Malic Acid Supplementation on Rumen Fermentation, Nutrient Digestibility, Performance and Carcass Traits in Lambs: A Meta-Analysis and Meta-Regression Considering Dietary Moderators

This meta-analysis and meta-regression evaluated how malic acid supplementation modulates rumen fermentation and its consequences for growth performance, nutrient digestibility, and carcass traits in lambs. Effect sizes (ES) were estimated using a random-effects model. Dietary composition was explored by meta-regression as a key source of heterogeneity, and subgroup analyses were used to compare free malic acid (FMA) and malate. Ruminal pH was not affected by malic acid supplementation. In contrast, total volatile fatty acid concentration increased with malic acid supplementation, particularly in studies using FMA. No effects were detected for propionate concentration, whereas acetate concentration increased (ES = 0.502; p = 0.036). A tendency toward a reduced ruminal acetate proportion was observed (ES = −0.683; p = 0.072). Malic acid supplementation tended (p = 0.057) to increase body weight gain (BWG; ES = 0.325) and final body weight (FBW; ES = 0.234). Malic acid supplementation did not affect carcass traits or overall nutrient digestibility. Meta-regression consistently identified fiber intake-related variables as major moderators of the effects of malic acid. Overall, the effects of malic acid supplementation on lamb performance appear to be primarily driven by its modulation of rumen fermentation and strongly conditioned by dietary context.