Exploring definitions of daily enteric methane emission phenotypes for genetic evaluations using a population of indoor-fed multi-breed growing cattle with feed intake data

Genetic selection has been identified as a promising approach for reducing enteric methane (CH4) emissions; a prerequisite for genetic evaluations; however, these are estimates of the necessary genetic parameters based on a population representative of where the genetic evaluations will be used. The objective of this study was, therefore, to derive genetic parameters for a series of definitions of CH4, carbon dioxide (CO2), and dry matter intake (DMI) as well as genetic correlations between CH4, CO2, and DMI in a bid to address the paucity of studies involving methane emissions measured in beef cattle using GreenFeed systems. Lastly, estimated breeding values (EBV) were generated for nine alternative definitions of CH4 using the derived genetic parameters; the EBV were validated against both phenotypic performance (adjusted for non-genetic effects) and the Legarra and Reverter method comparing EBV generated for a subset of the dataset compared to EBV generated from the entire dataset. Individual animal CH4 and CO2 records were available from a population of 1,508 multi-breed growing beef cattle using 10 GreenFeed Emission Monitoring systems. Nine trait definitions for CH4 and CO2 were derived: individual spot measures, the average of all spot measures within a 3-h, 6-h, 12-h, 1-d, 5-d, 10-d, and 15-d period and the average of all spot measures across the full test period (20 to 114 d on test). Heritability estimates from 1,155 animals, for CH4, increased as the length of the averaging period increased and ranged from 0.09 +/- 0.03 for the individual spot measures trait to 0.43 +/- 0.11 for the full test average trait; a similar trend existed for CO2 with the estimated heritability ranging from 0.17 +/- 0.04 to 0.50 +/- 0.11. Enteric CH4 was moderately to strongly genetically correlated with DMI with a genetic correlation of 0.72 +/- 0.02 between the spot measures of CH4 and a 1-d average DMI. Correlations, adjusted for heritability, between the adjusted phenotype and (parental average) EBV ranged from 0.56 to 1.14 across CH4 definitions and the slope between the adjusted phenotype and EBV ranged from 0.92 to 1.16 (expectation = 1). Validation results from the Legarra and Reverter regression method revealed a level bias of between -0.81 and -0.45, a dispersion bias of between 0.93 and 1.17, and ratio accuracy (ratio of the partial evaluation accuracies on whole evaluation accuracies) from 0.28 to 0.38. While EBV validation results yielded no consensus, CH4 is a moderately heritable trait, and selection for reduced CH4 is achievable. The aim of this research was to establish how much of the observed variability in enteric methane production and feed intake in growing cattle is attributable to the underlying genetic differences in these animals and if downstream genetic evaluations could be used to breed lower-emitting animals. Livestock production is a significant contributor to greenhouse gas emissions. Animal breeding programs have been proposed as a sustainable mitigation strategy to reduce enteric methane emissions in livestock production. Before creating a genetic evaluation for enteric methane production, it is important to estimate how much inter-animal genetic variability contributes to the observed differences in enteric methane production. The purpose of this study was to explore multiple enteric methane phenotypes and estimate how much phenotypic variation was due to genetic differences among 1,508 growing cattle of multiple breeds and crosses; also of interest was the extent of similarity in the genetic control of enteric methane, carbon dioxide, and feed intake (i.e., the genetic correlation) and to determine if selection of animals on the estimated genetic merit for methane emissions of their parents would manifest itself in differences in actual methane produced by those animals. Between 9% and 43% of the inter-animal differences in daily enteric methane production were due to differences in the genetic composition of those animals; the genetic control influencing methane production was similar to that of feed intake (i.e., a strong genetic correlation between methane emissions and feed intake of up to 0.72).