Abstract
The objectives were to evaluate the effects of fiber source and dietary starch level on growth performance, nutrient digestion, rumen parameters, and rumen bacteria in fattening Hu lambs. A total of 360 Hu lambs (BW = 24.72 ± 0.14 kg, 2 months old) were subjected to a 2 × 3 factorial arrangement. Lambs randomly assigned 6 treatments with 6 repetitions (10 lambs per repetition) of each treatment. Six treatments were formulated to include the fiber sources with three starch levels. The experiment lasted a 63 d. The amount of feed, orts, and total feces were sampled on the 42nd day of the experiment. Rumen fluid samples were collected after 2 h of morning feeding on day 56. Rumen contents were collected last day after the selected lambs were slaughtered. Increasing the starch content decreased the digestibility of neutral detergent fiber (NDF, P = 0.005). Increasing the starch level increased the proportions of propionate (P = 0.002) and valerate (P = 0.001) and decreased the proportion of acetate (P < 0.001) and the ratio of acetate to propionate (P = 0.005). The abundance of Fibrobacter succinogenes was affected by an interaction between the fiber source and the starch level (P < 0.001). Fibrobacter succinogenes tended to be greater in lambs fed SH than in lambs fed BP (P = 0.091), which was greater in lambs fed high starch levels than in lambs fed low starch levels (P = 0.014). Increasing the starch level increased Streptococcus bovis abundance (P = 0.029) and decreased total bacteria (P = 0.025). At the genus level, increasing the starch level reduced the abundance of Butyrivibrio_2 (P = 0.020). Nevertheless, the final body weight (BW) and acid detergent fiber (ADF) digestibility were greater (P < 0.01) in lambs fed soybean hull (SH) than in lambs fed BP. The proportion of butyrate was greater (P = 0.005), while the rumen pH was lower (P = 0.001) in lambs fed beet pulp (BP) than in those fed SH. The abundances of Succiniclasticum, Candidatus_Saccharimonas, Ruminococcus_1, and Christensenellaceae_R-7 were greater in lambs fed SH than in those fed BP (P < 0.050), whereas the abundance of Fibrobacter was lower (P = 0.011). The predominant microbial phyla in all of the groups were Firmicutes, Bacteroidetes, and Fibrobacteres. Changing the starch level for fiber sources mainly changed the rumen community in terms of the phylum and genus abundances. Lambs fed SH with low starch level increased the final BW without affecting total volatile fatty acids (TVFA) concentrations.