No abstract

The effect of increased postruminal supply of lysine and methionine was investigated in a production trial involving 64 dairy cows in early lactation. Within each of two basal rations, based on either corn silage or grass silage, rations were either naturally deficient in lysine or fortified with 24 g of lysine in a rumen-protected form and naturally deficient in methionine or fortified with 12 g of methionine in a rumen-protected form. The data were analyzed separately for the four lysine and the four methionine treatment groups. Milk production, body weight gain, and plasma concentrations of insulin-like growth factor-I, bovine somatotropin, insulin, glucose, nonesterified fatty acids, and urea were monitored over a 12-wk period. Supplementation with protected methionine led to increases in milk fat and protein contents of 2.4 and 1.8 g/kg of milk, respectively. Supplementation with protected lysine or methionine numerically increased protein yield comparable to values reported in the literature, but the treatment effects were not statistically significant. Efficiency of utilization of absorbed amino acids for milk protein synthesis and efficiency of utilization of metabolizable energy for milk production were not significantly altered in response to increased postruminal lysine and methionine flow, but a numerically increased efficiency of utilization of total amino acids was observed. No significant effect of lysine or methionine supplementation was observed on endocrine parameters nor on plasma metabolite concentrations. However, across treatment groups, high milk yield was correlated with low plasma insulin-like growth factor-I concentrations (r = -0.44) and partially with low plasma nonesterified fatty acids concentration and insulin levels (r = -0.26), while body weight gain was negatively correlated (r = -0.33) with elevated plasma bovine somatotropin concentrations.

Microbial protein produced in the rumen is not sufficient to meet the requirement for amino acids of the high producing dairy cow. The microbial protein thus need to be supplemented with a certain amount of undegraded protein which match the microbial protein both quantitatively and qualitative ly to enable an efficient utilization for milk production. This paper summarizes findings with respect to amino acid composition and digestibility of microbial amino acids. The main emphasis is however on recent findings with respect to degradability and digestibility of individual amino acids in feed proteins. The results seem to indicate that degradability and digestibility estimates obtained for nitro gen may not always apply to individual amino acids. It is concluded that more research is needed to identify the ideal profile of the amino acids absorbed from the intestine of lactating cows. Results of such efforts are needed to compose an ideal protein supplement eventually by using protected amino acids to balance the microbial protein to meet the amino acid requirement of dairy cattle with a minimum of rumen undegraded protein.