Effects of Intake and Postruminal Casein Infusion on Performance and Concentrations of Hormones in Plasma of Lactating Cows1

Effects of Intake and Postruminal Casein Infusion on Performance and Concentrations of Hormones in Plasma of Lactating Cows 1 W. S. COHICKfl J. L. V I C I N I , C. R, STAPLES, 3 and J. H. C L A R K Department of Amimal Sciences University of Illinois Urbana 61801 S, N. McCUTCHEON 4 and D. E. B A U M A N s Department of Animal Science

Cornell Universitv Ithaca, NY 14853-4801 ABSTRACT

ronine. Glucagon, prolactin, and thyroxine were not affected by intake. Casein infusion did not affect growth hormone, insulin, prolactin, triiodothyronine, or thyroxine. Increased milk and milk protein yields obtained with casein infusion were apparently not mediated through changes in circulating concentrations of these hormones; however, plasma glucagon was increased by casein infusions.

Four Holstein cows were utilized in a Latin square design with a factorial arrangement of treatments to examine the interaction between effects of dry matter intake (107 vs. 78% of energy requirements) and postruminal infusions (water vs. 395 g/d casein) on lactational performance, utilization of nitrogen and energy, and plasma concentrations of hormones. Yields of milk and milk protein were decreased by feed restriction and increased by casein infusion with no treatment interactions. Restricting feed intake decreased total nitrogen intake by 143 g/d and resulted in smaller quantities of fecal, absorbed, urinary, milk, and retained nitrogen compared with cows fed ad Iibitum. Casein infusion increased total nitrogen intake (55 g/d), absorbed nitrogen (54 g/d), urinary nitrogen excretion (28 g/d), and milk nitrogen (13 g/d). Casein by dry matter intake interactions were not significant for nitrogen utilization. Restricting feed intake increased plasma growth hormone and decreased concentrations of insulin and triiodothy-

INTRODUCTION

Received June 25, 1986. a Supported in part by University of Illinois Agricultural Experiment Station, Cornell University Agricultural Experiment Station and Church & Dwight Co., Inc. 2Present address: Department of Animal Science, Cornell University. 3Present address: Department of Dairy Science, University of Florida. 4On leave from Department of Animal Science, Massey University, Palmerston North, New Zealand. s Reprint requests. 1986 J Dairy Sci 69:3022--3031

Postruminal infusion of casein increases production of milk and milk protein by dairy cows (8). The mechanism by which casein elicits the improved performance has not been identified. Clark (8) suggested that the response may be attributed to an improved supply of limiting amino acids (for protein synthesis) or carbon (for gluconeogenesis) or to changes in the endocrine status of the animal. Amino acids limiting milk production (12, 34, 35) and the effects of abomasal casein infusion on glucose availability (9) have been investigated. However, little is known about the effects of postruminal infusion of protein on plasma hormone concentrations in lactating dairy cows. Administration of exogenous bovine growth hormone (somatotropin; GH) to dairy cows has demonstrated that this hormone is a key homeorhetic control responsible for coordinating metabolic processes in support of milk synthesis (4, 10). Concentrations of GH in plasma of ruminants were elevated by postruminal infusion of casein (26), by intravenous infusion of large amounts of individual amino acids (11, 19, 20, 31), and by feeding formaldehyde-treated protein supplements (27). In contrast, other studies with cows and goats have failed to demonstrate a GH response to

3022

INTAKE AND POSTRUMINAL CASEIN INFUSION postruminal casein infusion (17, 28, 32). These conflicting results might relate to the nutrient status of the animals. Most published studies involving postruminal infusion of protein or amino acids do not report energy and protein status of the animals. However, other studies involving arginine or thyroid-releasing hormone administration (iv) have shown that GH release to these challenges is much greater in animals in negative energy and protein balance (3, 23). The present investigation was undertaken to examine the interaction between effects of dry matter intake and of postruminal infusion of sodium caseinate on lactational performance, utilization of nitrogen and energy, and concentrations of hormones and metabolites in plasma of dairy cows. MATERIALS AND METHODS

Four rumen-fistulated Holstein cows averaging 82 d postpartum were used in a Latin square with a factorial arrangement of treatments involving abomasal infusions ( H 2 0 vs. sodium caseinate) and nutrient intake (ad libitum vs. 80% of digestible energy (DE) requirements). Treatments were maintained for 10 d. The initial 3 d of each period were used to adapt cows to treatment. Data on yield and composition of milk were collected during the remaining 7 d. Cows were fed a complete mixed diet containing 15% crude protein and consisting of 50% concentrate, 6 25% corn silage, and 25% alfalfa haylage on a dry matter basis. Diets were offered in ad libitum or restricted amounts at 0700 h and 1700 h daily. Quantities o f diet offered and refused were weighed and sampled daily. Dry matter percentages were determined for daily samples (feed and orts) by drying in a forced air oven at 55°C for 24 h. Feed samples were composited at the end of each period for subsequent analyses. Cows were milked at 0500 and 1600 h. Milk samples were taken at each milking, preserved

6Contains on a dry matter basis the following: ground shelled corn, 74%; soybean meal, 21%; dicalcium phosphate, 1.5%; limestone, .94%; sodium sulfate, .51%; trace mineral salt, 2.00%; and vitamin A and D supplement, .05%.

3023

with potassium dichromate, and stored at 4°C until analyzed for nitrogen and fat content. Milk fat was assayed b y Babcock method (1) and solids-not-fat according to the procedure of Golding (14). Energy in milk was calculated according to Tyrrell and Reid (36). A p p r o x i m a t e l y 6 L of water or a solution of sodium caseinate (7.5% wt/vol) were infused daily. Animals infused with casein received 395 ± 3 g sodium caseinate daily. Solutions were continuously infused via a 6-mm polyethylene tube that was passed through the rumen cannula and sulcus omasi into the abomasum. A one-way value enclosed in a 60-ml perforated bottle was affixed to the end o f the tube placed in the abomasum. Retraction from the abomasum was prevented by a rubber flange attached to the neck o f the bottle. Solutions were prepared daily and refrigerated at 4°C during infusion. Nitrogen balance and apparent digestibility were determined b y total collection of feces and urine on d 4 through 8. Urine was collected (via indwelling urinary catheters) in polyethylene containers under acidic conditions (200 ml 9 N HCI) and weighed and sampled daily. Urine samples were stored at 4°C and composited by volume at the end of each collection period. Three percent of the daily fecal output was preserved with t h y m o l and stored at 4°C until the end of the collection period when it was composited and analyzed for dry matter content. Dried feed and fecal samples were ground and saved for chemical analyses. Nitrogen content of feed, feces, urine, and milk was determined by the Kjeldahl method (1). Feed and fecal samples were analyzed for ash, ether extract, and acid detergent fiber (1, 13). Nitrogen-free extract was determined by difference. These analyses were used to calculate apparent digestibility coefficients and total digestible nutrient (TDN) content of the feeds ( 33). Blood samples were obtained via indwelling jugular cannulae at hourly intervals for 24 h on the last day of each period. Plasma was harvested by centrifugation and stored at - 2 0 ° C until analysis. Alternate hourly plasma samples were assayed for glucose and nonesterified fatty acids (NEFA). Glucose was determined by the glucose oxidase method (29) and NEFA by titration of heptane extract (21). Journal of Dairy Science Vol. 69, No. 12, 1986

3024

COHICK ET AL.

Plasma samples obtained at hourly intervals were assayed for concentrations of GH, insulin, and glucagon. Growth hormone was determined (15) using bovine GH for iodination (Miles lot 12) and reference standards (NIH-GH-B18). 7 Insulin was determined (24) using bovine insulin s as trace and for reference standards. Plasma glucagon was determined by a double antibody radioimmunoassay validated for bovine glucagon in the laboratory of E. N. Bergman, Cornell University, Ithaca, NY. Bovine glucagon (lot number 258-25J-120) 8 was used for reference standards. Intraassay and interassay coefficients of variation (CV) averaged 13.0 and 21.6% for GH, 10.8 and 13.7% for insulin, and 5.3 and 8.2% for glucagon. Plasma samples obtained at 4-h intervals commencing at 0300 h were assayed for prolactin, thyroxine (T 4), and triiodothyronine (T3). Prolactin was determined (7) using ovine prolactin as trace 9 and bovine prolactin for reference standards (NIH-PRL-B4). 7 ]ntraassay and interassay CV averaged 9.0 and 13.9%. Concentrations of T4 and T 3 were determined using solid-phase radioimmunoassay kits.l° Intraassay CV averaged 6.0 and 5.5% for T4 and T 3 assays. Data were analyzed by the general linear models procedure of the Statistical Analysis System computational package (18). Single degree of freedom comparisons were made to evaluate the effects of casein infusion, level of feed intake, and interaction of casein infusion and dry matter intake. RESULTS

Dry matter intake was not affected by casein infusion, but this treatment increased dry matter digestibility presumably because of the high digestibility coefficient of casein (Table 1). Reduced dry matter intake in the restricted fed group was also associated with increased dry matter digestibility. Digestible energy intakes were calculated from TDN values

7Donated by US Department of Agriculture Animal Hormone Program and the National Hormone and Pituitary Program. s Donated by M. Root; Eli Lilly Co., Indianapolis, IN. 9Amersham Corp., Arlington Heights, IL. ~0Diagnostic Products Corp., Los Angeles, CA. Journal of Dairy Science Vol. 69, No. 12,

1986

determined using total collection techniques. Digestible energy intakes relative to National Research Council requirements (25) in the restricted and ad libitum-fed groups averaged 78 and 107%, respectively. Calculated DE balances were --10.3 and 3.6 Mcal/d for restricted and ad libitum-fed cows, respectively. Restriction of feed intake depressed milk yield 13.5% and resulted in higher milk fat percentage and lower crude protein percentage (Table 2). Abomasal infusion of casein increased milk crude protein percentage and milk yield (9.2%), resulting in a 15.1% increase in yield of milk protein. Milk fat percentage was decreased by casein infusion. Compared with cows fed ad libitum, restricted-fed cows increased milk production more and decreased milk fat percentage less when infused with casein (Table 2). The net effect of these changes was a significant casein by dry matter intake interaction for milk fat yield. However, for most variables there were no significant interactions despite the substantial reduction in feed intake imposed in this study. Nitrogen intake and utilization data are in Table 3. No significant casein by dry matter intake interactions were detected. Restriction of feed intake decreased total nitrogen intake by 143 g/d and resulted in smaller quantities of fecal, absorbed, urinary, milk, and retained nitrogen compared with those of cows fed ad libitum. Casein infusion did not affect oral nitrogen intake, but total nitrogen intake (oral plus infused) was increased by 55 g/d. Cows infused with casein or water excreted similar quantities of fecal nitrogen. Thus, absorbed nitrogen was increased by casein infusion. This treatment also increased urinary nitrogen excretion (28 g/d) and milk nitrogen secretion (13 g/d) and tended to increase nitrogen retention. Concentrations of hormones and metabolites in plasma are in Table 4. Casein infusion increased concentrations of glucagon in plasma but did not affect GH, insulin, prolactin, T3, T4, NEFA, or glucose. Restriction of feed intake increased concentrations of GH and NEFA and decreased concentrations of insulin and T3. Concentrations of glucagon, prolactin, T4, and glucose in plasma w e r e n o t affected by intake. There were no significant casein by dry matter intake interactions for any of the hormones and metabolites.

TABLE 1. Least squares means for dry matter intake and other energy related parameters of cows fed two a m o u n t s of dry matter and infused postruminally with sodium caseinate or water. Significance of difference (P<)

Treatments Ad libitum ~ Parameter

Restricted ~

Water 2

Casein 2

Water 2

Casein 2

18.7

12.3 1213

12.7 .4 13.1

65.1 64.5 34.8 46.3

68.5 69.0 39.7 48.7

Dry matter intake, kg/d Mixed ration Sodium caseinate Total

1817

19.0 .4 19.4

Dry matter digestibility, % TDN, %s Digestible energy intake, ~ Mcal/d Digestible energy requirement, ~ Mcal/d

63.4 63.1 53.3 50.6

65.5 65.6 56.0 51.5

SE a

.5 15" .8 .8 1.9 .8

Water vs. casein

Ad libitum vs. restricted

Casein by intake interaction

NS 4 .0001 NS

.0001 NS .0001

NS NS NS

.02 .01 NS .08

.03 .03 .0003 .004

NS NS NS NS

o

ct~

x > C~

> o~

1 Refers to quantity of dry matter offered to the cows.

o

Refers to sodium easeinate or water which was infused postruminally. Standard error of individual treatment means. 4Nonsignificant (NS) at P>.10. s Total digestible nutrients (TDN) determined in total collection trials. 6 Digestible energy intake (Meal/day) = (% TDN/100) (kg dry matter intake) (4.41 Mcal/kg).

< o on ~o

z o

0o o~

Calculated from National Research Council (25).

o z

b~ Ox

o c/)

9.

?-

TABLE 2. Least squares means for milk p r o d u c t i o n and c o m p o s i t i o n of mi l k produced b y cows fed two a m o u n t s of dry m a t t e r and infused p o s t r u m i n a l l y with sodium caseinate or water.

Ox

Parameter

Water 2

Casein 2

Water 2

Casein 2

SE 3

Water vs. casein

Milk, kg/d Milk crude protein, %s Milk crude protein, kg/d s Milk solids-not-fat, % Milk solids-not-fat, kg/d Milk fat, % Milk fat, kg/d Milk energy, Mcal/d 6

25.4 3.03 .77 8.43 2.15 3.26 .83 16.62

27.2 3.18 .86 8.35 2.27 2.93 .80 16.92

21.5 2.92 .62 8.14 1.75 3.38 .72 14.10

24.0 3.07 .74 8.31 1.99 3.25 .78 15.60

.6 .03 .02 .05 .06 .07 .02 .35

.02 .005 .005 NS .02 .01 NS .05

O

70 00 O~

Significance of difference (P<)

Treatments

Z Ad l i b i t u m I

Restricted I

1 Refers to q u a n t i t y of d r y m a t t e r offered to the cows. 2 Refers to sodium caseinate or water which was infused postruminally. 3 Standard error of individual t r e a t m e n t means. 4 Nonsignificant (NS) at P>.10. SCrude protein = N × 6.38. Milk energy (kcal/kg) = 92.24 (% fat) + 49.14 (% solids-not-fat) - 5 6 . 3 9 . F rom Tyrrell and Reid (36).

A d lib itu m vs. restricted

Casein b y intake interaction

.001 .02 .001 .02 .001 .02 .006 .002

NS 4 NS NS .06 NS NS .02 NS

O ~v

r~

TABLE 3. Least squares means for nitrogen intake and u t i l i z a t i o n b y cows fed t w o a m o u n t s of d r y m a t t e r and infused p o s t r u m i n a l l y w i t h s o d iu m caseinate or water. Significance of difference (P<)

Tre a t me nt s Ad l i b i t u m 1 Parameter

Water:

Restricted I

Casein:

Water 2

Casein 2

SE a

Water vs. casein

Ad l i b i t u m vs. restricted

Casein b y intake interaction

(g/d) N Intake Oral Infused Total Fecal N Ab so rb ed N Urinary N Milk N Retained N Productive N s

420

426 48 474

276

285 47 332

12 1 12

NS 4 .0001 004

.0001 NS .0001

NS NS NS

179 249 136 124 -11 113

178 296 161 134 1 135

113 164 110 102 -48 53

106 226 140 118 -31 86

7 10 6 4 8 10

NS .003 .004 .02 NS .04

.0002 .0007 .009 .005 .006 .002

NS NS NS NS NS NS

55 52 --7 45

55 46 --1 45

62 52 --14 38

3 2 3 3

NS .004 .02 NS

.02 .004 .003 ,02

NS NS NS NS

> Pq w > Z O ,q

Z t" >

(% absorbed) o

Urinary N Milk N Retained N Productive N

68 62 --30 32

Z W

Refers to q u a n t i t y of dry m a t t e r offered to the cows. 2 Refers to sodium caseinate or water which was infused pos t rumi na l l y.

< o fox

a Standard error of individual t r e a t m e n t means.

Z

s Productive N = m i l k N plus retained N.

o

00 ¢h

4 Nonsignificant (NS) at P>.10.

to ~q

3028

COHICK ET AL. DISCUSSION

Postruminal infusion of casein increased yields of milk and milk crude protein in both ad libitum (107% of DE requirement) and restricted (78% of DE requirement) fed cows (Table 2). Similar increases have been observed in previous studies with abomasal casein infusion (8, 9). However, differences in response to casein infusion between ad libitum and restricted fed animals were not significant for most milk production variables, despite very large differences in energy intake. Therefore, responses to casein infusion were independent of energy status. Restricted and ad libitum-fed cows were in negative nitrogen balance ( - 4 0 and - 5 g/d, respectively). There was no evidence of interaction in regard to treatment effects on nitrogen utilization. However, there were additive effects of dry matter intake and casein infusion on the partitioning of absorbed nitrogen. When expressed as a percentage of absorbed nitrogen, milk nitrogen was increased and retained nitrogen was decreased by restricted feed intake. Therefore, more absorbed nitrogen was directed toward milk synthesis and less toward body nitrogen pools in restricted-fed cows. As a proportion of absorbed nitrogen, retained nitrogen was increased b y casein infusion and milk nitrogen was decreased. Thus, at both intakes, cows responded to casein infusion b y decreasing the extent to which they mobilized b o d y protein stores in support of milk synthesis. Cows offered restricted amounts of feed had higher mean plasma concentrations of GH and N E F A and lower concentrations of insulin compared with animals fed ad libitum. Plasma glucagon concentrations were not affected b y feeding (Table 4). Restriction of feed intake of lactating sheep b y 80% for 4 d decreased plasma insulin, but not glucagon, and increased GH and N F F A (16). Other studies have also demonstrated that concentrations of GH, NEFA, and insulin are related to energy status of ruminants (2, 23). Casein infusion had no effect on plasma concentrations of GH at either restricted or ad libitum dry matter intakes (Table 4). Previous studies have reported conflicting results. Postruminal infusion of casein had no effect on circulating concentrations of GH in studies by Gow et al. (17) with lactating goats (positive N Journal of Dairy Science Vol. 69, No. 12, 1986

balances, energy balance not given) and by Peel et al. (28) with lactating cows (positive energy and N balance). In contrast, Oldham et al. reported that plasma GH concentrations were increased by abomasal infusion of casein in a 1-d study with lactating goats (26) and by feeding formaldehyde-treated casein in a study with lactating cows (27). Reasons for the differences are not obvious. All studies utilized high concentrate diets. Although energy and protein balances were not reported in studies by Oldham et al. (26, 27), this could not relate to differences reported because we observed no effect of casein on plasma GH when cows were in positive (ad libiturn intake) or negative (restricted intake) energy and protein balances (Tables 1, 3, and 4). Interestingly, feeding formaldehyde-treated casein had no effect on yield of milk or milk components in the study of Oldham et al. (27), and only a trend toward increased lactational performance was obtained with abomasal infusion of casein in studies of Gow et al. (17) and Peel et al. (28). Thus, our study is the first that actually measures effects of casein infusion on circulating concentrations of GH under conditions where lactational performance had been significantly increased. Casein infusion also had no effect on mean plasma concentrations of insulin, prolactin, T s , T4, glucose, or NEFA, and there were no significant casein by dry matter intake interactions (Table 4). Previous workers have also reported no effect of casein infusion on plasma concentrations of these hormones and metabolites (17, 26, 28, 32). Therefore, the increases in milk and milk protein yields elicited b y abomasal infusion of casein were apparently not mediated through changes in circulating concentrations of these hormones and metabolites. However, casein infusion increased mean plasma concentrations of glucagon. This supports data of Rodriguez et al. (32) who reported increased glucagon concentrations in plasma of lactating goats infused with casein. Glucagon increased hepatic extraction of glucogenic amino acids and other glucose precursors (5, 6) and increased hepatic glucose output in fed sheep (5). Therefore, it is possible that gluconeogenic amino acids supplied during casein infusion increased rates of gluconeogenesis, thereby increasing glucose availability for milk synthesis. This has been

TABLE 4. Least squares means for concentrations of metabolites and hormones in plasma o f cows fed two amounts o f dry matter and infused postruminally with sodium caseinate or water. Significance of difference (P<)

Treatments

Plasma variable ~

Water a

Casein 3

Water a

Casein a

SE4

Water vs. casein

Nonesterfied fatty acids, ~zeq/L Glucose, rag/100 ml Prolactin, ng/ml Triiodothyronine, ng/ml Thryoxine, ng/ml Growth hormone, ng/ml Insulin, ng/ml Glucagon, pg/ml

237 60.0 12.8 .86 42.1 3.9 2.5 244

241 61.5 15.0 1.02 48.6 3.5 2.6 279

284 59.6 8.9 .75 42.6 5.2 1.9 241

290 61.6 13.0 .73 42.6 4.6 2.3 269

14 1.1 1.8 .07 2.9 .4 .2 8

NS s NS NS NS NS NS NS .008

Ad libitum 2

Restricted 2

Ad libitum vs. restricted

Casein by intake interaction

.01 NS NS .03 NS .04 .06 NS

NS NS NS NS NS NS NS NS

> t~ Z t7 O

Z > > Z

Plasma samples obtained at hourly intervals over 24 h. Analysis were on hourly samples (growth hormone, glucagon, insulin), on samples at 2-h intervals (glucose and free fatty acids), or at 4-h intervals (prolactin, triiodothyronine, and thyroxine). 2 Refers to quantity of dry matter offered to the cows.

O Z

3 Refers to sodium caseinate or water which was infused postruminally. 4 Standard error of individual treatment means. <

s Nonsignificant (NS) at P>.10.

o, Z o

o~

t~ ".o

3030

COHICK ET AL.

e x a m i n e d and results indicate t h a t no sign i f i c a n t increases in glucose t u r n o v e r rates o c c u r r e d in c o w s (9, 22) and g o a t s (30, 32) p o s t r u m i n a l l y infused w i t h casein. H o w ever, each o f t h e s e studies r e p o r t e d a t r e n d t o w a r d an increased glucose flux in r e s p o n s e t o casein infusion. T h e sensitivity o f r a d i o i s o t o p e t e c h n i q u e s and t h e use o f small n u m b e r s o f animals m a y m a k e d i f f e r e n c e s in glucose t u r n o v e r rates d i f f i c u l t t o d e t e c t . T h e r e f o r e , m o r e sensitive m e a s u r e s o f glucose t u r n o v e r rates in lactating animals infused w i t h casein must be obtained before concluding that the increased milk p r o d u c t i o n is n o t due t o additional g l u c o n e o g e n i c p r e c u r s o r s supplied b y casein.

12

13 14 15 16 17

REFERENCES

1 Association of Official Analytical Chemists. 1975. Official methods of analysis. 12th ed. Assoc. Offic. Anal. Chem., Washington, DC. 2 Bassett, J. M., R. H. Weston, and J. P. Hogan. 1971. Dietary regulation of plasma insulin and growth hormone concentrations in sheep. Aust. J. Biol. Sci. 24:321. 3 Bauman, D. E., R. M. Akers, L. T. Chapin, H. A. Tucker, and E. M. Convey. 1979. Effect of level of intake on serum concentrations o f prolactin and growth hormone in lactating cows. J. Dairy Sci. 62 (Suppl. 1):114. (Abstr.) 4 Bauman, D. E., P. J. Eppard, M. J. DeGeeter, and G. M. Lanza. 1985. Responses of high-producing dairy cows to long-term treatment with pituitary somatotropin and recombinant somatotropin. J. Dairy Sci. 68:1352. 5 Brockman, R. P., and E. N. Bergman. 1975. Effect of glucagon on plasma alanine and giutamine metabolism and hepatic gluconeogenesis in sheep. Am. J. Physiol. 228:1627. 6 Brockman, R. P., E. N. Bergman, P. K. Joo, and J. G. Manns. 1975. Effects of glucagon and insulin on net hepatic metabolism of glucose precursors in sheep. Am. J. Physiol. 229:1344. 7 Butler, W. R., P. V. Malven, L. B. Willett, and J. D. Bolt. 1972. Patterns of pituitary release and cranial output of LH and prolactin in ovarectomized ewes. Endocrinology 91 :793. 8 Clark, J. H. 1975. Lactational responses to postruminal administration of proteins and amino acids. J. Dairy Sci. 58:1198. 9 Clark, J. H., H. R. Spires, R. G. Derrig, and M. R. Bennink. 1977. Milk production, nitrogen utilization and glucose synthesis in lactating cows infused postruminaUy with sodium caseinate and glucose. J. Nutr. 107:631. 10 Collier, R. J., J. p. McNamara, C. R. Wallace, and M. H. Dehoff. 1984. Endocrine regulation of metabolism during lactation. J. Anim. Sci. 59:498. 11 Davis, S. L. 1972. Plasma levels of prolactin, growth hormone, and insulin in sheep following Journal of Dairy Science Vol. 69, No. 12, 1986

18 19

20

21

22

23

24

25 26

27

28

the infusion of arginine, leucine, and phenylalanine. Endocrinology 91: 549. Derrig, R. G., J. H. Clark, and C. L. Davis. 1974. Effect of abomasal infusion of sodium caseinate on milk yield, nitrogen utilization and amino acid nutrition of the dairy cow. J. Nutr. 104:151. Goering, H. K., and P. J. Van Soest. 1970. Forage fiber analyses. US Dep. Agric., Agr. Handbook 379, Washington, DC. Golding, N. S. 1959. A solids-not-fat test for milk using density plastic beads as hydrometers. J. Dairy Sci. 42:899. (Abstr.) Gorewit, R. C. 1981. Pituitary, thyroid and adrenal response to clonidine in dairy cattle. J. Endocrinol. Invest. 4:135. Gow, C. B., G. H. McDonald, and E. F. Annison. 1981. Control of gluconeogenesis in lactating sheep. Aust. J. Biol. Sci. 34:469. Gow, C. B., S.S.E. Ranawana, R. C. KeUaway, and G. H. McDonald. 1979. Responses to post-ruminal infusions of casein and arginine, and to dietary supplementation in lactating goats. Br. J. Nutr. 41:271. Helwig, J. T., and K. A. Council. 1979. SAS Users guide. Star. Anal. Syst. Inst. Inc., Cary, NC. Hertelendy, F., L. Machlin, and D. M. Kipnis. 1969. Further studies on the regulation of insulin and growth hormone secretion in the sheep. Endocrinology 84:192. Hertelendy, F., K. Takahashi, L. Machlin, and D. M. Kipnis. 1970. Growth hormone and insulin secretory responses to arginine in the sheep, pig and cow. Gem Comp. Endocrinol. 14:72. Ko, H., and M. E. Royer. 1967. A submicrometer assay for nonpolar acids in plasma and depot fat. Anal. Biochem. 20:205. Konig, B. A., J. D. Oldham, and D. S. Parker. 1984. The effect of abomasal infusion of acetate, palmitate and glucose kinetics in cows during early lactation. Br. J. Nutr. 52:319. McAtee, J. W., and A. Trenkle. 1971. Effect of feeding, fasting, and infusion o f energy substrates on plasma growth hormone levels in cattle. J. Anita. Sci. 33:612. McCann, J. P. 1980. The effect of an 8-day starvation period upon the metabolic and reproductive endocrinology of cyclic Holstein heifers. M. S. Thesis, Cornell Univ., Ithaca, NY. National Research Council. 1978. Nutrient requirements for dairy cattle. Natl. Acad. Sci., Washington, DC. Oldham, J. D., I. C. Hart, and J. A. Bines. 1978. Effect of abomasal infusions of casein, arginine, methionine, or phenylalanine on growth hormone, insulin, prolactin, thyroxine and some metabolites in blood from lactating goats. Proc. Nutr. Soc. 37:9A. (Abstr.) Oldham, J. D., I. C. Hart, and J. A. Bines. 1982. Formaldehyde-treated proteins for dairy cows effects on blood hormone concentrations. Br. J. Nutr. 48:543. Peel, C. J., T. J. Fronk, D. E. Bauman, and R. C. Gorewit. 1982. Lactational response to exogenous growth hormone and abomasal infusion of a glucose-sodium caseinate mixture in high yielding

INTAKE AND POSTRUMINAL CASEIN INFUSION dairy cows. J. Nutr. 112:1770. 29 Raabo, E., and J. C. Terkildsen. 1960. On the enzymatic determination of blood glucose. Scand. J. Clin. Lab. Invest. 12:402. 30 Ranawana, S.S.E., and R. C. Kellaway. 1977. Responses to postruminal infusions of glucose and casein in lactating goats. Br. J. Nutr. 37:395. 31 Reynaert, R., M. DePalpe, and G. Peeters. 1972. Influence of arginine on the blood level of growth hormone in lactating cows. Arch. Int. Pharmacodyn. 197:405. 32 Rodriguez, N. R., E. C. Prigge, S. Lough, and W. H. Hoover. 1985. Gluconeogenic and hormone responses to abomasal c~ein and ruminal volatile fatty acid infusions in lactating goats. J. Dairy Sci.

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68:1968. 33 Schneider, B. H., and W. P. Flatt. 1975. The evaluation of feeds through digestibility experiments. Univ. Georgia Press, Athens. 34 Schwab, C. G., L. D. Setter, and A. B. Clay. 1976. Response of lactating dairy cows to abomasal infusion of amino acids. J. Dairy Sci. 59:1254. 35 Spires, H. R., J. H. Clark, R. G. Derrig, and C. L. Davis. 1975. Milk production and nitrogen utilization in response to postruminal infustion of sodium caseinate in lactating cows. J. Nutr. 105: 1111. 36 Tyrrell, H. F., and J. T. Reid. 1965. Prediction of the energy value of cows' milk. J. Dairy Sci. 48:1215.

Journal of Dairy Science Vol. 69, No. 12, 1986