Effect of Ration Protein Content and Solubility on Milk Production of Primiparous Holstein Heifers

Abstract

Twenty-eight primiparous heifers were blocked at calving and assigned to four dietary treatments in a 2 .times. 2 factorial arrangement of two crude protein concentrations (15.4 and 20.7%) and two protein solubilities (22.8 and 53% of protein). Heat-treated soybeans replaced raw soybeans in low solubility rations. Rations were offered for the first 10 wk of lactation divided into two 5-wk periods. Feed was offered ad libitum in period 1 and restricted in period 2. Digestibility and N balance were measured at completion of the lactation study. Body weight change, DM intake, milk yield, milk protein percentage, rumen pH, and total VFA were not affected by treatments. Milk fat percentage was increased by higher CP content during period 1, resulting in a 16.2% increase in 4% FCM. Plasma urea N and rumen NH3 were elevated by higher CP content. Reducing protein solubility depressed rumen NH3. A protein .times. solubility interaction for the acetate to propionate ratio suggests that protein and solubilty combinations that optimzied plasma urea N and rumen NH3 provided a more favorable rumen environment and supported highest milk prouction. Nitrogen digestibility was influenced most by ration CP; however, lower solubility improved DM and N digestibilites. Both dietary CP content and solubility must be considered to maintain optimum rumen environment.