Effects of Soybean Meal Replacement on Growth Performance, Rumen Fermentation, Rumen Microorganisms, and Metabolites in Dumont Lambs

This study investigated the effects of replacing part of the soybean meal in the diet of Dumont lambs with urea, rapeseed meal, and cottonseed meal on their growth performance and rumen fermentation and combined rumen microbial metagenomics and metabolomics to explain the reasons for the changes in phenotypic data. Twenty-four healthy male Dumont lambs were divided into four groups: soybean meal group (T1, control group), group with 1.5% urea replacing 6.4% soybean meal (T2), group with 1% urea replacing 4.3% soybean meal (T3), and group with 1% urea + 6.6% cottonseed meal +5% rapeseed meal replacing all soybean meal (19%) (T4), following the principle of equal energy and nitrogen. Urea, rapeseed meal, and cottonseed meal have different degradation rates in the rumen, primarily stimulating arginine biosynthesis, sulphur metabolism, and carbon fixation in photosynthetic organisms through <i>Prevotella</i> genus mediation, thereby influencing the accumulation of metabolites such as 9,10-DiHOME, DG (PGJ2/a-15:0/0:0), isonicotinate and taxifolin, affecting rumen fermentation. Compared with the T1 group, the T2 group showed significantly increased ammonia nitrogen (NH₃-N) and microbial protein (MCP) content (<i>p</i> < 0.01) and improved fructose and mannose metabolic capacity (<i>p</i> < 0.05). The T3 group showed a significant increase in total volatile fatty acids (TVFA) and MCP content (<i>p</i> < 0.01), which facilitated the absorption of subsequent nutrients. In the T4 group, different degradation rates of nitrogen resources and rapeseed meal + cottonseed meal contained abundant and complementary amino acids, which improved rumen fermentation, enhanced rumen microbial and metabolite diversity, and optimized the synergistic metabolic efficiency of carbon, nitrogen and sulphur. However, the specific mechanisms of post-rumen metabolism and absorption require further investigation.