Dietary forage to concentrate ratio and sunflower oil supplement alter rumen fermentation, ruminal methane emissions, and nutrient utilization in lactating cows.

The effects of supplementing high- or low-concentrate diets with sunflower oil (SO) on rumen fermentation, nutrient utilization, and ruminal methane (CH) emissions in lactating cows were examined. Four multiparous Nordic Red dairy cows fitted with rumen cannulae were used in a 4 × 4 Latin square with a 2 × 2 factorial arrangement of treatments and 35-d periods. Experimental treatments comprised iso-nitrogenous total mixed rations based on grass silage with forage to concentrate ratio of 65:35 or 35:65 supplemented with 0 or 50 g/kg diet DM of SO. Apparent ruminal OM and starch digestibility was greater ( < 0.05) with high- than low-concentrate diets but was unaffected by SO. Inclusion of SO in high-concentrate diet decreased ( ≤ 0.05) apparent total tract OM, fiber, and GE, and apparent ruminal fiber digestibility. High-concentrate diets and SO shifted ( < 0.05) fiber digestion from rumen to the hindgut. High-concentrate diet resulted in a lower rumen pH and elevated total rumen VFA concentration compared with low-concentrate diet, whereas SO increased rumen pH and decreased rumen VFA concentration when included in high-, but not low-concentrate diet ( < 0.05 for interaction). High-concentrate diet reduced rumen ammonia-N ( < 0.01) and molar proportion of acetate to propionate ( < 0.01), and decreased ( < 0.05) ruminal CH emissions when expressed as g/d or g/kg OM digested in the rumen. With both low- and high-concentrate diets, SO reduced ( < 0.05) daily emissions of CH as g/d or g/kg OM digested in the rumen, but SO reduced CH emissions expressed as g/kg OM intake, OM digested in total digestive tract, energy-corrected milk or % of GE intake only with low-concentrate diet ( ≤ 0.05 for interaction). In conclusion, replacing grass silage with concentrates led to a reduction in daily ruminal CH emissions that were accompanied by a shift in rumen fermentation toward the synthesis of propionate, and decreases in rumen pH and fiber digestion. Sunflower oil was effective in reducing daily CH emissions in lactating cows which was accompanied by a noticeable lower feed intake with high- but not low-concentrate diet. Overall the effects of SO and greater proportion of concentrates in the diet on daily CH emissions were additive but the additivity declined or vanished when different indices of CH emission intensity were considered. Consequently, SO was more effective in reducing CH emissions when low-concentrate diet was fed.