TAKE HOME MESSAGES

• Administering bST to Jersey cows increased dry matter intake and production of milk, milk solids, milk protein, milk fat, and 3.5% fat corrected milk.
• Feeding rumen-protected I let on ne and Iysine increased concentrations quantities were absorbed from the small intestine.
• Milk protein percentage was increased when rumen- ote ted met nine and Iysine were fed to the cows, but milk protein yield was not increased because milk yield was not increased.
• Either meth nine and Iysine supplied by the diet was adequate to meet requirements or some other nutrient was limiting for synthesis of milk and milk protein.

INTRODUCTION

Dairy producers are paid for milk, milk protein, and milk fat, and the protein and fat contents of milk are the major determinants of the manufacturing properties of milk. Supplemental fat in the diets of cows has increased both the yield and percentage of milk fat but often has decreased milk protein percentage. Supplementation of fat to the diet has decreased intakes of dry matter and crude protein, which might decrease the availability of amino acids for synthesis of milk protein. Administration of bST has increased milk production by dairy cows, which increases the quantity of nutrients, including amino acids, required by cows. Efficiency of amino acid utilization can be improved, and amino acid deficiencies can be avoided, by supplying postruminally those amino acids that are limiting for milk and milk protein synthesis. The percentage of methionine and Iysine that are extracted by the mammary gland is higher than that of other amino acids, which emphasizes the importance of these two amino acids for milk production. If bST increases milk production and adequate quantities of amino acids are not supplied to meet the additional needs then maximum yields of milk and milk protein may not be attained. The objective of this experiment was to determine the responses of lactating Jersey cows to bST and rumen-protected methionine and Iysine (RPAA) when fed diets that contained supplemental fat.

MATERIALS AND METHODS

The experimental design was a replicated 4x4 Latin square; treatments were in a 2x2 factorial arrangement with 28-day periods. Eight multiparous and 4 primiparous Jersey cows at 92 days of lactation were used. All cows were fed for ad libitum intake a total mixed diet (Table 1). Treatments were 1) control (no bST or RPAA), 2) control + bST, 3) control + RPAA, and 4) control + bST + RPAA. Dry matter intake, milk production, and milk composition were measured during the last 2 weeks of each experimental period. Milk samples were analyzed for fat, crude protein, and total solids. Blood samples were drawn from the tail vein on the last day of each experimental period and plasma was analyzed for amino acids.

RESULTS AND DISCUSSION

Cows that were injected with bST consumed a mean of 2.2 Ib/day more dry matter and produced 3.85 Ib/day more milk than cows not injected with bST (Table 2). Milk produced by cows that were injected with bST tended to have higher contents of fat and total solids than milk produced by cows that were not injected with bST. The percentage of crude protein in milk was not altered by injection of bST. Yields of fat, 3.5% fat-corrected milk, crude protein, and total solids were increased by injection of bST.

Dry matter intake was not affected by RPAA (Table 2). Perhaps because cows were past peak milk production, RPAA did not increase milk yield. Also, cows that were injected with bST and fed RPAA had smaller increases in milk yield than did cows that were injected with bST but were not fed RPAA. Milk produced by cows that were fed RPAA was higher in contents of fat, crude protein, and total solids than milk produced by cows that were not fed RPAA. Yields of fat, 3.5% fat-corrected milk, crude protein, and total solids were not altered by RPAA because of the significant interaction between RPAA and bST for milk production, resulting in a lower milk yield for cows administered bST and fed RPAA than for cows administered bST only.

Concentrations of individual essential amino acids in plasma were not affected by administration of bST (Table 3). Dietary RPAA increased the concentration of methionine in plasma, tended to increase concentrations of arginine and Iysine, but did not affect concentrations of other essential amino acids. There were significant interactions of bST and RPAA for plasma concentrations of arginine, isoleucine, leucine, Iysine, phenylalanine, and valine and trends for interactions of bST and RPAA for plasma concentrations of histidine, met lionine, and threonine. Increased concentrations of methionine in plasma and the trend for increased concentrations of Iysine in plasma when RPAA were fed to cows indicate that RPAA increased absorption of methionine and Iysine from the small intestine.

TABLE 1. Ingredient and nutrient composition of the total mixed diet {dry matter basis).

TABLE 2. Least squares means for DMI and yield and composition of milk.

'Control = No bST or RPAA, bST = control plus bST, RPAA = control pius rum ally protected AA {Met and Lys; RPAA), and bS plus RPAA = control plus bST plus RPAA.

TABLE 3. Least squares means for concentrations of amino acids in plasma.

1Control = No bST or RPAA, bST = control plus bST, RPAA = control plus rum nally protected AA (Met and Lys; RPAA), and bST plus RPAA = control plus bST plus RPAA.