Responses of Dairy Cows to Supplemental Highly Digestible Rumen Undegradable Protein and Rumen-Protected Forms of Methionine (Report‪)‬

INTRODUCTION Due to increasing societal pressure to improve the environmental performance of livestock operations, dairy research is currently being directed towards improving the efficiency of nitrogen (N) use of lactating cows to reduce losses of N (NRC, 2001). One of the first steps in diet formulation for lactating dairy cows is to provide rumen-degradable protein (RDP) to meet the requirements of rumen microorganisms. The total metabolizable protein (MP) requirement of the cow is met by supplementing rumen-undegradable protein (RUP) when microbial protein synthesis alone is insufficient to meet the MP requirements (Kalscheur et al., 2006). Efficiency of N utilization could be improved by increasing post-ruminal digestibility and/or providing a pattern of absorbed amino acids (AA) that more closely matches the AA requirements for milk synthesis (Noftsger and St-Pierre, 2003). Erasmus et al. (1993) found significant differences among feedstuffs in the digestibility of RUP and the AA profile of RUP. Metabolizable protein supply and AA balance can be manipulated through selection of highly digestible RUP sources and protected methionine (Met) supplementation (Noftsger and St-Pierre, 2003). 2-Hydroxy-4-(methylthio)-butanoic acid (HMB), is a common source of bypass Met (Schwab, 1998). The most consistent response to feeding HMB has been an increase in milk fat percentage (Huber et al., 1984; Koenig et al., 2002).