Effect of Crude Protein and Ruminal Ammonia-N on Digestibility and Ruminal Outflow in Beef Cattle Fed Wheat Straw

The effect of dietary crude protein (CP) level and time of supplementation on ruminal dry matter disappearance of wheat straw (WS) diets was evaluated using beef cows fitted with ruminal and duodenal canulae. The dietary treatments were 8% (8×1) and 10% (10×1) CP with supplemental protein fed once daily and 10% (10×2) CP with supplemental protein fed twice daily. Wheat straw fed once daily was 82% of the diet. Using indigestible acid detergent fiber as the indicator, the percentage dry matter fermented in the rumen was 35.8, 37.0 and 44.5%, respectively, and was affected by diet (8×1 < 10×1, P<.05; 10×1 < 10×2, P<.001). Ruminal N digestibility was −22.5, −12.3 and 16.8% for diets 8×1, 10×1 and 10×2, respectively. Diets 10×1 and 10×2 differed (P<.05). Ruminal ammonia-N (NH3-N) measured every 3 h for 24 h was extremely low with all diets. Daily mean values differed (P<.05) and were .6, 3.4 and 1.8 mg/dl, respectively. A more uniform level of NH3-N was achieved when supplement was fed twice a day. Protein supplement fed in two equal portions 12 h apart (10×2) increased ruminal dry matter fermentation. In another experiment the effect of incremental levels of CP and supplemental energy on digestibility and microbial activity in beef cattle fed WS diets was evaluated in four cows fitted with ruminal and duodenal canulae. Three levels of dietary CP, 8, 10 and 12%, were compared. Two 12% CP diets were fed with supplement fed at 12 (low energy, 12a) or 18% (high energy, 12b) of dry matter intake. Treatments were continued after an initial 15-d period and 30 g urea was added directly to the rumen in two equal portions at 8 and 16 h after feeding to determine whether increased levels of NH3-N would stimulate microbial activity. There were no dietary or urea treatment effects on ruminal dry matter digestibility or outflow of nonammonia-N and microbial protein-N. Total tract apparent dry matter digestibility was improved (P<.01) when dietary CP content was increased from 10 to 12%. Mean dry matter digestibilities were 49.3, 49.7, 53.2 and 54.1% for diets 8, 10, 12a and 12b, respectively. Apparent ruminal-N digestion for diets without additional urea was −52.7, −19.8, −9.7 and −19.4%, respectively. Addition of urea to the rumen tended to improve N digestion (P<.06), with values of −48.1, −18.1, 3.5 and 5.1%, respectively. Ruminal NH3-N increased (P<.001) when dietary CP increased. In spite of low levels of ruminal NH3-N associated with the 8 and 10% CP diets, increasing ruminal NH3-N levels with urea did not improve dry matter digestibility or microbial protein production.