Efficacy of formic acid, enzymes, and microbial additives in silage on the performance of sheep: Systematic review and meta-analysis

This study aimed to carry out a systematic review and meta-analysis to evaluate the efficacy of chemical, enzymatic, and microbial additives on the silage quality and performance of sheep. After article selection using the Cochrane Library, Embase, Web of Science, LILACS and MEDLINE-PubMed databases and a manual search of scientific journals and the references of articles selected in the systematic review, 4257 articles published through May 2021 were identified. Fifty-four randomized control trials eligible for review were selected. Data were grouped into subgroups by the additive type (Mho = homofermentative microbial inoculant; Mhe = heterofermentative microbial inoculant; MMi = microbial inoculant mixture; Fa = formic acid; En = enzymes). The meta-analysis was performed using the “meta” package of R software. The data were analyzed using the random effects model and the raw mean difference with a 95% confidence interval (p < 0.05). Fa use reduced (p < 0.05) the silage crude protein digestibility (CPD) (−20.7 g/kg DM), and the ammonia nitrogen (NH₃–N) (−53.6 g/kg total N), and acetic acid (−5.06 g/kg DM) content and pH (−0.325) and increased the animal nitrogen retention (1.30 g/day), and the dry matter (DM) (14.1 g/kg NM), crude protein (CP) (20.4 g/kg DM) and water-soluble carbohydrate (WSC) (11.5 g/kg DM) content. En use reduced (p < 0.05) the silage neutral detergent fiber digestibility (NDFD) (−33.1 g/kg DM), CP (−10.8 g/kg DM), neutral detergent fiber (NDF) (−49.1 g/kg DM), NH₃–N (−25.5 g/kg total N) and acetic acid (−4.43 g/kg DM) content. Mho use increased (p < 0.05) the dry matter intake (DMI) (95 g/day), dry matter digestibility (DMD) (28.8 g/kg DM), CPD (26.6 g/kg DM), WSC (2.55 g/kg DM), nitrogen intake (2.23 g/day), retained nitrogen (2.19 g/day) and lactic acid (10.4 g/kg DM) content and reduced the mold count (−1.01 log₁₀ CFU/g), NH₃–N (−13.4 g/kg total N), acetic acid (−3.31 g/kg DM) content and aerobic stability (−33.8 h) of the silage. Mhe use increased (p < 0.05) the DMI (78.6 g/day), carcass yield (0.906%), DM (9.96 g/kg NM), aerobic stability (23 h) and acetic acid (19.9 g/kg DM) content and reduced the NDF (−20 g/kg DM) and ADF (−16.2 g/kg DM), yeast (−0.852 log₁₀ CFU/g) and mold (−0.469 log₁₀ CFU/g) counts. MMi use increased (p < 0.05) the average daily gain (17 g/day), DM (13.4 g/kg NM) and yeast (0.930 log₁₀ CFU/g) count and reduced the silage pH (−0.037). All additive types improved the silage quality, but only MMi improved weight gain in sheep. Mhe increased silage aerobic stability and can be used for this purpose on commercial farms.