Ruminant Nutrition: Protein, Energy and By-Products Supplementation II

To avoid nutrient limitations, MP requirements are often overestimated in the formulation of dairy cow rations. Overfeeding of N results in increased expenses for producers and increased N excretion to the environment. Thus, improvements in the efficiency of dairy cow feeding provide economic and environmental incentives. Previous improvements in feed efficiency have come via the dilution of maintenance through increased production. To achieve further improvements, it is necessary to explore approaches such as supplementation of limiting AA’s in rumen protected (RP) form to diets low in MP. The goal of this study was to better illustrate the mechanism of lactation response to RPAA, and to evaluate the effect of RPAA products on milk yield. Four diets were fed to 36 multiparous Holstein dairy cows in a 4 × 4 Latin square design. Diets were (1) positive control diet with 17.1%CP (+Con), (2) negative control diet containing 15.4%CP (−Con), (3) −Con diet supplemented with 20 g/d/cow of RP methionine (RPM), and (4) −Con diet supplemented with 20 g/d/cow of RP methionine and 60 g/d/cow of RP lysine (RP M/L). Data were analyzed using the MIXED procedure in SAS 9.2. Milk yield was not significantly different between groups, suggesting that protein was not sufficiently limiting to affect production. Concentrations of MUN from cows fed –Con, RPM and RP M/L diets were significantly lower than that of cows fed +Con diet (P < 0.0001), suggesting improved efficiency of N metabolism in cows fed diets low in MP. The 2001 NRC formulation software was used to formulate diets. The predicted metabolizable protein balance of the –Con diet was −262 g/d, and predicted a 17.5% decrease in MP Allowable Milk compared with the +Con. This inaccuracy illustrates the need for improvement in dairy nutrition models. Model inaccuracies contribute greatly to inefficiencies in dairy cow feeding, and must be evaluated in the effort to improve dairy cow feed efficiency. Such improvements could act synergistically with RPAA supplementation to more closely match nutrient composition in feed to nutrient requirements of dairy cows.