Genetic parameters for repeatedly recorded enteric methane concentrations of dairy cows.

Animal breeding techniques offer potential to reduce enteric emissions of ruminants to lower the environmental impact of dairy farming. The aim of this study was to estimate the heritability and repeatability of methane (CH) concentrations, using the largest data set from long-term repeatedly recorded CH on cows to date, and to evaluate (1) the accuracy of breeding values for different CH traits, including using visits or weekly means, and (2) recording strategies (with varying numbers of records and recorded daughters per sire). The data comprised of long-term recording of CH and carbon dioxide (CO), from 1,746 Holstein Friesian cows, on 14 commercial dairy farms throughout the Netherlands. Emissions were recorded in 10- to 35-s intervals, between 64 and 436 d, depending on farms. From each robot visit, CH and CO concentrations were summarized into various traits, averaged per visit and per week: mean, median, mean log, and mean CH/CO ratio. Genetic parameters were estimated with animal repeatability models, using a restricted maximum likelihood procedure, and a relationship matrix based on genotypes and pedigree. The heritability was equal for mean and median CH per visit (0.13) but lower for logCH and CH/CO (0.07 and 0.01, respectively). Phenotypic and genetic correlations were high (≥0.78) between the CH traits, apart from the genetic correlations with the CH/CO trait, which were negative. To achieve a minimum reliability of 50% for the estimated breeding value of a bull, 25 records on mean CH, measured on 10 different daughters, were sufficient. Although the heritability and repeatability were higher for weekly (0.32 and 0.68, respectively) than for visit mean CH (0.13 and 0.30, respectively), the reliabilities of estimated breeding values from visit or weekly means were equal; thus, we found no advantage in averaging records to weekly means for genetic evaluations.