Phenotypic variation in responses to methane-reducing additives in early-lactation first parity Holstein cows

The phenotypic traits that determine between-cow variability in response to CH₄ mitigating feed additives, such as nitrate and 3-nitrooxypropanol (3-NOP) in dairy cows, require further investigation. This study examined the relationship between CH₄ emissions and several phenotypic traits in dairy cows supplemented with nitrate or 3-NOP, to identify which ones affected the individual responses to these additives. A total of 201 first-lactation Holstein cows were used in a 3 × 3 Latin square design with 3 treatments: control diet (CON), diet containing nitrate (NIT; 8.57 g NO₃^-/kg of DM), and diet containing 3-NOP (NOP; 60 mg/kg of DM). Each 21-d period included 2 adaptation weeks and 1 sampling week. Enteric gas emissions, feed intake, milk production, and BW were recorded. Additionally, ruminal content samples were collected to determine fermentation parameters, milk samples to assess composition, and blood samples to monitor methemoglobin in response to nitrate. Statistical differences were determined using a linear mixed model, where treatment, period, their interaction, and the week of experiment were included as fixed effects and cow as a random effect. Random cow estimates (RCE) of different variables and residuals for CH₄ yield (g/kg of DMI), DMI (kg/d), and ECM yield (kg/d) were extracted and correlated to determine each cow's deviation from model predictions and explore phenotypic differences in response to the additives. The yield of CH₄ was reduced by 19% and 29% when the NIT and NOP diets were fed, respectively, while CH₄ intensity (g CH₄/kg of ECM) decreased by 20% and 28%. When the NIT diet was fed, DMI was maintained, but decreased by 4% when cows were fed the NOP diet compared with CON, contributing to a greater BW loss with NOP. This was reflected in milk production, which decreased with NOP but remained stable with NIT. Correlations with RCE revealed that cows with a high CH₄ yield exhibited higher CO₂ yield, milk fat concentration, total ruminal VFA, acetic and butyric acids proportions, and ruminal ammonia. Conversely, cows with low enteric CH₄ emissions showed higher DMI, milk, and ECM yields, and higher proportions of propionic and valeric acids. Regression analyses revealed that cows with higher DMI, milk yield, ECM yield, or ECM/BW exhibited greater CH₄ yield reduction with NOP, while NIT was more effective in reducing CH₄ yield in cows with high CH₄ yield, milk yield, or BW. Cows with high ruminal propionic acid proportions evidenced poorer CH₄ yield reduction responses to NIT. Ruminal phenotypes also influenced the effect of additives on DMI; NOP reduced DMI more in cows with high ruminal butyrate proportions, whereas NIT reduced DMI more in cows with low ruminal ammonia concentrations. Effects on ECM yield varied similarly, with higher ECM yield when NOP was fed to cows with high milk fat or lactose content, but lower in cows with high CH₄/ECM or ammonia concentrations. In conclusion, both NIT and NOP were more efficient in reducing CH₄ emissions in high-performing cows, while variability in DMI and ECM responses due to the additives depended on ruminal fermentation phenotypes.