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

SIMPLE SUMMARY: Methane (CH(4)) produced by ruminants contributes as a source of anthropogenic greenhouse gases (GHG). Plant-derived bioactive compounds have been investigated for their potential to reduce CH(4) emissions from ruminant livestock. Garlic contains bioactive organosulphur compounds, which have been reported to be effective in reducing CH(4) emissions, but they have demonstrated inconsistent effects in reducing CH(4) production in the rumen. This might be because different types of garlic-based supplements vary in their concentrations of bioactive compounds. Therefore, further investigation is needed, such as the mode of action and persistence of the bioactive compound, to determine whether these compounds can be used successfully to inhibit rumen methanogenesis. The present review discusses garlic and its potential contribution to reducing CH(4) production by ruminant animals and discusses how differences in the diet and the concentration of bioactive compounds in garlic might contribute to inconsistent CH(4) mitigation potential of garlic. ABSTRACT: Methane (CH(4)) 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 CH(4) emissions from ruminants. In recent years, plant-derived bioactive compounds have been investigated for their potential to reduce CH(4) emissions from ruminant livestock. The organosulphur compounds of garlic have been observed to decrease CH(4) emission and increase propionate concentration in anaerobic fermentations (in vitro) and in the rumen (in vivo). However, the mode of action of CH(4) 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 CH(4) emissions by ruminants. Additionally, this review aims to provide insight into garlic and its bioactive compounds in terms of enteric CH(4) mitigation efficacy, consistency in afficacy, possible mode of action, and safety deriving data from both in vivo and in vitro studies.