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.