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 based upon the proportion of it which is expected to degrade in the rumen (DIP) vs. that which is expected to escape the rumen undegraded (UIP). However characteristics of dietary proteins can impact the nutritional 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% whole crop barley silage, and 44.2% grain-based concentrate (DM basis). This 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 supplementation substantially improved animal performance and measured parameters of N metabolism are consistent with a hypothesis that rumen bacterial growth and outflow, rumen escape of intact dietary protein, and the ability of animal metabolic processes to utilize absorbed energy yielding nutrients for productive purposes were all enhanced. Urea and blood meal supplementation both improved animal performance to a similar extent; although to a lesser degree than soybean meal. Measured parameters of N metabolism suggest that urea stimulated rumen bacterial growth and outflow leading to increased intestinal delivery of protein whereas blood meal supplementation enhanced intestinal delivery of protein due to its very high proportional escape from the forestomachs. Key words: Duodenum, amino acids, protein, protozoa, phosphatidylcholine