Impact of dietary rumen degradable protein level on growth performance, rumen fermentation, and nitrogen utilization...

It is critical to supply adequate rumen degradable protein (RDP) to maintain ruminal fermentation of carbohydrates and microbial protein synthesis in sheep. Our objective was to investigate the effect of dietary RDP content on growth performance, rumen fermentation, and nitrogen utilization in growing Hu sheep. The study was conducted for 11 wk (i.e., 2-wk adaptation period and 9-wk experimental period) with 44 3-month-old intact male Hu sheep averaging body weight (BW) of 22.7 ± 3.32 kg at the beginning of the trial. Following the adaptation period, sheep were blocked by BW and randomly assigned to 1 of 4 experimental diets. Dietary RDP concentrations of 11.0%, 10.0%, 9.0%, and 8.0% were achieved by replacing solvent soybean meal and wheat middlings with coarsely cracked corn and extruded soybean meal, and experimental diets were fed as pelleted total mixed rations. Treatment had no effect on BW, average daily gain, dry matter intake, feed conversion ratio, or income over feed cost. When decreasing dietary RDP, body height decreased (P = 0.017) linearly, chest depth tended (P = 0.079) to reduce linearly, and chest width responded (P = 0.008) quadratically. There were (P ≤ 0.018) quadratic effects of decreasing RDP concentration for ruminal concentrations of total volatile fatty acids (VFA), acetate, butyrate, and branched-chain VFA (i.e., isobutyrate, valerate, and isovalerate), with maximal values at 10.0% RDP. Apparent total-tract digestibilities of crude protein and acid detergent fiber decreased (P ≤ 0.001) and that of starch increased (P = 0.003) linearly as dietary RDP concentration decreased from 11.0% to 8.0%. Decreasing RDP tended (P = 0.051) to have a quadratic effect on NDF digestibility. Urinary urea-nitrogen and total-nitrogen excretion showed (P < 0.001) linear decreases when dietary RDP concentration declined. There was (P = 0.027) a cubic effect of decreasing RDP on fecal nitrogen output. The ratio of retained nitrogen to nitrogen intake was maximized at 10.0% RDP (quadratic effect, P = 0.016). In summary, even though reducing dietary RDP level from 11.0% to 8.0% did not modify growth performance and linearly reduced urinary N output, ruminal fermentation, apparent total-tract digestibility of NDF, and nitrogen utilization were optimal when feeding the 10.0% RDP diet.