Use of 3-nitrooxypropanol in early-lactation dairy cows fed a high forage total mixed ration: Effect on enteric methane emissions, performance, and milk carbon isotopic signature

The aim of this study was to test the effect of 3-nitrooxypropanol (3-NOP) on methane emissions, animal performance, milk composition, and rumen fermentation in early-lactation dairy cows fed a forage-rich diet. A second objective was to assess the ¹³C isotopic signature of milk as a potential proxy for methane-emission status. Twenty-seven cows with similar BW and age in early lactation (7-11 DIM) and fed a 75% forage-based diet were selected and distributed to 2 balanced groups in a randomized block design. The treatment group (n = 13) received 3-NOP (60 mg/kg DM basis) in a TMR for 105 d and the control group (n = 14) received a placebo. The 3-NOP additive was included in a supplement formulated with propylene glycol and adsorbed on silicon dioxide. The control group received the same supplement without the additive. Individual daily methane emissions were quantified using the GreenFeed system throughout the study. Intake and milk production were recorded daily, and milk composition (fat, protein, lactose, urea) twice a week. Natural ¹³C abundance (δ¹³C) in milk and feed samples was determined using isotope-ratio MS coupled with elemental analysis for each cow at wk 3, 7, 11, and 15. Methane emissions, methane yield, and methane intensity were lower in the treatment group throughout the 105 d (on average -31% g/d, -24% g/kg DMI, and -30% g/kg ECM). By monitoring methane emissions throughout the day, we found that the main effect occurred after feeding, preventing the postprandial peak in methane emissions. Intake was reduced by ∼7% with 3-NOP, whereas milk production was similar between groups (34.7 kg ECM/d). Consequently, feed conversion efficiency in animals tended to increase with 3-NOP (1.39 vs. 1.48 kg milk/kg DMI on average). Milk composition did not vary between groups. Although no differences were found in dietary δ¹³C across the treatments, milk from the treatment group had lower δ¹³C than the control group throughout the experimental period. The results of this work with medium-producing dairy cows confirm the efficacy of the additive on diets relatively high in NDF, which broadens its applicability to less intensive production systems. The results for milk δ¹³C, suggesting its potential as a qualitative proxy for methane emissions, merit further investigation, as it could be used in breeding and in monitoring, reporting, and validation systems.