Garlic and Its Bioactive Compounds: Implications for Methane Emissions and Ruminant Nutrition

Methane (CH₄) emission from enteric fermentation of ruminant livestock is a source of greenhouse gases (GHG) and has become a significant concern for global warming. Enteric methane emission is also associated with poor feed efficiency. Therefore, research has focused on identifying dietary mitigation strategies to decrease CH₄ emissions from ruminants. In recent years, plant-derived bioactive compounds have been investigated for their potential to reduce CH₄ emissions from ruminant livestock. The organosulphur compounds of garlic have been observed to decrease CH₄ emission and increase propionate concentration in anaerobic fermentations (in vitro) and in the rumen (in vivo). However, the mode of action of CH₄ reduction is not completely clear, and the response in vivo is inconsistent. It might be affected by variations in the concentration and effect of individual substances in garlic. The composition of the diet that is being fed to the animal may also contribute to these differences. This review provides a summary of the effect of garlic and its bioactive compounds on CH₄ emissions by ruminants. Additionally, this review aims to provide insight into garlic and its bioactive compounds in terms of enteric CH₄ mitigation efficacy, consistency in afficacy, possible mode of action, and safety deriving data from both in vivo and in vitro studies.