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.

Increased recycling of nitrogen to the forestomachs on low nitrogen rations was found. On high CP rations the AAN at duodenum was greater than or equal to the N intake. Apparently absorbed TN in the small intestine was increased on the SBM supplemented ration. No significant difference was found for absorbed AAN fractions.Key words: Nitrogen, cattle, metabolism, gastro-intestinal tract Results by Lambert et al. (1975) showed that increased nitrogen fertilization to grass, 0-400 kg N/ha, raised the amount of crude protein from 8.6 to 20.6% in DM and the amount of NPN from 0.8 to 5.2Vo, respectively. As a proportion of total CP the amount of NPN varies from 9 to 25Vo in fresh grasses (Lambert et al. 1975) For personal use only.