Guanidinoacetic acid as a feed additive in ruminants: A systematic review and meta-analysis

Guanidinoacetic acid (GAA) is an emerging feed additive that has been studied in ruminant feeding systems since 2020. This systematic review and meta-analysis provide a quantitative summary of in vivo studies in ruminants investigating the effects of GAA supplementation on production performance, meat quality, nutrient metabolism, rumen fermentation, and blood biochemical indices. A total of 34 peer-reviewed studies published between 2020 and 2026 were selected according to the PRISMA guidelines, comprising 23 studies in cattle and 11 in lambs. The results using a meta-analytic approach indicated that the response to GAA supplementation varied among animal categories, with greater improvements in average daily gain (ADG) were found in bulls (+16.91%; 95% CI = 12.20–21.61%; P < 0.001) and lambs (+6.97%; 95% CI = 4.91–9.04%; P < 0.001) whereas no effects were observed in steers ( P = 0.731), gestating cattle ( P = 0.817), or lactating cattle ( P = 0.537). Feed conversion ratio was improved in bulls (-12.04%; 95% CI = −16.25 to −7.83%; P < 0.001) and lambs (-6.63%; 95% CI = −10.23 to −3.04%; P = 0.001). Carcass weight increased in lambs (+6.97%; 95% CI = 1.69–12.25%; P = 0.001) but was unaffected in steers ( P= 0.511) and bulls ( P = 0.879). Supplementation with GAA increased nutrient digestibility, total volatile fatty acid concentration, plasma total protein, albumin, creatine, creatinine, and circulating GAA, while glucose remained unaffected. Dose-response analysis showed that the standard inclusion level (0.06–0.12% DM) resulted in the greatest increase in ADG and feed efficiency. Meta-regression analysis further demonstrated a quadratic relationship between GAA level and growth response, with a significant quadratic relationship observed in cattle (R² = 0.235; P = 0.021), but not in lambs (R² = 0.145; P = 0.296). In conclusion, GAA supplementation has the potential to influence growth performance, nutrient utilization, and metabolic responses in ruminants in a context-dependent manner. More consistent positive responses were observed in bulls and lambs, whereas evidence in steers, gestating cattle, and lactating dairy cows remains comparatively limited or inconsistent for several outcomes. The magnitude and direction of response may depend on animal category, physiological status, dietary composition, supplementation strategy, ruminal metabolism, and methyl donor availability.