Effects of Replacing Alfalfa with Sesbania cannabina Hay on Growth Performance, Rumen Fermentation, and Rumen Microecology in Fattening Lambs.

This study evaluated the effects of replacing alfalfa (Medicago sativa) with Sesbania cannabina hay on growth performance, nutrient digestibility, and ruminal microecology in fattening lambs. Forty 3-month-old Aohu crossbred lambs (initial BW: 22.08 ± 0.31 kg) received five isoenergetic, isonitrogenous diets in which S. cannabina hay replaced 0% (CON), 25%, 50%, 75%, or 100% of alfalfa during a 60-day trial. Growth traits, nutrient digestibility, rumen fermentation, enzyme activities, serum biochemical parameters, ruminal microbiota, and metabolomic profiles were evaluated. Substitution did not affect final body weight, average daily gain, or feed conversion ratio (p > 0.05), but nutrient utilization and ruminal fermentation differed among replacement treatments. The 50-100% replacement groups showed higher digestibility of crude protein, ether extract, and fiber, together with higher ruminal protease, cellulase, hemicellulase, and α-amylase activities (p < 0.05). Total volatile fatty acid concentrations were higher in the 50-100% replacement groups, whereas the acetate-to-propionate ratio was lower in all substituted groups. Serum biochemical parameters indicated changes related to protein metabolism without evidence of adverse health effects. Multi-omics analyses revealed distinct differences in ruminal bacterial communities and metabolic profiles among replacement treatments. The 25% replacement group showed relatively limited microbial and metabolic changes. The 75% replacement group exhibited greater bacterial diversity, enrichment of Prevotellaceae_NK3B31_group and Streptococcus, coordinated microbe-metabolite associations, and changes in bile acid and cholesterol metabolism. Complete substitution was associated with more extensive microbial remodeling and lower abundances of several functional metabolites, without additional benefits to growth performance.