two fold difference between minimum and maximum values for both DM and CP effective degradability. This difference may have resulted from the majority of samples being grass mixtures rather than pure varieties. In addition, differences in N fertilization may have affected the results, as it causes an increase in forage CP content, particularly in the soluble CP fraction. The latter causes a decrease in the proportion of total CP available for escape into the small intestine (Van Straalen and Tamminga, 1990). In grasses with high initial CP content, less CP will be bound to the structural carbohydrate fraction (Tamminga et al., 1990). This will result in an increase in the fractional rate of degradation of the CP in the rumen since less time is required for the microbes to break down the fiber fraction to release it. Increasing the fractional rate of passage will cause a decrease in the effective degradability of both DM and CP. In such cases where the rate of passage through the digestive tract is faster than the 6% h-’ used in this study it is likely that there will be less time for microbial attachment and subsequent digestive action to occur. It may be assumed that an increased fractional rate will have a greater impact on the extent to which the degradable B fraction is ultimately degraded and have a limited effect on the soluble A fraction.

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Grass hays exhibited a two-fold difference between minimum and maximum values in both DM and CP effective degradability in the rumen (Table 4). Although the observed mean value was similar to the 66% effective protein degradability value (k, of 6.0%) reported by APRC (1993) the range observed between individual grass hays tested in this experiment must not be overlooked. These differences may result from many of the samples being grass mixtures rather than a single grass species. Increased proportions of soluble CP will result in a decrease in the amount of total protein N available for escape into the small intestine (Van Straalen and Tamminga, 1990). Although the difference in effective DM and CP degradabilities exhibited in the alfalfa hays (Table 4) was significant (P < 0.05) the variability was much less than that observed in the grass hays. Although the alfalfa hays were all of one species they may have differed in terms of variety as well as date of harvest, fertilization practices and general forage management techniques. Christensen and Fehr (1993) stated that alfalfa hay has very different nutrient profiles depending on the region where it is grown. The grass hay and grass silage samples were intentionally not separated into species or varieties since the majority of the samples received by the Feed Manufacturing Industry from farms are not identified in this manner and are consequently, referred to only as grass hay. From these results it is evident that differences in the amount of soluble CP and degradable CP may account for differences in the effective protein degradability within a feed type (Vik-Mo, 1989). In contrast to the work of Vik-Mo (1989), who reported that the higher the CP content of a forage the higher the effective protein degradability, there appeared to be no relationship between any of the rumen degradation parameters and CP content of the forages in the present study (Table 5). Interestingly, only the initial NDF content of grass silages showed a significant relationship with the degradable Fraction B and effective CP degradability. It is evident from this study that considerable variation exists in the rumen degradability of DM and CP from individual samples of forages and that this variation was carried over to the amount of material eligible for disappearance in the small intestine. This was particularly true in the case of CP. In contrast to many concentrates, a much smaller proportion of the initial forage CP is degraded in the intestine. This difference is probably a result of the leaf protein in forages being highly degradable in the rumen and that much of the forage CP appearing in the small intestine is associated with the cell wall portion (i.e. stems) and therefore not readily available for digestion (Van Straalen and Tamminga, 1990). Prior to determining intestinal degradability, all samples were pre-incubated in the rumen for 12 h based on the recommendations of De Boer et al. (1987) and Hvelplund et al, (1992). It has been well established that pre-incubation significantly increases the total disappearance of protein in the intestines when compared to samples without pre-incubation (Hvelplund et al., 1992). Whether the 12 h pre-incubation time in the rumen as recommended by De Boer et al. (1987) prior to insertion into the duodenal cannula is representative of actual rumen retention time is questionable. Varvikko and Vanhatalo (1991) pre-incubated forage samples for 16 h and stated that this was probably too short. De Boer et al. (1987) assumed that rumen incubation of both cereals and alfalfa hay for 8 h reflected rumen residence time. Although this may be appropriate

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for the cereal grains it is questionable whether 8 h is long enough in the case of forages. It is likely that the 12 h pre-incubation time used in this experiment was too short rather than too long. If rumen retention time was indeed longer than 12 h, the actual intestinal disappearance values would be lower than those presented in Tables 6 and 7. The pooling of the mobile nylon bag residues per feed and cow prior to N analysis was considered valid based on the work reported by Van Straalen et al. (1993). In the present study the mobile nylon bag contents were also not corrected for microbial contamination since Hvelplund (1985) and Kohn and Allen (1992) both reported limited microbial contamination for mobile bag studies. In the present study all bags were thoroughly washed prior to DM and N analysis. Washing has been reported to remove essentially all endogenous and bacterial contamination. Therefore, the total tract disappearance values can be considered to reflect true digestibility rather than apparent digestibility (De Boer et al., 1987). The intestinal disappearance of CP in all cases was high compared to that of DM. The failure to treat the mobile nylon bags with abomasal pepsin-HCl following rumen pre incubation must be addressed. A significant increase in intestinal CP disappearance for formaldehyde treated soybean meal incubated in mobile nylon bags was reported following a decrease in pH prior to intestinal incubation (Varvikko and Vanhatalo, 1991). Lack of this treatment in the experimental protocol followed in the present study should not, however, seriously affect the ranking of the present results since all forages were treated in a similar manner. The percentage CP escaping rumen degradation (rumen undegradable protein (UDP)) which is subsequently available for digestion in the small intestine was also very variable. NRC (1989) reports a UDP value of 3 1% for corn silage based on three determinations. A mean value of 10.56% (Table 7) was observed in this experiment; however, values for the 12 corn silages ranged from 3.36 to 23.78%. The total tract disappearance values for grass and alfalfa hays were slightly higher than in vivo digestibility results reported by von Keyserlingk and Mathison (1989). This was not unexpected since the mobile nylon bag technique gives an estimate of true digestibility (De Boer et al., 1987) rather than apparent digestibility which is obtained using conventional in vivo digestibility determinations. As corn silages are highly degradable in the rumen it was not unexpected that the proportion of the initial silage DM that disappeared in the small intestine was minimal. The significant differences in intestinal DM degradability between individual grass silage samples were likely a result of the many factors discussed above as well as the fermentation process during ensiling. The mobile nylon bag data indicated that in the corn silage samples, approximately 10% of the original nitrogenous material disappeared in the intestine while in grass silage, alfalfa hay and grass hay more than 20% of the original nitrogenous material disappeared in the intestine (Table 7). NRC (1989) reported a value for rumen undegradability of crude protein of 23% for alfalfa which is slightly higher than the mean calculated for intestinal disappearance (Table 7). It is of interest that the values in CP intestinal disappearance ranged from 9.20 to 48.68%. Although the NRC (1989) value is calculated in a slightly different manner, it is clear that incorporating a forage containing only 9.2% UDP into a ration will result in a very different ration than

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incorporating a forage containing 23% UDP. The intestinal disappearance values for CP from all forages was relatively higher than the DM disappearance values. Varvikko and Vanhatalo (1991) reported a higher total tract CP disappearance from grass silage than from grass hay which they attributed to the higher proportion of soluble CP in the silage. The latter researchers freeze dried the silage samples whereas the silages in the present experiment were dried at 50°C. Oven drying of samples, depending on maximum temperatures, decreases the solubility, extent of degradation of the protein (Hristov, 1992) as well as the fractional rate constant for the initial disappearance of CP (Vik-Mo, 1989). Freezing the samples in a chest freezer until the onset of the experiment may have caused lower buffer solubility and higher neutral detergent insoluble N which is attributed to precipitation of the protein during the initial freezing process (Van Straalen et al., 1993). Due to its ease of use, the nylon bag technique has become the accepted method for assessing rumen degradability (Broderick, 1994). It is however recognized that this technique is not feasible for measuring the large numbers of samples that need to be routinely screened by the feed manufacturing industry. In view of the results obtained in this study it is evident that considerable effort should be made to find a screening method which can be used on a routine basis by the feed manufacturing industry to test individual silages.

5. Conclusions

Results obtained from this study clearly established that there are significant differences in the degradability characteristics between individual samples of corn silage, grass silage, alfalfa hay and grass hay from different farms. In its formulation equations, NRC (1989) requires that a degradable protein value and an undegradable protein value be specified for each feedstuff included in the ration. It is evident that, in the case of forages, the present use of average values for forages by the Feed Manufacturing Industry can lead to inaccurate feed formulations since they may not reflect the particular forage being used in the ration. The results presented in this study confirm those of Hoffman et al. (1993) who stated that the use of mean values without taking into consideration the range between forage species and maturities may have a major impact on ration formulation and feed cost. Consideration of the contribution of forage protein entering the intestines must also not be over looked. It is evident that in the case of dairy cattle rations where as much as 50% of the ration consists of forages, that this source of protein is vital. The contribution of protein by the forage component may also become increasingly important as environmental pressures increase to reduce overfeeding of protein which leads to excessive N pollution.

Acknowledgements The assistance of Dr. J.W. Hall from Agriculture Agri-Food Canada with the statistical analysis is greatly appreciated. Funding for this study was provided by Pro

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Form Feeds Inc., Ritchie Smith Feeds, Buckerfields, B.C. Ministry of Agriculture, Fisheries and Food (Research Partnership Program, NSERC-Industry Partnerships and Warsaw Agricultural University.

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