Divergent Effects of 3-Nitrooxypropanol on Enteric Methane Emissions in Holstein and Brown Swiss Cows, and Its Lack of Synergy with Acacia Mearnsii Tannin Extract

The objectives of this study were to investigate the combined effects of 3-nitrooxypropanol (3-NOP) and Acacia mearnsii tannin extract (TAN), and their interactions with dairy cattle breed [Brown Swiss (BS) vs. Holstein Friesian (HF)], on lactational performance, and enteric methane (CH4) emissions. Sixteen multiparous mid-lactation cows, including 8 BS and 8 HF cows were used in a split-plot design, with breed as the main plot. Cows within each subplot were arranged in a replicated 4 x 4 Latin Square design with a 2 x 2 factorial arrangement of treatments across four 24-d periods. The experimental diets were: 1) CON (basal total mixed ration), 2) 3-NOP (60 mg/kg DM), 3) TAN (3% of DM), and 4) 3-NOP + TAN. Spot samples of urine, feces, and gas emissions (via GreenFeed) were collected at the end of each period 8 times over 3 days. No 3-NOP x TAN x Breed interactions were observed for DM intake (DMI), milk production, or enteric gas emissions, except for CH4 yield (g/kg DMI) and CO2 production. Breed influenced DMI, milk production, and component yields, with HF cows consuming 3.7 kg/d more DMI, producing 9.3 kg/d more milk, and achieving greater feed efficiency and higher milk component yields than BS cows. Milk yield and energy-corrected milk (ECM) tended to increase in HF but tended to decrease in BS cows by 3-NOP. Cows fed TAN had 1 kg/d lower DMI with the tendency for 3-NOP x TAN showed greater reduction when TAN was fed alone, but milk yield, ECM, and feed efficiency remained unchanged. Cows fed TAN exhibited 18% lower milk urea nitrogen (N) concentration and 23.0% lower urinary N but 36.7% greater fecal N excretions as a percentage of daily N intake. A 3-NOP x Breed interaction was observed in CH4 production (g/d), with a 21.7% reduction in HF, and a 13.0% reduction in BS. Similarly, there were 3-NOP x Breed tendencies in CH4 yield and intensity (g/kg ECM), with reductions in HF cows of 21.8% and 23.4%, respectively, compared to 11.0% and 10.8% in BS cows. In conclusion, there were no synergistic or additive effects between 3-NOP and TAN on enteric CH4 mitigation. The enteric CH4 emission mitigating effect of 3-NOP was more pronounced in HF cows than in BS cows. Further research is needed to understand breed-specific responses and to optimize CH4 mitigation strategies for inclusion in national greenhouse gas inventories.