Genetic variation in cows' response to methane-mitigating feed additives

Feed additives such as 3-nitrooxypropanol (3-NOP) can reduce a dairy cow's enteric methane (CH4) emissions. Our study aimed to examine whether there is genetic variation in the cow's response in reduction of CH4 emission after receiving 3-NOP feed additives. Data were available from a project in which the CH4-mitigating effect of 3-NOP (Bovaer, DSM-Firmenich) was tested for Norwegian dairy cows. The one-year trial took place in a commercial dairy herd and included 79 Norwegian Red cows. The cows' CH4 emissions were measured by a GreenFeed unit. A total of 14,166 daily CH4 measures were analyzed, and the overall mean (SD) was 387 (96) g of CH4 per cow per day. Cows were divided into 2 groups, where 54 cows received between 1.2 and 1.5 g of 3-NOP per day and 25 cows were in a control group without any feed additives. The trait of interest, CH4-response, was defined as the cow's change in CH4 after introducing 3-NOP in the diet compared with the base level CH4 in periods without 3-NOP. The base level period consisted of 3 wk in September of 2023 and 4 wk in January and February of 2024. The trait daily CH4 (g/d) was analyzed with a linear animal repeatability model with fixed effects of 3-NOP group, parity, lactation week, and test day, and random animal genetic and permanent environment effects. Fixed effect solutions from this model were used to compute yield deviations for CH4 (YD_CH4), a corrected phenotype (i.e., daily CH4 corrected for effects of parity, lactation stage, and test day). For cows fed 3-NOP, we calculated their individual base level CH4 as the average YD_CH4 from the periods without 3-NOP. The trait CH4-response was then calculated as YD_CH4 minus base level CH4. The new CH4-response trait had a total of 7,293 daily records from 42 cows and was analyzed with a linear animal repeatability model with the fixed effect of test day and random additive genetic effect of cow. Although this is a small dataset and results should be interpreted with caution, the estimated heritability of 0.15, with an SE of 0.03, suggests that genetic variation in response in the reduction of CH4 emission after receiving 3-NOP feed additives exists for Norwegian Red dairy cows.