A meta-analysis of the effect of dietary fat on enteric methane production, digestibility and rumen fermentation in sheep, and a comparison of these responses between cattle and sheep

The objective of this study was to investigate the effect of dietary fat supplementation on methane production, digestibility and rumen fermentation in sheep by means of a meta-analysis, and subsequently to compare the results with response in cattle. A dataset was constructed compiling data from ten published publications containing 41 dietary treatments and 259 observations on sheep. Fat supplementation decreased methane production (expressed as g/day, g/kg dry matter (DM) intake, g/kg digestible DM intake, % of gross energy intake) linearly (P<0.05; R2=0.49 to 0.78). Inclusion of fat did not (P=0.21) affect DM intake. However, digestibilities of DM (P=0.04; R2=0.24) and neutral detergent fiber (P=0.09; R2=0.16) reduced linearly with increasing fat concentrations. Conversely, fat digestibility increased quadratically (P=0.03; R2=0.65) with increasing fat contents. Total volatile fatty acids and acetate percentage in rumen fluid were not altered (P>0.10) by dietary fat supplementation. Percentage of propionate increased linearly (P=0.06; R2=0.17), while butyrate percentage (P=0.06; R2=0.21), acetate to propionate ratio (P=0.05; R2=0.22) and ammonia concentration (P=0.02; R2=0.39) in rumen fluid decreased linearly with increasing fat concentrations. Supplementation of fat had greater suppressive effects on methane production expressed as digestible DM intake in sheep than cattle, but had similar responses on digestibility and rumen fermentation. In conclusion, low concentration of fat in the diet may decrease methane emission without adversely affecting rumen fermentation; but it may lower fiber digestibility at high concentrations.