Effects of 3-nitrooxypropanol on enteric methane emissions and milk production characteristics in dairy cows fed a high corn-silage diet in different environmental conditions.

This study examined the effect of 3-nitrooxypropanol (3-NOP) on enteric GHG emissions, milk production, and the blood metabolic profile of lactating Holstein dairy cows across 2 experiments, one conducted during the summer and the other in fall. Thirty mid lactation multiparous cows, with an average (mean ± SD) of 190 ± 72 DIM and a milk yield of 36.8 ± 7.5 kg/d, were included in the summer experiment. In the fall experiment, cows averaged 158 ± 74 DIM and a milk yield of 36.8 ± 4.6 kg/d. Experiments were structured as longitudinal studies with repeated measurements in a completely randomized design, with an initial 14-d covariate period, a 63-d treatment period, and a 14-d washout period. Cows were randomly assigned to 1 of 2 treatment groups: (i) the control group (CTR), receiving a corn silage-based diet supplemented with a placebo, and (ii) the 3-NOP group, receiving the same diet supplemented with 60 mg/kg DM of 3-NOP. Cows were housed in a freestall barn, with individual monitoring of feed intake, and fed a TMR ad libitum delivered 1 time a day. The enteric CH₄, CO₂, and H₂ emissions were measured using a GreenFeed unit. The 3-NOP reduced enteric CH₄ production (46%), yield (46%), and intensity (44%), regardless of the season, but increased H₂ and CO₂ emissions. In the summer, 3-NOP cows reduced DMI (5.7%) and meal size and increased the feed (4.8%) and dairy (4%) efficiencies but not in the fall. Adding 3-NOP increased the apparent OM, CP, and NDF digestibility, independently of the season. There was no effect of 3-NOP on milk yield, although supplementing 3-NOP increased the milk protein and casein and decreased milk urea contents. Saturated fatty acids (FA) increased with 3-NOP, regardless of the phase of the study. In the summer season, 3-NOP supplementation increased de novo FA (7.3%) and decreased mixed FA (6.7%), whereas in the fall, SFA decreased (2.3%) and MUFA increased (6.8%) compared with CTR. Supplementing 3-NOP affected the molar proportions of rumen VFA. Acetate was reduced, whereas butyrate, valerate, isovalerate, and caproate were increased by 3-NOP. Plasma concentrations of urea, nonesterified FA, Zn, and advanced oxidation protein products decreased with 3-NOP, but blood cholesterol and myeloperoxidase increased. In conclusion, including 3-NOP in a high corn-silage and high-starch diet almost halved enteric CH₄ emissions without adversely affecting milk yield and composition.