Evaluation of fresh garlic and its bioactive compounds for mitigating rumen methane emissions: in vitro and in vivo analyses

The inhibitory effect of garlic extract on rumen methane (CH 4 ) production has been well-documented, but comparative studies evaluating fresh garlic and its active compounds, as well as their long-term effects in beef cattle, remain limited. This study evaluated the effects of fresh garlic, allicin (S 2 O), and diallyl trisulfide (S 3 ) at five addition levels (0, 30, 60, 120, and 240 mg/L of incubation medium) on in vitro rumen fermentation. Total gas production (TGP) and kinetic parameters over 48 h, and pH, in vitro dry matter degradability (IVDMD), total volatile fatty acids (TVFA), volatile fatty acid profiles (VFAs), gas composition, and rumen archaea community at 24 h were measured. A 152-day feeding trial was then conducted with 24 Simmental crossbred bulls. Animals were assigned to a basal diet or diets supplemented with 0.5% or 1.0% dry matter (DM) fresh garlic, to assess growth performance and rumen archaea responses. In vitro , supplementation with S 3 , S 2 O, and 120-240 mg/L fresh garlic remarkably lowered CH 4 ( P < 0.001) with significant additive × dose interaction effects ( P < 0.001). Fresh garlic at 60-120 mg/L increased the fermentation rate, TGP, TVFA, and propionate, while reducing pH and the acetate-to-propionate ratio (A/P) ( P < 0.05). However, 240 mg/L garlic reduced IVDMD ( P = 0.001). Additionally, 60 mg/L garlic significantly decreased the relative abundance of Euryarchaeota ( P < 0.001) but increased Thermoplasmatota ( P < 0.01) . In vivo , 0.5% DM fresh garlic tended to increase average daily weight gain (ADG) ( P = 0.062) and had the lowest feed-to-gain ratio (F/G), while reducing the relative abundance of norank_f_Methanobacteriaceae and norank_f_Methanomethylophilaceae ( P < 0.05). Taken together, supplementation with 0.5% DM fresh garlic appeared to be the most favorable level under the present conditions for modulating rumen methanogenic communities while maintaining animal performance.