Effects of Selenium Supplementation on Growth Performance, Antioxidant and Metabolic Status in Fattening Lambs: A Systematic Review and Meta-Analysis

Selenium (Se) is an essential trace element that supports cell membrane integrity and regulates antioxidant activity. This systematic review and meta-analysis aimed to analyze the overall effects of Se supplementation on productive performance, antioxidant status, metabolic profile, and methane emissions in lambs. Five independent researchers conducted the literature search, three extracted the data, and two cross-checked the database. Effect sizes were estimated using standardized mean difference (SMD) and raw mean difference (RMD), and heterogeneity was quantified with I² statistic. For variables with substantial heterogeneity (I² > 50%), meta-regression models were applied considering breed, Se source (organic, inorganic, yeast, and nano), dose, body weight, and age. Results, from a total of 28 studies included, show that Se improves average daily gain (RMD = + 0.02 kg/d; p = 0.0001) and reduces feed conversion ratio (RMD = -0.88 kg/kg; p = 0.0001). Average daily gain heterogeneity (I² = 56.4%) was mainly attributed to the Se source, with Se nanoparticles being the best source across most outcomes. No significant effects were observed for dry matter intake (RMD = + 0.004 kg/d; p = 0.71) and blood glucose concentration (RMD = + 1.21 mg/dL; p = 0.56). Glutathione peroxidase activity increased with Se supplementation (SMD = + 1.44; p = 0.0001), while methane emissions remained unaffected (SMD = + 0.19; p = 0.53). Overall, dietary Se supplementation enhances growth efficiency and antioxidant status in lambs, however, the magnitude of responses is modulated by Se source, production level, age, and basal diet composition, reflecting the complex interaction between animals and supplementation strategies.