INFLUENCE OF SUPPLEMENTAL PROTEIN-QUALITY ON RUMEN FERMENTATION, RUMEN MICROBIAL YIELD, FORESTOMACH DIGESTION, AND INTESTINAL AMINO-ACID FLOW IN LATE LACTATION HOLSTEIN COWS

The quality of dietary protein for dairy cows is generally assessed ba sed upon the proportion of it which is expected to degrade in the rume n (DIP) vs. that which is expected to escape the rumen undegraded (UIP ). However characteristics of dietary proteins can impact the nutritio nal value of the DIP and UIP. Four lactating Holstein cows were fed a low protein (9.8% CP of DM) ration of 28.6% timothy silage, 27.2% whol e crop barley silage, and 44.2% grain-based concentrate (DM basis). Th is was supplemented with 1.3 kg d(-1) of ground barley with either no additional protein supplement, 1.11 kg d(-1) of soybean meal, 0.67 kg d(-1) of blood meal or 0.20 kg d(-1) of urea. All cows were judged to have underperformed relative to expectations, with performance limited by supplies of DIP and/or UIP with all diets. Soybean meal supplement ation substantially improved animal performance and measured parameter s of N metabolism are consistent with a hypothesis that rumen bacteria l growth and outflow, rumen escape of intact dietary protein, and the ability of animal metabolic processes to utilize absorbed energy yield ing nutrients for productive purposes were all enhanced. Urea and bloo d meal supplementation both improved animal performance to a similar e xtent; although to a lesser degree than soybean meal. Measured paramet ers of N metabolism suggest that urea stimulated rumen bacterial growt h and outflow leading to increased intestinal delivery of protein wher eas blood meal supplementation enhanced intestinal delivery of protein due to its very high proportional escape from the forestomachs.