Functional Feed Additives in Intensive Sheep Production: From Mechanism to Practical Application

In ruminant nutrition, functional feed additives are defined as non-nutritive or bioactive substances added to diets to intentionally alter rumen microbial populations, enhance nutrient digestibility, or modulate metabolic pathways beyond standard baseline nutrition. This review evaluates four additive classes used to address the feed-efficiency gap in sheep: gluconeogenic precursors, β-adrenergic agonists (β-AAs), exogenous enzymes, and phytochemicals. We place specific emphasis on their comparative productive performance, carcass and meat quality, economic feasibility, environmental impact, and regulatory status. Among the compounds reviewed, calcium propionate and zilpaterol hydrochloride yield the largest increases in growth rate and lean yield, though these effects remain breed-, diet-, and dose-dependent. Exogenous enzymes and phytochemicals offer smaller performance gains but generally face fewer regulatory restrictions; notably, phytochemicals reduce enteric methane emissions by 10–25% and improve meat oxidative stability, aligning well with modern sustainability goals. While combination strategies across additive classes show mechanistic promise, they remain empirically under-tested. To guide practical adoption, this review provides a cross-additive synthesis alongside qualitative cost–return profiles. Addressing critical research gaps—specifically multi-additive interactions, breed-specific validation, and the development of on-farm tools from precision-nutrition biomarkers—will determine the long-term viability of these strategies under growing market and environmental pressures.