The effect of rumen available protein amino acids and carbohydrates on microbial protein synthesis, amino acid flow and performance of high yielding dairy cows was studied. A significant relationship between the effective degradabilities of OM in feedstuffs and the in vivo ruminal OM degradation of diets of dairy cows was found. The in situ method enabled the prediction of ruminal nutrients degradability response to processing of energy and nitragenous supplements. The AA profile of the rumen undegradable protein was modified by the processing method. In a continuous culture study total N and postruminal AA flows, and bacterial efficiency, is maximal at rumen degradable levels of 65% of the CP. Responses to rumen degradable non carbohydrate (NSC) were linear up to at least 27% of DM. Higher CP flow in the abomasum was found for cows fed high ruminally degradable OM and low ruminally degradable CP diet. It appeared that in dairy cows diets, the ratio of rumen degradable OM to rumenally degradable CP should be at least 5:1 in order to maximize postruminal CP flow. The efficiency of microbial CP synthesis was higher for diets supplemented with 33% of rumen undegradable protein, with greater amounts of bacterial AA reaching the abomasum. Increase in ruminal carbohydrate availability by using high moisture corn increased proportions of propionate, postruminal nutrients flow, postruminal starch digestibility, ruminal availability of NSC, uptake of energy substrates by the mammory gland. These modifications resulted with improvement in the utilization of nonessential AA for milk protein synthesis, in higher milk protein yield. Higher postruminal NSC digestibility and higher efficiency of milk protein production were recorded in cows fed extruded corn. Increasing feeding frequency increased flow of N from the rumen to the blood, reduced diurnal variation in ruminal and ammonia, and of plasma urea and improved postruminal NSC and CIP digestibility and total tract digestibilities. Milk and constituent yield increased with more frequent feeding. In a study performed in a commercial dairy herd, changes in energy and nitrogenous substrates level suggested that increasing feeding frequency may improve dietary nitrogen utilization and may shift metabolism toward more glucogenesis. It was concluded that efficiency of milk protein yield in high producing cows might be improved by an optimization of ruminal and post-ruminal supplies of energy and nitrogenous substrates. Such an optimization can be achieved by processing of energy and nitrogenous feedstuffs, and by increasing feeding frequency. In situ data may provide means for elucidation of the optimal processing conditions.
The effects of L-type amino acid transporter 1 on milk protein synthesis in mammary glands of dairy cows
