The objective of this trial was to investigate the effects of different dietary crude protein levels (10.5, 12, 13.5, and 15%) on ruminal pH, ruminal concentration of ammonia nitrogen, ruminal passage rate, nitrogen balance, and microbial protein synthesis in Holstein x Zebu steers. Diets contained 65% of Tifton 85 bermudagrass hay and 35% of concentrate. Four castrated steers averaging 487.3 kg of initial body weight and fitted with ruminal and abomasal cannulas were randomly assigned to a 4 x 4 Latin square design. Each experimental period lasted 20 days with 10 days for diet adaptation and 10 days for sample collection. Ruminal fluid was collected before (0 h) and at 2, 4, 6 and 8 h after feeding for determination of both ruminal pH and ammonia-N. Ruminal passage rate was determined according to a unicompartmental model using chromic oxide as the external marker while abomasal purine bases (RNA) were used as the internal markers for measuring microbial protein synthesis. Ruminal pH and ammonia-N concentration were affected by sampling time with measured maximum values of 6.54 and 17.43 mg/dL at 3.62 and 4.17 h after feeding, respectively. Digesta passage rate did not differ significantly averaging 3.69 %/h across diets. Similarly, abomasal flow of N compounds as well as microbial efficiency were not affected by different dietary crude protein levels while nitrogen balance expressed in g/day increased linearly when the crude protein content increased from 10.5% to 15% in the diet.
microbial efficiency; ammonia nitrogen; ruminal pH; passage rate
