Enteric methane emission from Jersey cows during the spring transition from indoor feeding to grazing

Organic dairy cows in Denmark are often kept indoors during the winter and outside at least part time in the summer. Consequently, their diet changes by the season. We hypothesized that grazing might affect enteric CH₄ emissions due to changes in the nutrition, maintenance, and activity of the cows, and they might differentially respond to these factors. This study assessed the repeatability of enteric CH₄ emission measurements for Jersey cattle in a commercial organic dairy herd in Denmark. It also evaluated the effects of a gradual transition from indoor winter feeding to outdoor spring grazing. Further, it assessed the individual-level correlations between measurements during the consecutive feeding periods (phenotype × environment, P × E) as neither pedigrees nor genotypes were available to estimate a genotype by environment effect. Ninety-six mixed-parity lactating Jersey cows were monitored for 30 d before grazing and for 24 d while grazing. The cows spent 8 to 11 h grazing each day and had free access to an in-barn automatic milking system (AMS). For each visit to the AMS, milk yield was recorded and logged along with date and time. Monitoring equipment installed in the AMS feed bins continuously measured enteric CH₄ and CO₂ concentrations (ppm) using a noninvasive "sniffer" method. Raw enteric CH₄ and CO₂ concentrations and their ratio (CH₄:CO₂) were derived from average concentrations measured during milking and per day for each cow. We used mixed models equations to estimate variance components and adjust for the fixed and random effects influencing the analyzed gas concentrations. Univariate models were used to precorrect the gas measurements for diurnal variation and to estimate the direct effect of grazing on the analyzed concentrations. A bivariate model was used to assess the correlation between the 2 periods (in-barn vs. grazing) for each gas concentration. Grazing had a weak P × E interaction for daily average CH₄ and CO₂ gas concentrations. Bivariate repeatability estimates for average CH₄ and CO₂ concentrations and CH₄:CO₂ were 0.77 to 0.78, 0.73 to 0.80, and 0.26, respectively. Repeatability for CH₄:CO₂ was low (0.26) but indicated some between-animal variation. In conclusion, grazing does not create significant shifts compared with indoor feeding in how animals rank for average CH₄ and CO₂ concentrations and CH₄:CO₂. We found no evidence that separate evaluation is needed to quantify enteric CH₄ and CO₂ emissions from Jersey cows during in-barn and grazing periods.