Forage-free diet in finishing cattle: Effects on performance, ruminal fermentation, and enteric methane mitigation

• Forage-free diet with plant-based additive improved feed intake and growth in finishing cattle. • Ruminal health was maintained despite lower pH and microbiota shifts. • Forage-free diet was associated with reduced estimated methane intensity. Ruminant production is largely pasture-based, and although this system is beneficial during the cow-calf and growing phases, it often becomes nutritionally limiting during finishing, delaying slaughter and increasing greenhouse gas emissions. High-grain diets accelerate weight gain and may reduce enteric methane (CH 4 ) emissions per unit of animal output, but they increase the risk of ruminal acidosis. Ionophores are effective in mitigating this disorder, yet their use is restricted in several countries, stimulating interest in sustainable alternatives such as plant-derived bioactive compounds. This study evaluated the effects of low-grain, high-grain, and forage-free diets supplemented with a functional additive (LM+), a blend of rumen-modulating secondary metabolites, on animal performance and ruminal fermentation under in vivo finishing conditions. Forty-five crossbred Girolando cattle were assigned to three diets for 147 days. Animal performance, ruminal pH, short-chain fatty acids, lactate concentration, and selected microbial groups were assessed in vivo . In addition, an in vitro gas production assay was conducted at the end of the in vivo trial, using ruminal fluid from adapted animals, to provide complementary mechanistic information on organic matter degradability and fermentation partitioning. The forage-free diet resulted in the highest average daily gain (1.43 kg day -1 ) compared to low-grain (0.49 kg day -1 ) and high-grain diets (1.26 kg day -1 ; P < 0.001). Increasing grain inclusion reduced ruminal pH (lowest in the forage-free diet: 5.59; P < 0.001) and increased short-chain fatty acid and lactate concentrations (P < 0.001). Quantitative PCR showed decreases in fibrolytic bacteria and methanogenic archaea with increasing grain levels (P < 0.01). Principal component analysis indicated clear dietary separation, associating the forage-free diet with greater final body weight and distinct ruminal fermentation patterns. These findings indicate that forage-free diets supplemented with plant-derived additives improved cattle performance and altered ruminal fermentation under finishing conditions.