J. Dairy Sci. 86:958–969 American Dairy Science Association, 2003. Supplementation of Methionine and Selection of Highly Digestible Rumen Undegradable Protein to Improve Nitrogen Efficiency

Metabolizable protein (MP) supply and amino acid bal-ance were manipulated through selection of highly di-gestible rumen-undegradable protein (RUP) sources and methionine (Met) supplementation. Effects on produc-tion efficiency and N utilization of lactating dairy cows were determined. Thirty-two multiparous (647 kg) and 28 primiparous (550 kg) Holstein cows were assigned during the fourth week of lactation to one of four dietary treatments. Treatments were 1) 18.3 % crude protein (CP) with low estimated intestinal digestibility of RUP (HiCP-LoDRUP), 2) 18.3 % CP with high digestibility RUP (HiCP-HiDRUP), 3) 16.9 % CP with high digestibil-ity RUP (LoCP-HiDRUP), and 4) 17.0 % CP with high digestibility RUP and supplemental Met (LoCP