Effect of source and level of protein supplementation on rice straw utilization by Brahman steers

Seven ruminally cannulated Brahman steers were used in a 7 × 4 incomplete block design to determine the effects of cottonseed meal (CSM; 43.9% CP, 82.9% RDP) or dried distillers’ grains (DDG; 27.5% CP, 43.6% RDP) supplementation on rice straw utilization (47 g/kg CP and 681 g/kg NDF). Treatments consisted of a negative control receiving no supplement (control) and 3 levels (60, 120, and 180 mg N/kg BW) of either CSM or DDG. Periods were 14 d with 8 d for adaptation and 6 d for data collection. Steers had ad libitum access to rice straw and were fed supplements daily. Increased supplementation resulted in a linear increase (P ≤ 0.06) in forage OM intake from 13.5 g/kg BW by controls to 15.5 and 16.1 g/kg BW for 180 mg N/kg BW of DDG and CSM, respectively. No differences between sources were observed (P = 0.84). Total digestible OM intake was increased by supplementation (linear, P < 0.01) from 6.9 g/kg BW (control) to 10.0 and 11.2 g/kg BW for 180 mg N/kg BW of CSM and DDG, respectively. A greater response was observed for DDG (P = 0.05) due to greater provision of supplement (g DM/d) to achieve isonitrogenous treatment levels. Total tract OM digestion tended to increase with DDG supplementation (linear, P = 0.08) but not CSM supplementation (P = 0.19). Both supplements did not affect NDF digestion (P > 0.40) or calculated forage NDF digestibility (P > 0.40). Ruminal ammonia concentrations peaked 4 h after supplementation/feeding with the greatest concentration (4.0 mM) observed for 180 mg N/kg BW of CSM and the lowest concentration at 4 h observed in the control (0.8 mM). Provision of CSM resulted in a linear increase (P < 0.01) in average ruminal ammonia, in contrast to the quadratic response (P = 0.02) observed with DDG supplementation. Total VFA production linearly increased for both CSM and DDG supplementation (P = 0.09 and P = 0.01, respectively). Protein supplements containing high and low levels of RDP were effective at improving intake and utilization of rice straw by Brahman steers.