Effect of somatotropin administration and duodenal infusion of methionine and lysine on lactational performance and nutrient flow to the small intestine

Effects of jugular-infused lysine, methionine, and branched-chain amino acids on milk protein synthesis in high-producing dairy cows

In addition to lysine and methionine, current ration-balancing programs suggest that branched-chain amino acid (BCAA) supply may also be limiting in dairy cows. The objective of this study was to investigate whether BCAA, leucine, isoleucine, and valine become limiting for milk protein synthesis when methionine and lysine supply were not limiting. Nine multiparous Holstein cows with an average milk production of 53.5±7.1 kg/d were randomly assigned to 7-d continuous jugular infusions of saline (CTL), methionine and lysine (ML; 12 g and 21 g/d, respectively), or ML plus leucine, isoleucine, and valine (ML+BCAA; 35 g, 15 g, and 15 g/d, respectively) in a 3×3 Latin square design with 3 infusion periods separated by 7-d noninfusion periods. The basal diet consisted of 40% corn silage, 14% alfalfa hay, and a concentrate mix, and respectively supplied lysine, methionine, isoleucine, leucine, and valine as 6.1, 1.8, 4.7, 8.9, and 5.3% of metabolizable protein. Dry matter intake (23.9 kg/d), milk yield (52.8 kg/d), fat content (2.55%), fat yield (1.33 kg/d), lactose content (4.77%), lactose yield (2.51 kg/d), and milk protein efficiency (0.38) were similar across treatments. Protein yield and protein content were not significantly different between ML (1.52 kg/d and 2.88%, respectively) and ML+BCAA (1.51 kg/d and 2.83%, respectively), but they were significantly greater than that of CTL (1.39 kg/d and 2.71%). Cows that received ML+BCAA had less milk urea nitrogen content (10.9 mg/dL) compared with milk of CTL cows (12.4 mg/dL) and ML cows (11.8 mg/dL). Whereas high-producing cows responded positively to methionine and lysine supplementation, no apparent benefits of BCAA supplementation in milk protein synthesis were found. Infusion of BCAA may have stimulated synthesis of other body proteins, probably muscle proteins, as evidenced by decreased milk urea nitrogen.

Effects of dietary supplements of folic acid and rumen-protected methionine on lactational performance and folate metabolism of dairy cows

The present experiment was undertaken to determine the interactions between dietary supplements of folic acid and rumen-protected methionine on lactational performance and on indicators of folate metabolism during one lactation. Fifty-four multiparous Holstein cows were assigned to 9 blocks of 6 cows each according to their previous milk production. Within each block, 3 cows were fed a diet calculated to supply methionine as 1.75% metabolizable protein, equivalent to 70% of methionine requirement, whereas the 3 other cows were fed the same diet supplemented with 18 g of a rumen-protected methionine supplement. Within each diet, the cows received 0, 3, or 6 mg/d of folic acid per kg of body weight. Rumen-protected methionine increased milk total solid concentration but not yield. Supplementary folic acid increased crude protein and casein concentrations in milk of cows fed no supplementary methionine and the effect increased as lactation progressed; it also decreased milk lactose concentration. Folic acid supplements had the opposite effects on milk crude protein, casein, and lactose concentrations in cows fed rumen-protected methionine. Milk and milk component yields and dry matter intake were unchanged. Folic acid supplementation increased serum folates and this response was greater at 8 wk of lactation. It decreased serum cysteine in cows fed rumen-protected methionine, whereas it had no effect in cows fed no supplementary methionine. The highest serum concentrations of cysteine but the lowest of vitamin B(12) were observed at 8 wk of lactation. Serum clearance of folic acid following an i.v. injection of folic acid was slower at 8 wk of lactation. During this period, the high concentrations of serum folates and cysteine, the low serum concentrations of vitamin B(12) and methionine, and the slow serum clearance of folates strongly suggest that the vitamin B(12) supply was inadequate and interfered with folate use. It could explain the limited lactational response to supplementary folic acid observed in the present experiment.

Milk Protein Synthesis as a Function of Amino Acid Supply

Most prediction schemes of milk protein secretion overestimate milk protein yield from dairy cows at high protein intakes, thereby overestimating milk protein yield response to protein supplementation. This study was conducted to determine factors contributing to such an overestimation. Using published studies, a database was constructed that was limited to amino acid (AA) infusion studies, as then only the digestible amino acid of dietary origin needed to be estimated, whereas the amount infused was known exactly, thereby reducing the dependence on estimated values. Although milk protein yield was positively related with total energy supply, and both digestible duodenal supply and infused AA, in this database there was no relationship between milk protein yield response above control treatments and the nutrient status of the cows (energy or protein). Total milk protein yield was defined as a function of individual AA supply, using a segmented-linear and a logistic model to obtain estimates of the efficiency of conversion of AA into milk protein. Except for Lys and Met supply, the segmented-linear model yielded lower root mean square error and better correlation, but both models were similar in their reliability. For both models, the estimated efficiency of conversion of AA to milk differed among AA. Estimations of the ideal profile of AA for lactating dairy cows were similar between models, with requirements for Lys and Met in line with 2001 National Research Council recommendations. The major difference is that the segmented-linear model yields a constant efficiency of conversion of an AA until requirements are met, with zero efficiency beyond this point. The logistic model allows for an estimation of the decreasing marginal efficiency of conversion of AA as the supply approaches the requirements. The use of variable efficiency factors should improve our ability to predict protein yield in response to supplemental protein.

Lactational and systemic responses of dairy cows to postruminal infusions of increasing amounts of methionine

Five multiparous Holstein cows were used in a study with a 5 x 5 Latin square design to measure the effects of postruminal infusion of Met on lactational performance and plasma metabolites. The treatments were duodenal infusions of 1) 10 g/d of Lys (control), 2) 10 g/d of Lys plus 6 g/d of Met, 3) 10 g/d of Lys pus 12 g/d of Met, 4) 10 g/d of Lys plus 18 g/d of Met, and 5) 10 g/d of Lys plus 24 g/d of Met. The cows were fed a diet of 61% maize silage, 31% concentrate, and 5% dehydrated alfalfa. The DMI were similar among treatments. Milk yield, 4% FCM, and milk fat yield and content were not affected by infusions. In contrast, milk protein yield and content were increased linearly as Met infusion increased, which was true also for plasma Met and Cys concentrations. Using measurements of AA flow to the duodenum and assumed intestinal digestibilities of 0.8 for digesta and 1.0 for infused AA, estimated concentrations of Lys and Met in total AA absorbed in the small intestine were 7.3% for Lys and 1.52, 1.73, 1.94, 2.15, and 2.36% for Met for diets 1 through 5, respectively. The substantial linear increases in milk protein yield and content indicated that postruminal Met supply was not adequate over the entire range of Met infusions. In conclusion, the extent of Met limitation in this study could be defined only as that exceeding 2.4% of total AA absorbed in the small intestine.