Methane emissions from dairy cows fed grass or corn silage diets supplemented with rapeseed oil were evaluated. The partial replacement of GS with MS decreased the total dry matter (DM), nutrients intake, milk yields or nutrients digestibility, among others, but did not affect the daily CH4 emissions or strongly affect the microbial composition. The supplementation of rapeseed oil also decreased total DM, nutrients intake and altered the milk composition, but also decreased the CH4 emissions. Analysis of the microbial composition revealed an increase in the relative abundance of Methanobrevibacter and changes in the bacterial population.

The inclusion of maize silage and oilseeds in dairy cattle diets has been reported, separately, to decrease enteric methane (CH4) emissions. The aim of the study was to investigate the effect of the partial replacement of grass silage with maize silage on milk production and CH4 emissions by dairy cows fed diets with or without rapeseed oil supplementation. Twenty Nordic Red cows averaging (mean ± standard deviation) 71 ± 37.2 days in milk and 34.2 ± 5.26 kg milk/d pretrial were blocked by parity and milk yield (MY) and assigned to a replicated 4 × 4 Latin square designed with four 28 d periods consisting of14 d of adaptation and 14 d of sampling and data recording. Dietary treatments were in a 2 × 2 factorial arrangement and were grass silage (GS), GS supplemented with rapeseed oil (GSO), GS plus maize silage (GSMS), and GSMS supplemented with rapeseed oil (GSMSO). The diets were fed ad libitum as total mixed ratios and CH4 production was measured daily using the GreenFeed emissions monitoring system (C-Lock Inc., Rapid City, SD, USA). The partial replacement of GS with MS decreased the total dry matter (DM) and nutrients intake, MY, energy-corrected milk (ECM), yields of milk components, and nutrients digestibility, but without impairing the daily CH4 emissions or CH4 yield. The MS in diet did not alter the archaea ruminal and discretely affected the bacteria ruminal population. The rapeseed oil supplementation decreased total DM and nutrients intake, ECM, composition and yields of fat and protein, milk urea nitrogen, and nutrients digestibility excepted for crude protein. Further, the rapeseed oil in the diets decreased the daily CH4 emissions, yield and intensity, while increased Methanobrevibacter and altered the relative abundance of bacteria ruminal population. In conclusion, the forage type did not alter methane, but 4 g/kg DM of rapeseed oil in the diet to dairy cows reduced CH4 emissions by 22.5%.