Partial replacement of soybean meal with alfalfa meal in reduced-protein diets for finishing pigs: effects on muscle nutrient composition, nitrogen utilization, and gut microbiota

This study evaluated a dietary reformulation involving partial replacement of soybean meal with 15% alfalfa meal and a moderate reduction in dietary crude protein (CP), focusing on muscle nutrient composition, nutrient digestibility, nitrogen (N) excretion, and gut microbiota in finishing pigs during an 80-day feeding trial. A total of 225 barrows (Duroc × Landrace × Large White, initial body weight 55.7 ± 0.05 kg) were assigned to three diets: a control diet (CK, 14.5% CP), an alfalfa meal diet (AM, 14.5% CP with 15% alfalfa meal) and a low-protein alfalfa meal diet (AMLP-I, 12.5% CP with 15% alfalfa meal). Samples of muscle, feces and colonic contents were collected for subsequent analyses. All diets were balanced for lysine, methionine, threonine and tryptophan. Compared with CK, both the AM and AMLP-I groups showed higher muscle methionine levels, while tryptophan was higher in AMLP-I than in both CK and AM ( P < 0.05). Total and essential amino acid contents were unaffected. Both the AM and AMLP-I groups showed higher muscle polyunsaturated fatty acid (PUFA) levels, particularly C18:2n6 and C18:3n3, than the CK group ( P < 0.05). Crude fat digestibility was higher in both AM and AMLP-I groups than in CK, whereas phosphorus digestibility was higher in AMLP-I than in CK ( P < 0.05). Fecal, urinary and total N excretions were lower in both AM and AMLP-I groups, with the lowest values observed in AMLP-I ( P < 0.05). N retention as a percentage of intake was higher in AMLP-I than in CK and AM ( P < 0.05). Colonic acetate concentrations were higher in both AM and AMLP-I than in CK ( P < 0.05), whereas total short-chain fatty acid (SCFA) concentrations were higher in AMLP-I than in CK pigs ( P < 0.05). The AM and AMLP-I groups also showed higher microbial α-diversity and altered colonic microbial composition, with higher relative abundances of UCG-005 , unclassified_f_Lachnospiraceae and Christensenellaceae_R-7_group observed in AM than in CK ( P < 0.05). Christensenellaceae_R-7_group abundance was positively correlated with PUFA and negatively correlated with several saturated fatty acids (SFAs) and the n-6/n-3 ratio ( P < 0.05). UCG-005 was negatively associated with the protein fermentation-related metabolites isobutyrate and valerate ( P < 0.05). The dietary reformulation containing 15% alfalfa meal was associated with lower N excretion, higher crude fat digestibility, increased muscle PUFA levels, and altered colonic microbial composition. In the AMLP-I formulation, soybean meal inclusion was reduced by 67.3% relative to CK (14.2% to 4.65% of the diet), together with a 2-percentage-point reduction in dietary CP. Despite the lower N intake, N deposition was maintained, while total N excretion was reduced by approximately 25.0% and N retention (% of intake) was higher in the AMLP-I group. These findings support the potential of dietary reformulation incorporating alfalfa meal as part of a sustainable low-protein feeding strategy for pig production, although the observed responses should be interpreted as the combined effects of the overall formulation changes.