E F F E C T OF F E E D I N G B U F F E R S TO D A I R Y C O W S G R A Z I N G

P E A R L M I L L E T OR S U D A N G R A S S 1

R. W. MILLER, 2 R. W. HEMKEN, AND J. tI. VANDERSALL University of Maryland, College Park

A]~D

D. R. WALDO, M. OKAMATO, AND L. A. MOORE Dairy Cattle Research Branch, Animal Husbandry Research Division

ARS, USDA, Beltsville, Maryland

ABSTRACT

Twelve Holstein cows were assigned in a 2 × 3 factorial design to pearl millet or Sudan grass pasture. Two cows on each forage served as controls, two were supplemented with 1.00 lb KHCO~ and two with 0.42 lb MgCO3 per day.

There were no significant differences among any of the experimental treat- ments for daily milk production, per cent solids-not-fat and protein in the milk, or for changes in body weight. Cows grazing pearl millet had lower (P < .01) milk fat tests than did those grazing Sudan grass. The buffers had no effect on milk fat tests.

Blood serum values for calcium, nmgnesium, sodium, potassium, phosphorus, and chloride were within the normal range. Both the KHCO.~ and the MgC08 increased the total alkalinity of the urine as compared to the nonsupplemented ration. Animals grazing pearl millet had lower (P < .05) molar per cents of rumen acetic, butyric, and isovaleric acids and higher (P < .01) molar per cents of propionic and n-valeric acids than did cows grazing Sudan grass. These molar per cents of volatile fatty acids were not influenced by addition of KtICO~ or MgCO~ to the rations.

The problem of milk fat depression when cows are fed high-concentrate, low-roughage, and finely ground rations has been recognized for a number of years (20). There have been reports of milk fat depression when aninmls are grazing certain pastures (6, 17). A possible relationship is indicated between grass tetany (hyponmgnesemia) and milk fat depression, as both of these phenomena have been shown to occur when cows are first turned out to early spring pastures (21, 24). French workers (14) have shown that the fall in serum magnesium of cows grazing a young grass pasture could be prevented by feeding 2 kg of rough hay; how- ever, if the hay was finely ground, this effect was not evident.

Received for publication May 19, 1965.

1Data taken from a thesis presented by R. W. Miller in partial fulfillment of the requirements for the M.S. degree, University of Maryland. Scientific Article no. A 1187, Contribution no. 3681 of the Maryland Agricultural Experiment Station, Department of Dairy Science.

2Address: Dairy Cattle Research Branch, Ani- mal Husbandry Research Division, ARS, USDA, Beltsville, Maryland.

Recently, t temken et al. (11) reported that cows grazing pearl millet had a depression in milk fat test that did not occur with cows grazing Sudan grass or sorghum-Sudan grass hybrid. I t was reported that the cows on pearl millet had a lower molar per cent of rumen acetic acid and a higher molar per cent of pro- pionic acid than did those grazing Sudan grass or the hybrid.

In 1961, Emery and Brown (7) reported that the depression in milk fat test caused by the feeding of a high-concentrate, low-roughage ra- tion could be prevented by including 1 lb of NaHC03 or KHCO~ in the ration. Similar re- sults have been reported by Davis et al. (5) and by Loosli et al. (15).

I f the mechanism for the depression in milk fat test of animals grazing pearl millet is sim- ilar to that when cows are fed a high-concen- trate, low-roughage ratio~ b then feeding buffers similar to those used by Emery and Brown (7) should prevent this depression. I f the milk fat depression on pearl millet is secondarily related to hypomagnesemia, then MgC03 might be especially suitable for correcting the depression. Also, if hypomagnesemia were the problem,

1319

1 3 2 0 a . w . M I L L E R ET AL

KHC03 should further aggravate the condition (12). A preliminary report of this work was presented earlier (18).

EXPERIMENTAL PROCEDURE

Twelve Holstein cows were placed in two replicates on the basis of age and randomly as- signed in a 2 X 3 factorial design to pearl mil- let (Pennisetum glaucum L.) or Sudan grass (sorghum sudanense [Piper] Stapf) pasture. Two cows on each forage served as controls, two were supplemented with 1.00 lb KHCO~ per cow, per day, and two with an equal equiva- lent weight of (0.42 lb) MgC03 per day. These supplements were added to the concentrate mix- ture, fed at a level of 16 lb per day for the replicate of 2-yr-olds (lower milk production) and 18 lb for the replicate of mature cows (higher milk production). I f there were re- fusals of the buffered concentrate, an equal amount of unsupplemented concentrate was fed to the cows, so that the grain intake was con- stant throughout the experiment. The animals were turned out to their respective pastures daily and a rotational grazing system was prac- ticed. The cows were on pasture all day except for about 6 hr when they were in the barn for grain and milking. The experiment lasted from July 10 to August 31.

The cows were weighed three consecutive days at the start and end of the experiment for cal- culation of body weight changes.

Milk from individual cows was weighed at each milking. One-day composite milk samples were taken twice weekly for milk fat~ protein, and solids-not-fat (SNF) analyses. Methods used for the analyses were Babcock, Orange-G dye binding (1), and Watson lactometer (28), respectively.

Blood samples were obtained weekly by jug- ular puncture and the following analyses per- formed on the serum: calcium, magnesium, phosphorus, chloride, sodimn, and potassium. A modification of the method of Patton and Reeder (19) was used for the determination of calcium and total bivalent ions. Magnesium was calculated by difference. Phosphorus was determined by a modification of the Bell-Doisy phosphate method described by Briggs (3) and chloride was determined with an Aminco-Cot- love Chloride Titrator by a method described by Cotlove et al. (4). Sodium and potassium analyses were performed by flame spectro- photometry.

During a preliminary period all cows were brought into the barn and a 24-hr total urine collection taken. Urinary creatinine was deter- mined on each urine (10). Since the crcatinine

excretion in the urine of the dairy cow is rela- tively constant (9), the total daily output of urine was estimated by analyzing weekly urine grab samples for ereatinine. Titratable alka- linity of the urine was determined by a modifi- cation of the method described by Hawk, Oser, and Summerson (10). From these two deter- minations, the total urine a lka l in i ty was calculated.

Three times during the experiment, rumen samples were taken by stomach tube approxi- mately 8 hr after the morning concentrate feed- ing, immediately following grazing. After the rumen samples were withdrawn, they were filtered through a double layer of cheese cloth and pH measurements taken. A 20-ml sample was preserved with mercuric chloride and frozen for later volatile fatty acid (VFA) analyses. These analyses were performed using flame ionization, gas chromatography.

Statistical analysis was performed on the data using methods described by Snedecor (23). Covariance analyses of the milk production and the per cent of milk constituents were con- ducted, using data from a preliminary period when all animals were grazing a grass legume pasture, to adjust the values for initial animal differences.

RESULTS AND DISCUSSION

There were some refusals of the buffered con- centrate mixture; therefore, the average con- sumption of KttC03 was 0.87 lb per day and that of MgCO~, 0.40 lb.

Table 1 shows the adjusted means for milk production, the per cents of fat, SNF, and pro- tein in the milk and, also, the average daily changes in body weight. There were no sta- tistically significant differences for any of the above criteria except for the difference (P .01) in milk fat test between the cows on pearl millet and Sudan grass. This confirms the earlier work of Hemken et al. (11). The reason for the greater difference (0.79 vs. 0.35%) in milk fat test between cows on pearl millet and Sudan grass in this experiment as compared to that of Hemken et al. (11) is not known.

Neither the KHCO~ nor the MgCO~ prevented the lower fat test of animals grazing pearl mil- let. This is in contrast to earlier work (5, 7, 8, 15), where the depressions in milk fat test of cows on high-concentrate, low-roughage ra- tions was prevented by the feeding of KtICO, or NaHC03, or both. A possible explanation for this phenomenon is that cows grazing pearl millet may not have a decrease in saliva flow. This is indicated by the higher tureen pH of

BUFFERS FED TO GRAZING COWS 1321

TABLE 1 Adjusted means for milk production and milk constituents; average body weight changes

S.E. Sudan grass Pear] millet of a

Control KHCOs MgCOa Control KHCO8 MgCO3 mean

Milk (lb.~day) 44.6 47.7 44.0 41.0 46.3 43.2 3.4 F a t ( % ) 3.59 3.51 3.82 2.83 2.82 2.90 0.13 ~ SNE ( % ) 8.53 8.62 8.62 8.40 8.70 8.71 0.22 Protein (%) 3.25 3.26 3.32 3.28 3.36 3.45 0.13 Body wt change

(lb/day) --0.20 +0.]6 +0.23 --0.66 --0.84 --0.04 0.45

" Sudan grass and pearl millet significantly different (P ~ .01).

an imals g raz ing pear l mil let (Table 4) com- p a r e d to those on a h igh-concent ra te , low- roughage r a t i on (7) . W h e n the buffers are f ed to an imals on a h igh-concen t ra t ion ra t ion , a more normal r u m e n f e r m e n t a t i o n p a t t e r n is re- stored, r esu l t ing in milk not be ing depressed in f a t content . Since the rumen p H of an imals g raz ing pea r l mil let is higher , this would not be the case, and the exp lana t ion f o r this t ype f a t depress ion mus t be sought elsewhere.

The average values fo r se rum calcium, mag- nesium, sodium, potass ium, phosphorus , a n d chloride of cows du r ing the expe r imen t are shown in Table 2. There were no s ta t is t ica l ly s ignif icant differences in concen t ra t ion of these serum electrolytes among any of the exper i - menta l rat ions. This would seem to rule out the poss ibi l i ty of the depress ion milk f a t tes t of an imals g raz ing pear l mil let be ing associated wi th a n absolute deficiency or lack of avai l- abi l i ty of any of the minera ls determined. These resul ts t end to confirm the work of DeMan, Sjol lema, and Grashuis (16) and McClymont (17) , who a t tempted , wi th negat ive results , to

associate depress ions in mi lk f a t tests of cows on pas tu res wi th a deficiency of calcium or mag- nesium, or both.

Es t ima tes of the to ta l u r ine a lka l in i ty are p resen ted in Table 3. Bo th Kt tCO~ and MgCO~ increased ( P < .05) the to ta l u r ine a lka l in i ty as compared to the controls. There was a dif- ference ( P < .05) between fo rages ; however, as shown in Table 3, the addi t ion of the supple- ments a p p e a r e d to be responsible , as the values for the controls on pear l mil let and Sudan grass were abou t the same. The reason the supple- ments increased the u r ine a lka l in i ty to a g rea te r ex tent in cows g raz ing pear l mil let as compared to those on Sudan grass is not known, bu t the minera l composi t ion of the forages migh t be involved. The effects jus t discussed do not ap - pea r to have any re la t ionsh ip wi th the lower milk f a t tes t of cows graz ing pear l millet, since the buffers were wi thou t effect in p r e v e n t i n g the drop in milk f a t test.

Resul ts ob ta ined f r o m ana lyz ing r u m e n fluid for V F A are shown in Tables 4 and 5. As in- dicated in Table 4, where the V F A resul ts a re

TABLE 2 Blood serum electrolytes

S.E. Sudan grass Pearl millet of a

Control KHCO3 MgCO~ Control KHCO3 MgCO3 mean

(~ng/1OOml) Calcium 11.20 10.72 11.08 10.81 10.93 10.98 0.19 Magnesium 2.48 2.52 2.51 2.58 2.72 2.60 0.15 Sodium 337 328 338 334 330 338 5 Potassium 18.6 17.9 18.3 17.8 17.4 20.3 0.9 Phosphorus 5.62 4.65 4.81 5.64 5.14 6.52 0.16 Chloride 370 364 370 371 366 368 5

TABLE 3 Milliliters 0.1 N tIC1 required to t l t ra te 24-hr output of urine to p i t 7.4

Sudan grass Pearl millet S.E. Control KHCO.~ MgCO8 Control KHCO.~ MgCO3 of a mean

900 2,294 7,394 1,090 15,987 13,412 2,262 a'b

a Sudan grass and pear] millet significantly different (P ~ .05). b Control and buffered rations significantly different (P ~ .05).

1 3 2 2 n . w . M I L L E R ET AL

Control

Acid C~ 5.24 C3 1.68 C, 1.01 C~ (iso) 0.14 C5 0.08 Total 8.15

pH 6.66

" Sudan grass and pearl b Sudan grass and pearl

TABLE 4 Concentration of rumen VFA and pH.

Sudan grass Pearl millet KHCOa MgCO8 Control KHCO3

(meg/lO0 qnl rumen fluid) 5.58 6.04 4.50 4.32 1.68 1.69 2.64 2.36 0.93 1.11 0.66 0.71 0.14 0.14 0.10 0.10 0.06 0.07 0.20 0.24 8.39 9.05 8.10 7.73 6.66 6.60 6.80 6.72

millet significantly different (P < .05). millet significantly different (P < .01).

S . ~ , MgCOa of a mean

4.67 0.38" 2.53 0.14 b 0.77 0.12" 0.14 0.01 0.20 0.02 b 8.31 0.60 6.88 0.04 b

expressed as mi l l iequivalent p e r 100 ml of rumen fluid, there were s ta t i s t ica l ly s ignif icant differences be tween cows on pea r l mil let and Sudan grass fo r acetic, p ropionic , butyr ic , and n-valer ie acids. I n the l i te ra ture , the p resen ta - t ion of rumen V F A da ta in the fo rm of concen- t r a t i on is the except ion r a t h e r t h a n the rule, a l though concen t ra t ion is p r o b a b l y a be t t e r means of eva lua t ing the amounts of V F A avail- able to the animal . As seen in Table 4, there was a decrease ( P < .05) in the concen t ra t ion of rumen acetic acid of an imals g raz ing pear l millet compared to those g raz ing Sudan grass. V a n Soest (26) , however, expresses doubt t h a t an acetate deficiency is respons ib le for the milk f a t depress ion caused by h igh-concent ra te , re- s t r ic ted roughage diets.

The decrease in the molar pe r cent of acetic acid and the increase in the molar pe r cent of p rop ion ic acid in cows graz ing pear l mil let con- firms the resul t s of o ther workers wi th cows on fa t -depress ing diets (11, 13, 22, 25).

To see i f decreases in the ace t i c :p rop ion ic ra t ios could account for the decreases in the milk f a t tes ts of cows graz ing pear l millet , a covar iance analys is us ing the ace t i c :p rop ion ic acid ra t ios as the X var iab le and milk f a t tests was conducted. A f t e r the milk f a t tests were ad jus ted fo r changes in the ace t i c :p rop ion ic acid rat ios, the differences in f a t tests be tween an imals g raz ing pear l mil let and Sudan grass were nonsignif icant . These resul ts suggest tha t ,

in this exper iment , the mechanism by which pear l millet decreased milk f a t tes t was t h rough changes in the ra t ios of acetic and p rop ion ic acids.

The decrease in the molar pe r cent of bu tyr ic acid is in ag reement wi th some prev ious l i tera- tu re (27) , bu t disagrees wi th o ther workers who r e p o r t no change or an increase in the molar pe r cent bu tyr ic acid of cows on f a t -dep res s ing diets (2, 22, 25).

As shown in Table 4, cows g raz ing pear l mil- let had a small, bu t s ignif icant ( P ~ .01), in- crease in rumen p H compared to those on Sudan grass. Whi le i t has been r epo r t ed t h a t b ica rbona tes will ra ise the r u m e n p H in ani- mals on h igh-concent ra te , low-roughage diets (7, 8) , th is effect was not observed, p r o b a b l y because the rumen p H of cows on these pas- tures were h igher t h a n fo r cows fed a high- concentra te , low-roughage ra t ion .

F r o m resul ts obta ined in this exper iment , i t is concluded t ha t a n absolute deficiency or lack of ava i lab i l i ty of magnes ium or any of the o ther sermn electrolytes de te rmined p robab ly was not the cause of the lower milk f a t tests of cows graz ing pear l millet. Also, t h a t since ne i the r the K H C 0 ~ nor the MgC0~ ra ised the milk f a t pe r cent of cows on pea r l mil let pas- ture, i t is suggested t ha t the n a t u r e of the f a t depress ion caused by the feeding of a high- concentra te , low-roughage diet and t h a t caused by pear l millet m a y be different.

TABLE 5 Per cent rumen VFA

Sudan grass Pearl millet S.E. Control KHCO.~ MgCO3 Control KHCOs MgCOs of a mean

Acid (molar % of total) C~ 64.4 66.5 66.8 55.3 56.0 56.4 1.4 b C3 20.6 20.1 18.7 32.9 30.6 30.4 1.4 b C~ 12.4 11.0 12.2 8.0 9.2 9.2 1.0" Cs (iso) 1.7 1.7 1.5 1.2 1.3 1.6 0.1 a C5 0.9 0.7 0.8 2.6 3.0 2.4 0.2 b

" Sudan grass and pearl millet significantly different (P ~ .05). b Sudan grass and pearl millet significantly different (P ~ .01).

BUFFERS FED TO GRAZING COWS 1323

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