Effect of Body Weight Changes During Lactation on Estimated Enteric Methane Emissions in New Zealand Dairy Cows

The New Zealand greenhouse gas inventory methodology for estimating enteric methane emissions (CH 4 ) in dairy cows assumes, for simplicity of the prediction, that lactating cows maintain a constant body weight (BW) throughout each lactation. This study evaluated the effect of BW changes during lactation on estimated dry matter intake (DMI) and its consequences on estimated CH 4 emissions in grazing dairy cows. Daily records for walk‐over body weights and milk metre milk yields and herd tests for fat and protein percentages in milk were collected from 3957 cows during the 2023–2024 production season from three farms. Random regression models fitted the trajectories of these traits over the first 270 days in milk (DIM). Metabolisable energy (ME) requirements and DMI and CH 4 were predicted either assuming a constant BW over lactation (S1) or considering daily BW changes (S2). CH 4 per cow was 63–92 g/day higher for S1 than for S2 during the first 50 DIM. This relationship reversed between 50 and 270 DIM when CH 4 was 18–21 g/day lower in S1 than S2. Parity, breed and farm affected the total predicted CH 4 per lactation under the two scenarios modelled. Considering BW changes during lactation influenced the prediction of CH 4 emissions.