Plant oil supplements reduce methane emissions and improve milk fatty acid composition in dairy cows fed grass silage-based diets without affecting milk yield.

Several nutritional and management strategies can be effective in mitigation of greenhouse gas (GHG) emissions from animal production systems. Plant oil and oilseeds are one way and have been shown to lower enteric methane and therefore GHG consistently over time without compromising the performance of growing or lactating cattle. Depending on level of inclusion they may, however, depress feed intake. Lipid supplements, of course, have the additional advantage of increasing dietary energy density to meet the high energy requirements of cows during early and mid lactation. One or more mechanisms are thought to contribute to the decrease in methane production of dietary lipid supplements, including lowered digestion of feed in the rumen, shifting of rumen fermentation from acetate toward propionate production, and direct inhibitory effects on methanogens (methane-producing bacteria) and protozoa. Lower feed intake and replacement of fermentable feed by lipids also contribute to lower methane production. The changes in the rumen microbial community associated with decreased ruminal methane production to lipid supplements are, however, not well characterized and this was therefore a theme of the study by Dr A.R. Bayat and colleagues, which they published in the Journal of Dairy Science, Volume 101 of 2018, page 1136 to 1151. The title of their paper was: Plant oil supplements reduce methane emissions and improve milk fatty acid composition in dairy cows fed grass silage-based diets without affecting milk yield. 

They used four lipid supplements which varied in chain length or degree of unsaturation, and studied their effects on milk yield and composition, ruminal methane emissions, rumen fermentation, nutrient utilization and microbial ecology in lactating dairy cows. Five cows fitted with rumen cannulae were used in a 5 × 5 Latin square with five 28-day periods. Treatments comprised total mixed rations based on grass silage with a forage-to-concentrate ratio of 60:40 supplemented with no lipid (Control-CO) or 50 g per kg of feed DM of myristic acid (MA), rapeseed oil (RO), safflower oil (SO), or linseed oil (LO).         

Feeding MA resulted in the lowest DM intake and feeding RO reduced DM intake compared with CO. Feeding MA reduced the yields of milk, milk constituents and energy-corrected milk. The plant oils, however, did not influence milk yield and milk constituents, but reduced milk protein content. Compared to CO, all treatments had no effect on rumen fermentation characteristics, other than an increase in ammonia-nitrogen (N) concentration. The lipid supplements reduced daily ruminal methane emission; however, the response was to some extent a result of lower feed intake. They, furthermore, modified the ruminal microbial community composition without affecting total counts of bacteria and ciliate protozoa. The treatments also had no effect on the apparent total tract digestibility of organic matter, fibre and gross energy and did not affect either energy secreted in milk as a proportion of energy intake or efficiency of dietary N utilization. All lipids lowered fatty acid synthesis in the mammary gland. The plant oils increased the proportions of the milk fatty acids 18:0, cis 18:1, trans and the mono-unsaturated fatty acids, and decreased the saturated fatty acids compared with CO and MA. Both SO and LO increased the proportion of total poly-unsaturated fatty acids, total conjugated linolenic acid, and cis-9,trans-11 conjugated linoleic acid. Feeding MA increased the Δ9 desaturation of fatty acids. 

The results provide compelling evidence that plant oils supplemented to a grass silage-based diet reduce ruminal methane emission and milk saturated fatty acids, and increase the proportion of unsaturated fatty acids and total conjugated linoleic acid while not interfering with digestibility, rumen fermentation, rumen microbial quantities, or milk production.