Combined Effects of Type of Forage Fed, of Concentrate Ingredients, and of Pelleting Concentrates on Rumen Fermentations, Milk Yield, and Milk Composition of Dairy Cattle

Combined Effects of Type of Forage Fed, of Concentrate Ingredients, and of Pelleting Concentrates on Rumen Fermentations, Milk Yield, and Milk Composition of Dairy Cattle GEORGE E. HAWKINS and JOE A. LITTLE

Department of Dairy Science, Auburn University Agricultural Experiment Station, Auburn, Alabama Abstract A study was made to determine if differences in responses of dairy cattle to oat vs. corn silage, corn grain-E cottonseed meal vs. corn distillers dried grains with solubles (CDDGS), and pelleted vs. nonpelleted concentrates were accumulative. Responses measured were a) concentrations and molar percentages of V F A in rumen fluid, b) milk yield, c) milk energy yield, d) milk-fat per cent, e) milk protein per cent, £) energy per kilogram of milk, and g) milk energy produced daily as a percentage of gross energy in concentrates eaten. Oat pasture increased concentration and molar per cent of propionic acid and concentration of total V F A over corn silage; whereas, CDDGS depressed the concentration and nmlar per cent of acetic acid and the concentration of total VFA, as compared with the corn concentrates. Pelleting the corn concentrate increased the molar per cent of propionic acid and decreased that of acetic acid, with differences being significant only within the oat pasture forage; whereas, pelleting of the CDDGS concentrate had no effect on these responses. Within the corn concentrates, pelleting decreased energy per kilogram of milk and milk fat per cent and increased milk yield, with responses being magnified on oat pasture as compared with corn silage. The data suggest that the effects of grazing oat pasture and of feeding pelleted corn on the molar percentages of acetic and propionic acids and on milk fat per eent~ energy per kilogram of milk, and total milk yield were essentially additive. I n relation to the concentrate energy consumed, cows fed CDDGS concentrates produced more milk energy than those fed the corn concentrates.

produced (5, 9, 11, 16, 20-22). Therefore, it is important to characterize all nutrient variables that affect rumen fermentation and to establish whether the effects of these variables are additive. Dietary variables that reportedly affected rumen fermentation included the form in which starchy concentrates were fed (5, 9, 20), ingredients pelleted (9), and the quality, type, or form of roughage (2-4, 8, 11, 16) consumed by the cows. Also, the low concentrations of acetic and lactic acids found in silage to which corn distillers dried grains with solubles had been added indicated that this feedstuff affected anaerobic fermentations of bacteria (15) and possibly would alter rumen fermentation. Studies on the accumulative effects of two or more dietary variables producing like responses on rumen fermentation and on milk composition are few in number. For these reasons, a study was made to determine whether the effects of forages, concentrate ingredients, and pelleting of concentrate ingredients are additive, as measured by concentrations and molar percentages of volatile fatty acids in rumen fluid and milk composition. Experimental Procedure

Eight Holstein cows producing between 22.7 and 32.7 kg of milk daily were the experimental subjects. They had calved a minimum of 30 days before start of the standardization period. Prior to assignment to experimental treatments, the cows were standardized on a ration of alfalfa hay ( A I t ) fed at 5 p~ and corn silage (CS) fed at 5 A~t, with each representing 25% of the ration on an air-dry equivalent basis. The other 50% of the ration consisted of a 4:1 mixture of ground yellow corn and cottonseed meal (41% protein grade), fed at 5 A~ and 5 PM. The ration provided estimated net energy (ENE) for maintenance, plus 120% of ENE allowance recommended for milk production (17). The ENE allowance established during the standardization period was fed throughout the four experimental periods. The experimental design was a 4 × 4 Latinsquare with two groups which also could be analyzed as a 2 × 2 × 2 factorial. Periods were 28 days in length, with the first seven

Several research reports have established relationships between the molar ratios of volatile fatty acids (VFA) produced in the rumen of lactating cows and the composition of milk Received for publication1 August 9, 1966. 62

T Y P E S OF F E E D S

AND R U M E N F E R M E N T A T I O N S

days considered as changeover. The roughage ration for cows in Group I was the same as that fed during the standardization period. I n Group I I , oat pasture (OP) replaced the corn silage fed during the standardization period, and was grazed from approximately 7 A~ to 2 PM daily. Alfalfa hay and CS for Group I, and A H for Group I I were fed at times indicated for the standardization period. Throughout the remainder of this report, Group I will be referred to as the CS group and Group I I as the OP group. The four concentrates fed to cows in both roughage groups were 80% ground corn and 20% cottonseed meal (GC), 80% pelleted corn and 20% cottonseed meal ( P C ) , corn distillers dried grains with solubles (CDDGS), and pelleted corn distillers dried grains with solubles (PCDDGS). Each concentrate was fed at the same rate and time as that fed during the standardization period. Aliquots of milk produced by each cow were collected at eight consecutive milkings during a four-day period each week. These milk aliquots were combined to form a weekly composite for each cow and were analyzed for milk fat per cent by the Babcock procedure, and for milk protein per cent by micro-KjeldahI. In addition, gross energy of milk freeze-dried 24 hr was determined by oxygen bomb calorimetry. Weights of the 24-hr freeze-dried samples were high in comparison with the fat percentage and energy per kilogram of hhe dried milk, thus indicating that all of the moisture was not removed. F o r this reason, total solids percentages are not reported herein. One of the cows in the OP group was off-feed during parts of the final three periods; therefore, substituted values determined by missing plot procedure (19) were used for calculating milk and milk energy yields. Rumen fluid samples were collected from each cow by stomach tube at 6-hr intervals during one 24-hr period in the last week of each 4-wk experimental period. Mercuric chloride (0.5 ml) was added to each sample as a preservative. The four collections were combined to form a 24-hr composite for each cow and processed by filtering, adding I ml of 25% metaphosphoric acid per 5 ml, and centrifuging. Subsequently, the samples were analyzed for concentrations of acetic, propionic, butyric, and higher acids by a colmnn partition chromatogr a p h y procedure (18). Feed samples were analyzed for proximate composition (1) and for cold- and hot-water solubles. Milk yield and milk-composition data were adjusted by covariance analyses (19), to take

63

into account initial differences among cows. Results

Percentages of cold and hot water solubles, respectively, in the four concentrates, on the dry matter basis, were GC, 18.8 and 46.1; PC, 19.6 and 53.1; CDDGS, 19.2 and 27.0, and PCDDGS, 21.1 and 30.0. Pelleting of the corn d-cottonseed meal mixture or corn distillers dried grains with solubles concentrates increased the solubility of dry matter of these feeds in both cold and hot water. The small increase in cold-water solubles of PC, as compared with that of GC, indicates that dextrinization of pelleted corn was incomplete. Yet, the 7% increase in hot-water solubles in PC, as compared with that of GC, suggests that some denaturization of the starch granule of corn occurred during pelleting. F o r the purpose of calculating the composition of the rations fed, it was assumed that OP dry matter intakes during the experiment (Group I I ) were the same as that of CS during the standardization. The basis for this assumption is presented in the discussion. Most cows (81%) refused a portion of their concentrate allowance when fed CDDGS and PCDDGS concentrates, with refusals averaging 9.17% (range 0- to 38.25%). In contrast, only one cow refused the GC a n d PC concentrates, with refusals being 2.25 and ]8.06%, respectively, of their allowances. Estimated crude protein contents of the total rations of cows in the OP group were GC, or PC, 18.9%; and CDDGS, or PCDDGS, 22.3%. Rations fed cows in the CS group averaged 3.5% lower in crude protein than that of cows grazing OP, with differences being the result of the high crude protein content in the pasture. Crude fiber contents of rations fed to cows grazing OP were GC or PC, ]3.1%; and CDDGS or PCDDGS, 16.5%. Rations of cows fed the same concentrates and CS contained 0.2% more crude fiber than that of those grazing OP. Ash contents of the rations were not affected significantly by the concentrate fed, and averaged 6.1% for cows receiving CS, compared to 7.3% for those grazing OP. Mean concentrations of V F A in rumen fluid as related to rations fed are given in Table 1. Concentrations of propionic acid and of total Y F A in rumen fluid from cows grazing OP were greater (P ~ 0.05 and P ~ 0.01) than from those fed CS, Section A. As shown in Table ], Section B, pelleting of the concentrates did not affect the concentrations of individual or total Y F A in rumen fluid from the experimental cows (P ~ 0.05). Nevertheless, concentrations of acetic and total J. DAII~¥ SCIEI~CE VoIJ. 50, 1N[O. 1

HAWKINS AND LITTLE

64

TABLE 1 Mean concentrations of volatile f a t t y acids in rumen fluid as related to rations fed Volatile f a t t y acids ~ Section

Ration variable

Acetic

Propionic

Butyric

Higher

Total

-(mMoles/lO0 ml) A

Corn silage Oat pasture

4.82 5.03

1.45 ~ 2~10e

1.20 1.33

0.43 0.48

7.90 ~ 8.942

B

Nonpelleted Pelleted

4.94 4.92

1.71 1.84

1.29 1.24

0.46 0.45

8.40 8.45

C

Corn CDDGS

5.176 4.685

1.87 1.68

1.29 1.23

0.45 0.45

8.786 8.045

D

GC PC CDDGS PCDDGS

5.15 5.20 ~ 4.73 4.635

1.71 2.03 1.71 1.65

1.31 1.28 1.26 1.19

0.44 0.46 0.47 0.44

8.61 8.97 ~ 8.17 7.915

CS-GC CS-PC CS-CDDGS CS-PCDDGS OP-GC OP-PC OP-CDDGS OP-PCDDGS

5.15 5.17 4.58 4.39 5.11 5.23 4.88 4.88

1.375 1.48 ~ 1.52 1.43 2.058 2.58 s'8 1.91 1.88 ~

1.23 1.33 1.19 1.04 1.39 1.23 1.34 1.35

0.47 0.40 0.47 0.37 0.42 0.53 0.47 0.51

8.22 8.38 7.76 7.23 9.01 9.57 8.60 8.62

~8

( p ~ 0.01) ; and 5 ~ , and ~ s

( p ~ 0.05).

V F A in r u m e n fluid f r o m cows r e c e i v i n g the corn c o n c e n t r a t e s were g r e a t e r ( P < 0.05) t h a n in t h a t f r o m cows r e c e i v i n g the corn distillers dried g r a i n s w i t h solubles c o n c e n t r a t e s , Table 1, Section C. M o s t of this difference w a s att r i b u t a b l e to t h e P C vs. P C D D G S c o m p a r i s o n , Section D. T h e m e a n m o l a r p e r c e n t a g e of acetic acid

w a s decreased a n d t h a t of p r o p i o n i e acid w a s i n c r e a s e d in r u m e n fluid f r o m cows g r a z i n g oats, in c o m p a r i s o n w i t h t h a t o f cows f e d corn silage ( P ( 0 . 0 5 ) , T a b l e 2, Section A. T h e r e w a s n o over-all effect o f p e l l e t i n g t h e concent r a t e s ( S e c t i o n B ) , or o f c o n c e n t r a t e i n g r e d i e n t s ( S e c t i o n C) on the m o l a r p e r c e n t a g e s o f V F A in r u m e n fluid f r o m cows r e c e i v i n g

TABLE 2 Relationship between molar percentages of volatile f a t t y acids in rumen fluid and ration fed Volatile f a t t y acids" Section

Ration variable

Acetic

Propionlc

A

Corn silage Oat pasture

61.2~ 56.75

18.4s 23.2~

15.1 14.7

5.3 5.2

B

/~onpelleted Pelleted

59.5 58.6

20.1 21.5

15.0 14.8

5.4 5.1

C

Corn CDDGS

59.6 58.5

20.9 20.8

14.5 15.4

5.1 5.3

D

GC PC CDDGS PCDDGS

60.8 58.4 58.2 58.7

19.5 22.2 20.7 20.8

14.5 14.4 15.5 15.2

5.2 5.0 5.6 5.3

CS-GC CS-PC CS-CDDGS CS-PCDDGS OP-GC OP-PC OP-CDDGS OP-PCDDGS

63.42, 6 61.6 ~ 59.1 ~ 60.88 58.11'1° 55.5 ~'8 57.3 56.67

16.41' ~ 18.03 19.6 s 19.4 22.63,0 26.3 ~'1° 21.7 22.13

14.4 15.86 15.4 14.9 14.6 13.05

5.8 4.6 5.9 4.9 4.7 5.2

--.(Molar

1~8, and 8 ~ , ( p ~ 0.01) ; and 5 ~ , ~ s , and 9 ~ o ( p ~ 0.05). J. DAIRy SOZE~OE ~rOL. 50. NO. i

Butyric

Higher

%)

15.6

5.4

15.6

5.7

T Y P E S OF F E E D S A N D R U M E N

these feeds (P ~ 0.05). Similarly, across forages (Section D ) , there were no significant differences in the molar percentages of V F A in rumen fluid f r o m cows fed GC vs. P C or C D D G S vs. P C D D G S . The absence of detectable over-all effects of pelleting on the molar percentages of acetic and propionic acids was related to interactions between ingredients, pelleting, and forages. However, within the CS group (Table 2, Section E ) , the molar p e r cent of acetic acid was lower and that of propionic acid was higher ( P < 0.05) in rumen fluid f r o m cows fed C D D G S than in rumen fluid from those fed GC. Also, within the OP group, the molar percentage of acetic acid was lower and that of propionic acid was higher ( P < 0.05) in tureen fluid from cows fed P C than f r o m those fed GC. Thus, the subration data (Section E ) indicate that the molar percentages of acetic and propionic acids in r a m e n fluid were affected by concentrate ingredients, by pelleting of concentrates, and by the f o r a g e fed. I n contrast, the only significant difference in molar percentages of butyric acid was a f o r a g e effect ( P < 0.05) within the P C concentrate (Table 2, Section E ) , and the higher f a t t y acids were not affected significantly ( P ~ 0.05) by any treatment. Daily milk and milk-energy production p e r cow and milk-composition data are presented in Table 3. There were relatively large variations in values for milk production p e r cow

FERMENTATIONS

65

daily, associated with treatment comparisons. The true significance of the milk production responses associated with the concentrates, Section D, was obscured by interactions. Among subrations, Section E, feeding P C increased milk production, as compared with GC (P 0.05). I n contrast, within the OP group feeding P C D D G S depressed milk production as compared with C D D G S (P ~ 0.05). Milkenergy production was not affected (P ~ 0.05) by treatments. Nevertheless, when fed the corn concentrates, the milk energy secreted by cows was qual to 33.75% of the gross energy in the concentrate consumed, as compared with 39.5% when fed the corn distillers dried grains with solubles concentrates, and the difference was significant (P < 0.05). Milk fat percentage and energy per kilogram of nfilk were affected significantly by one or more ration variables; whereas, milk crude protein ( M C P ) was not affected by any ration variable (i.e., P ~ 0.05), Table 3. Compared with feeding CS, the grazing of OP depressed milk f a t percentage (P ~ 0.10) but had no effect on energy p e r kilogram of milk (P 0.10), Table 3, Section A. The over-all effect of pelleting the concentrates depressed milk f a t percentage (P ~ 0.05) and energy per kilogram of milk ( P < 0.05), Section B. Milk f a t percentage (P < 0.05) and energy per kilog r a m of nfilk ( P < 0.01) were higher for cows fed corn distillers dried grains with solubles

TABLE 3 Relationship between rations fed and milk yield per cow daily and milk composition Milk ~ields a, b Section

Ration variables

(leg~day) A

Corn silage Oat pasture

24.4 25.4

]3

Nonpelleted Pelleted

C D

E

Milk

Milk composition ~' b Fat

MCP

Energy

(megcal/day)

(%)

(%)

(kcal/kg)

15.259 15.737

3.571~ 3.411~

3.24 3.22

623 625

24.7 25.1

15.786 15.210

3.64~ 3.34 ~

3.23 3.23

6426 6065

Corn CDDGS

25.2 24.5

15.416 ]5.580

3.35 ~ 3.64 ~

3.54 3.22

606 ~ 642 B

GC PC CDDGS PCI)DGS

24.3 26.2 23.9

15.521 ]5.310 16.051 15.110

3.60 ~ 3.09 ~ 3.72 3.56 ~

3.32 3.16 3.]2 3.31

634 ~ 572 ~ 645 6406

CS-GC CS-PC CS-CDDGS CS-PCDDGS OP-GC OP-PC OP-CDDGS OP-PCDDGS

23.0 ~'~ 25.3 ~'7 24.36'9 24.7 ~ 25.6 -0'~3 27.04's'~* 25.94'~° 23.13'n

14.823 14.883 ]5.663 15.667 16.222 15.737 16.440 14.552

3.60 ~ 3.16 ~ 3.896 3.652 3.604 3.028 3.555 3.47 *

3.28 3.17 3.26 3.24 3.36 3.14 3.00 3.38

6406 578 ~ 639 6346 638 s 565 ~'~ 650 6462

25.1

Energy

, ~ 2 , and 8~4 ( p ~ 0.01) ; ~ 6 , ~ s , 9~o, n ~ 1.~, ~3~1, ( p ~ 0.05; and 15~16 ( p ~ 0.10). b Data adjusted by covari~nee analysis, to take into account initial differences among cows. ~'. DA1RY SCIENCE VOL. 50, NO. 1

6~

H A W K I N S AND L I T T L E

concentrates than for those fed the corn concentrates, Section C. These components were significantly lower in milk from cows fed PC than in milk from those fed PCDDGS (P < 0.05), Sections D and E. Body weight changes associated with rations fed did not differ significantly (P > 0.05). Discussion

The lower concentrations of V F A in 1-amen fluid from cows fed the CDDGS and PCDDGS concentrates, as compared with that of cows fed the GC and PC concentrates, are similar to the reductions in concentrations of organic acids observed in silages preserved with CDDGS (15). Nevertheless, other reports (7, 12) on in vitro studies indicated that distillers solubles contained some factor stimulatory to cellulose digestion. An increased rate of digestion of cellulose would be expected to result in a higher concentration of total V F A in lalmen fluid; whereas, the opposite was observed. Time of sampling the tureen fluid in relation to the time the forages were consumed may have been a factor in differences in concentrations of Y F A associated with type of roughage. As indicated in the procedure, corn silage was fed 6 and 12 hr prior to the daytime collections of rumen fluid, whereas cows grazing oat pasture had access to their forage up to the time of sampling. However, studies by Bath et al. (4) have shown that ruminal-VFA production was higher by cows grazing herbage than by others fed the same herbage after clipping, thus suggesting that differences in V F A concentrations associated with forages were true forage effects. It is unlikely that the peak concentrations of ruminal-VFA in cows fed corn distillers grains with solubles concentrates, which were lowest in water solubles, occurred earlier than in cows fed the corn concentrates, which were highest in water solubles. Findings in the present study suggest that the corn distillers dried grains with solubles concentrates failed to provide some nutrient(s) (probably a readily available source of energy) needed by ruminal microorganisms for a sustained high level of metabolic activity. Findings by Huff and Hawkins (10), that molar peerentages of V F A in the tureen fluid of grazing cattle varied at intervals during the day, suggest that the differences in molar percentages of V F A in tureen fluid, associated with CS and OP, were because of variations in the eating habits of cows fed the two forages. Yet, the possibility that differences were absolute forage effects cannot be ruled out, since forage species have been shown to affect the ~. DAIRY SCIENCE VOL. 50, NO, 1

molar percentages of V F A in rumen fluid (8, 11). Several reports (5, 9, 20) have shown that heat treatment of starchy concentrates changes the molar percentage of V F A similarly to that observed for the PC concentrate. The higher fiber and lower N F E and hot-water so]ubles percentages in the corn distillers grains concentrates, as compared with the corn concentrates, indicate that the starch content of the corn distillers grains concentrates was low Pelleting of a low-starch concentrate would be expected to produce a minimal effect on molar percentages of ruminal VFA, such as that observed for the PCDDGS concentrates in this study. Differences in the molar percentages of acetic or propionic acids in rumen fluid associated with subrations indicate that the effects of pelleting corn and of grazing OP on these V F A were essentially additive. Thus, the forage, the concentrates, and the ingredients in pelleted concentrates when fed to ruminants may be expected to affect the molar percentages of V F A produced by microbial fermentation in the rumen. 5iilk production responses to feeding pelleted concentrates have been variable. Increases in milk production associated with feeding PC as compared with GC were similar to responses reported by others (5, 6) when cows were fed a pelleted concentrate. In contrast, within the OP group, the lower level of milk production of cows fed PCDDGS vs. those fed CDDGS is an unusual response to feeding of pelleted concentrates. Most of the differences in amounts of milk production associated with subrations were related to the varied energy contents of the milks produced. This is indicated by the absence of detectable significant differences in the amounts of milk energy secreted daily by cows fed the test rations and toe significant (P < 0.01) correlation between milk fat per cent and milk energy. Both milk energy and milk fat per cent were correlated ( r = 0.44 and --0.31, respectively) with the molar percentages of propionic acid in rumen fluid. However, the correlation between the molar percentages of propionic acid and these milk components was significant only within the 0 P subrations ( r = -- 0.983). Thus, there was evidence that the molar percentages of propionic acid produced in the rumen affected milk composition in this study, as reported in other investigations (5, 9, 11, 16, 20-22). Lack of a significant correlation between milk fat per cent or milk energy and the molar percentage of propionic acid within the CS group probably was because of differences in the time at which

T Y P E S OF F E E D S

AND R U M E N F E R M E N T A T I O N S

p e a k c o n c e n t r a t i o n s of tiffs acid occurred on the f o u r CS subrations. Decreases in f a t cont e n t of milk f r o m cows g r a z i n g O P were similar to p r e v i o u s findings (13, 14) a n d were related to h i g h m o l a r p e r c e n t a g e s of p r o p i o n i c acid. The d e p r e s s i n g effect of P C a n d o f O P on milk f a t p e r cent was essentially additive. W i t h decreases in the p e r c e n t a g e o f milk fat, there was not a c o n c o m i t a n t change in the percentage of milk p r o t e i n . Thus, t h e r e were significant shifts in r a t i o s of milk f a t to milk protein, a c c o m p a n i e d by significant changes in the p e r c e n t a g e of e n e r g y p e r k i l o g r a m of milk derived f r o m milk fat. Since total milk e n e r g y p r o d u c t i o n p e r cow daily a n d body weight changes were no~ a f fected significantly by the r a t i o n variables, it seems h i g h l y p r o b a b l e t h a t total e n e r g y intakes by cows in the O P g r o u p were similar to t h e i r e n e r g y intakes, t h r o u g h o u t the s t a n d a r d i z a t i o n period, d u r i n g which time they were fed CS instead of g r a z i n g 0 P . Differences in gross e n e r g y intakes f r o m c o n c e n t r a t e s b y i n d i v i d u a l cows p r o b a b l y cont r i b u t e d to v a r i a t i o n in the response criteria measured. This v a r i a t i o n possibly m a s k e d the significance of some true t r e a t m e n t effects.

(8)

(9)

(10)

(11)

(12)

(13)

(14)

References (1) Association of Official Agricultural Chemists. 1960. Official Methods of Analysis. 9th ed. Washington, D. C. (2) Balch, C. C., Broster, W. H., t~ook, J. A. F., and Tuck, Valeric, J. 1965. The Effect on Growth Rate and on Milk Yield and Composition of Finely Grinding the Hay and Cooking (Flaking) the Maize in Mixed Diets for Growing and Milking Heifers. J. Dairy Research, 32: 1. (3) Balch, C. C., and Rowland, S. J. 1957. Volatile F a t t y Acids and Lactic Acid in the Rumen of Dairy Cows Receiving a, Variety of Diets. Brit. J. Nutrition, 11: 288. (4) Bath, L H., Rook, J. A. F., and l~owla~d, S. J. 1962. Effects of Grazing on the Ruminal Production of Volatile F a t t y Acids in Relation to the Protein and F a t Contents of Milk. Proc. X V I t h Intern. Dairy Congr., A : 49. (5) Bishop, S. E., Loosli, J. K., Trimberger, G. W., and Turk, K. L. 1963. Effect of Pelleting and Varying Grain Intakes on Milk Yield and Composition. J. Dairy Sci., 46 : 22. (6) Blosser, T. H., and Shaw, A. 0. 1959. Effect of Pelleting on Rate of Concentrate Consumption by Dairy Cows. J. Dairy Sci., 42 : 936. (7) Burroughs, W., Long, J., Gerlaugh, P., and Bethke, R. M. 1950. Cellulose Digestio~ by Rumen Microorganisms as Influenced by Cereal Grains and Proteln-rich Feeds Corn-

(15)

(16)

(17)

(18) (19) (20)

(21)

(22)

67

monly Fed to Cattle Using the Artificial Rumen. J. Animal Sci., 9: 523. Coats, E. J., Hawkins, G. E., and Rollins, G. K. 1965. Comparison of a Commercial Concentrate and Pelleted Oats for Grazing Cows. J. Dairy Sci., 48: 810. Hawkins, G. E., Paar, G. E., and Little, J. A. 1963. Physiological Responses of Lactating Dairy Cattle to Pelleted Corn and Oats. J. Dairy Sci., 46: 1073. Huff, J. W., and Hawkins, G. E. 1964. Differences in Levels of Volatile F a t t y Acids in l~umen Fluid from Cows Grazing Coastal Bermudagrass and Millet. J. Dairy Sci., 47 : 703. King, R. L., and Hemken, R. W. 1962. Composition of Milk Produced on Pelleted Hay and Heated Corn. J. Dai~7 Sci., 45 : 1336. Little, O. C., Mitchell, G. E., Jr., and Bradley, N. W. 1964. Rumen Stimulatory Factors in Corn Distillers Dried Solubles. Pxoc. Distillers Feed ]%eseaxch Councfi~ 19 : 43. Mayton, E. L., Hawkins, G. E., Blackstone, J. It., and Little, J. A. 1965. Forage Systems Compared for High Producing Cows. Auburn Univ. Agr. Expt. Sta., Bull. 363. McClymont, G. L. 1950. The Relation of the Type and Quality of Roughage and Grazing to the F a t Content of Milk. Australlan Vet. J., 26: 111. McCullough, M. E. 1964. Improving the Energy Value of Silage Through Controlled Fermentation. Proc. Distillers' Feed Reseaxch Council, 19: 12. Miller, R. W., Waldo, D. R., Okamoto, M., Hemken, R. W., Yandersall, 5. H., and Clark, N. A. 1963. Feeding of Potassium Bicarbonate or Magnesium Carbonate to Cows Grazed on Sudangrass or Pearl Millet. J. Dairy Sci., 46: 621. National Research Council. 1958. Nutrient Requirements of Domestic Animals. 3. Nutrient Requirements of Dairy Cattle. Natl. Aead. Sei., Washington, D. C., Publ. 464. Ramsey, H. A. ]963. Separation of Organic Acids in Blood by Partition Chromatography. J. Dairy Sci., 46: 480. Snedecor, G. W. 1946. Statistical Methods. 4th ed. The Iowa State College Press, Ames. Storr}', J. E., and l~ook, J. A. F. 1966. The Relationship in the Cow Between Milk-Fat Secretion and Ruminal Volatile F a t t y Acids. Brit. J. Nutrition, 20: 17. Van Soest, P. J. 1963. Ruminant F a t Metabolism with Particular Reference to Factors Affecting Low Milk F a t and Feed Efficiency. A Review. J. Dairy Sci., 46. 204. Van Soest, P. J., and Allen, N. N. 1959. Studies on the Relationship Between Rumen Acids and F a t Metabolism of Ruminants Fed on Restricted Roughage Diets. J. Dairy Sci., 42: ]977. J. DAIRY SCIENCE VOL. 50, NO. 1