Across study evaluation of enteric methane emissions from dairy cattle for spot sampling schemes using simulation approaches

Measuring daily enteric methane emissions from dairy cattle using spot sampling systems such as GreenFeed requires careful consideration of sampling frequency and timing. Previous research shows that measurement accuracy increases with sampling frequency, and a minimum number of observations is essential, particularly for restricted-fed dairy cattle. The present study evaluated the accuracy of various sampling schemes using simulation approaches across multiple experiments. Diurnal methane emission patterns from 6 in vivo respiration-chamber experiments were compiled. Diets included grass herbages, corn and grass silages and linseed oil and 3-NOP supplements. Cattle were fed either restrictively at 80–95% of ad libitum intake or fully ad libitum. Methane emissions were recorded over two or three 24-hour periods at ≤ 20-minute intervals. For each animal, the mean of all observed diurnal emission rates was converted to daily methane production (g/d) and treated as the reference. Fourteen preset sampling schemes were evaluated, including 10 evenly spaced intervals (e.g., every 0.5 to every 8 hours) and 4 uneven intervals based on prior literature. Daily methane production calculated for each sampling scheme was statistically compared with the reference using mixed models, with sampling and dietary treatment included as fixed effects and cow as a random effect. To further assess sampling precision, generalized additive models were fitted to diurnal methane profiles, and areas under the curve were compared with reference means. Using the best-fitting spline for each profile, resampling under three schemes was performed 1,000 times to estimate means and standard deviations of methane production, and next construct 95% confidence intervals relative to sample size. Results show that hourly or specific 2- or 3-hour sampling schedules provide accurate estimates of daily methane production, especially in restricted-feeding systems. Although increasing sample size narrows confidence intervals, the choice of sampling scheme consistently influences precision across experiments.