Effects of feeding grass silage- and corn silage-based diets in phenotypically low and high methane-emitting Holstein-Friesian dairy cows.

The objectives of this study were to determine (1) whether replacing grass silage with corn silage reduced methane (CH4) emissions equally effectively in dairy cows with a low or high CH4 emission level, (2) whether low or high CH4-emitting cows remained low and high CH4 emitters irrespectively of the diet fed, and (3) whether the diet and CH4 emission level affected lactation performance, feed intake, and rumen microbiome. Emissions of CH4 of 192 lactating dairy cows were measured with the GreenFeed system (C-Lock Inc.) in a screening phase to select the 12 highest and the 12 lowest CH4-emitting cows (i.e., 15.9 ± 1.49 vs. 24.0 ± 1.43 g CH4/kg DMI). These 24 cows were subsequently enrolled in a crossover design trial with a grass silage-based diet (GS) and a corn silage-based diet (CS). The GS diet consisted of 58.0% grass silage, 19.3% corn silage, and 22.7% concentrate, and CS consisted of 19.3% grass silage, 58.0% corn silage, and 22.7% concentrate (DM basis). Treatment periods lasted 4 wk and consisted of a 2-wk adaptation period followed by a 2-wk measurement period. No interaction between diet and CH4 emission level was observed. Hence, the reduction in CH4 emissions by CS compared with GS did not differ between the low (-23%) and high (-18%) CH4-emitting cows. The CH4 emission level of low-emitting cows was 27% lower than that of high-emitting cows, and was persistent irrespective of the type of diet fed. The community structure and diversity of the rumen microbiome responded to diet and CH4 emission level, but no interactions were observed except for 1 low-abundance bacterial genus accounting for less than 0.6% of total bacterial relative abundance. Replacing grass silage with corn silage resulted in increased milk yield and DMI, decreased milk fat content and CH4 emissions, and a shift from acetate to propionate. The latter aligned with the changes in rumen microbiota, shifting from a more fibrolytic, acetate-oriented community with GS toward a more amylolytic, propionate-oriented community with CS. Other than in CH4 emissions, the low and high CH4-emitting cows did not differ from each other in feed intake level, lactation performance, and body measures. Despite these similar production responses, low CH4-emitting cows had distinct ruminal bacterial and archaeal communities compared with high CH4 emitters across both diets. Low CH4 emitters were enriched in Succinivibrionaceae_UCG-001, which was positively associated with ruminal molar proportion of propionate. High CH4 emitters showed higher archaeal diversity and a greater relative abundance of Methanomethylophilaceae. The molar proportions of butyrate and carbon dioxide (CO2) yield were higher in high-emitting cows compared with low-emitting cows. In conclusion, this study shows that replacing grass silage with corn silage effectively reduces CH4 emissions regardless of the cow's CH4 emission level, and that CH4 emission levels in Holstein-Friesian cows are persistent regardless of diet fed.