Effect of precision feeding high- and low-quality forage with different rumen protein degradability levels on nutrient utilization by dairy heifers.

One approach to improving efficiency of protein and nitrogen use for optimal productivity in dairy heifers is to provide adequate levels of protein with differing rumen degradabilities. The objective of this experiment was to determine effects of manipulating RDP in diets of dairy heifers precision fed low- and high-quality forage. Eight rumen-cannulated Holstein heifers (448 ± 19 kg BW) were randomly used and assigned to an 85% forage diet with 2 forage qualities, low quality (LQ, 48% NDF) and high quality (HQ, 38% NDF), and to 4 RDP levels (62%, 68%, 74%, and 81% of CP as RDP) within forage quality administered according to a split-plot two 4 × 4 Latin square design (20-d periods). Similar N intake was provided (1.70 g N/kg BW0.75), and alterations of RDP levels were made by exchanging canola meal with heat-treated soy protein with slow-release urea. Heifers offered the HQ diet had greater apparent total tract DM and OM digestibilities than those fed the LQ diet. Apparent N digestibility tended to be higher for the HQ diet but was not affected by the levels of RDP in diets. No difference was observed for rumen VFA concentration between forage treatments, but acetate to propionate ratio was higher for LQ-fed heifers. No difference in rumen carbohydrate fermentation was detected with increasing levels of RDP. Feeding HQ forage improved DM, OM, and N apparent digestibility. Increasing RDP in LQ and HQ had no effects on apparent digestibility of any measured parameter or N dynamics in dairy heifers.