Association of Ruminal Volatile Fatty Acids with Average Daily Gain in Growing Sheep and Goats: A Systematic Review and Meta-Regression Analysis

This study assessed how ruminal volatile fatty acid (VFA) biomarkers—acetic acid, propionic acid, the acetate-to-propionate ratio (A:P), and total VFA—relate to average daily gain (ADG) in growing sheep and goats, and their added predictive value over covariates. A systematic search identified 84 unique studies (290 treatment groups; complete data: 82 studies, 283 groups). Two analytic strategies were used: inverse-variance-weighted multilevel meta-regression and an unweighted mixed model, adjusting for dry matter intake, initial body weight, crude protein, neutral detergent fiber, and species. Under inverse-variance weighting with cluster-robust inference, acetic acid (β = 0.00088; robust p = 0.003), propionic acid (β = 0.00188; robust p < 0.001), and total VFA (β = 0.00088; robust p < 0.001) were positively related to ADG, whereas A:P was not significant (robust p = 0.074) and behaved inconsistently (p = 0.015). The mixed linear framework gave consistent results (marginal R2 = 0.610–0.642). Leave-one-out cross-validation showed propionic acid gave the largest predictive gain (overall ΔCV-R2 = 0.124, 95% CI 0.012–0.326; RMSE reduced 10.84%). In conclusion, acetic acid, propionic acid, and total VFA are robustly associated with ADG, but their incremental explanatory power is modest; propionic acid offers the largest cross-study predictive increment, whereas A:P is unreliable.