Effects of dietary crude protein level on rumen microbial composition and the fermentation parameters of finishing Hu lambs

The objective of this study was to evaluate the effects of dietary CP levels on the rumen and its contents, pH, and microbial and volatile fatty acid (VFA) concentrations in finishing Hu lambs. A total of 54 three month old male Hu lambs (32.25 ± 2.50 kg) were randomly divided into three groups based on their weights. The dietary CP levels were 11.5%, 14%, and 16.5%, respectively. Each group contained 18 replicates, and a replicate consisted of one lamb. The pre-test period was 7 days, and the formal experiment lasted 63 days. A total of 10 lambs were selected for slaughter in each group at the end of the test period. Then, the slaughter performance was measured and the rumen contents were collected so that the rumen microbial DNA could be extracted. The results showed that 1) the different CP treatments had no significant effect on live weight before slaughter, carcass weight, slaughter rate, the total weight of the rumen, rumen tissue weight, and tail fat weight; 2) the ammonia nitrogen in the rumen fluid of the 11.5% CP group was significantly lower than in the rumen fluid of the 14.0% CP and 16.5% CP groups (P < 0.001), but there was no significant difference between the 14.0% CP group and the 16.5% CP group ( P > 0.05). Rumen pH increased as dietary protein levels rose ( P < 0.1). 3) The TVFA concentration, and the acetate, propionate, isobutyrate, butyrate, and valerate ratios were not affected by the CP level ( P > 0.05). However, the isovalerate ratio tended to increase ( P = 0.098) as the CP level in the diet rose. 4) Butyrivibrio fibrisolvens levels in the rumens from the different treatment groups were higher than the other bacteria, but there were no significant differences among the treatment groups (P > 0.05). The Ruminococcus albus, Fibrobacter succinogenes, and Prevotella brevis contents were not affected by CP level (P > 0.05), whereas the Ruminococcus flavefaciens levels were significantly lower in the 11.5% CP group than in the 14.0% CP group (P < 0.05). The 16.5% CP group was not significantly different from 11.5% CP and 14.0% CP groups. These results indicated that the different CP levels did not affect the slaughter performance of the Hu lambs, but could affect the pH and ammonia nitrogen content of the rumen fluid. Furthermore, the isovalerate proportion increased as the CP levels rose, and the Ruminococcus flavefaciens levels were highest in the 14.0% CP group.