Nitrogen losses in urine and feces cause great economic cost as well as negative environmental problems. It is critical to find solution to minimize nitrogen loss. Presentations in the Ruminal Nutrition-Protein and Amino acids Session in the Wednesday morning mainly focus on dietary crude protein levels, supplementation of essential amino acids to low protein diet, and accessing bioavailability of supplemented amino acids.
Nitrogen use efficiency deceases linearly with increasing dietary CP level, as reported by Letelier et al. from the University of Wisconsin-Madison. They determined the effects of different dietary crude protein levels (13.5%, 15%, 16.5%, and 18%) on lactation performance, feed efficiency, and nitrogen use efficiency. The Feeding dietary crude protein higher than 16.5% of the dry matter did not improve cow performance in early lactation and penalized cow milk production in late lactation.
Low protein diet (15% CP) results in less milk in comparison with a reference diet (17% CP), shown by Vandaele et al. from ILVO at Melle in Belgium. Moreover, supplementation of extra rumen-protected (RP) amino acids (i.e., Lys and Met) to low protein diet increases milk production to an intermediate level and decreases nitrogen losses. Zang et al. from the University of New Hampshire showed that substitution of ground corn with soyhulls and RP-fat improved feed efficiency and milk fat yield but appeared to reduce N utilization.
Several methods are developed to measure the bioavailability of amino acids from diet or ruminal microbes. By using in situ nylon bag technique and a modified 3-step in vitro technique, Francia et al. from Univeristat Autonoma de Barcelona compared the bioavailability of three RP-Lys sources, highlighting the necessity to evaluate ruminal degradability and intestinal digestibility for evaluation of RP-Lys. Whitehouse et al. from the University of New Hampshire determined and differentiated the bioavailability of four RP-amino acids supplements using the plasma free amino acid dose-response technique. Moreover, Huang et al. from the Virginia Tech adopted a stable isotope-based approach to determine the availability of essential amino acids from microbial protein and RUP. This approach can avoid errors associated with estimates of ruminal microbial crude protein and RUP outflow, amino acids composition, and intestinal amino acids digestibility, but efforts are needed to validate the approach by comparing it to traditional in vivo methods.
Overall, the poor nitrogen efficiency in dairy cattle are gaining more attention. Further efforts are necessary to increase nitrogen efficiency and reduce nitrogen excretion to environment.
Bo Zhang is a PhD student in the Department of Animal Science at UC Davis. He earned his M.S. from the University of Chinese Academy of Sciences. Under the guidance of Dr. Timothy Hackmann, he is working on how rumen microbes ferment feed and produce protein digested by cattle.