The effect of dietary fats on methane emissions, and its other effects on digestibility, rumen fermentation and lactation performance in cattle: A meta-analysis

The objective of this study was to investigate the effects of fats in diets of cattle on methane emissions, and associated impacts on digestibility, milk yield and composition, and rumen fermentation by using meta-analysis. For this purpose, a dataset was constructed compiling data from 29 experiments (27 publications) containing 105 dietary treatments arising from 1339 observations in cattle. Fat supplementation decreased methane production (expressed as g/day, g/kg dry matter (DM) intake, g/kg digestible DM intake, g/kg milk, % of gross energy intake) linearly (P<0.01) with moderate degree of relationship (R2=0.38 to 0.63). Fatty acids C12:0 and C18:3 had marked inhibitory effect on methanogenesis compared with other fatty acids in diets. Methane emissions were not considerably affected by total concentration of saturated fatty acid, but were depressed (P<0.05) by total concentrations of mono- and poly-unsaturated fatty acids in diets. Among other nutrient composition of diets, only non-fibrous carbohydrate content affected the response of fats on methane suppression. Methane yield was influenced by the interaction of fatty acid composition and neutral detergent fiber content of diets. Fats showed a quadratic response (P=0.03) on DM intake, although the relationship was very low (R2=0.15). The digestibilities of DM (R2=0.30) and neutral detergent fiber (R2=0.51) reduced linearly (P<0.01) with increasing fat concentrations. Conversely, fat digestibility increased quadratically (P=0.04; R2=0.53) with increasing fat contents. Milk yield increased quadratically (P<0.01; R2=0.41) with increasing fat supplementation. Milk production reached plateau levels within a range of 3.9-6.0% fat concentrations, and then decreased with increasing fat concentrations. Total volatile fatty acids and acetate percentage in rumen fluid were not altered (P>0.10) by fats, but percentage of propionate increased linearly (P=0.03) with increasing fat concentrations in diets. Fats tended to decrease percentage of butyrate (P=0.10) and acetate to propionate ratio (P=0.07) linearly, but degrees of relationship were very low. From this analysis, it can be concluded that fats with high concentrations of C12:0, C18:3 and polyunsaturated fatty acid up to 6% of dietary DM should, in general, be considered when developing effective feeding strategies for methane mitigation without compromising the productivity in dairy cattle.