Factors affecting recording methane emission phenotypes of composite and crossbreed beef cattle grazing tropical and subtropical rangelands of Northern Australia

Globally agricultural emissions contribute 10%-12% of all anthropogenic greenhouse gas emissions. Ruminant livestock account for a considerable proportion of these emissions, with cattle grazing in tropical and subtropical regions representing a major source. Enteric methane production has climatic implications and represents a loss of energy from the animal that could be directed toward growth. This study focuses on cattle grazing tropical and subtropical rangelands in Australia, aiming to identify the factors that influence the recording of methane emissions in these environments. Short-term methane production was recorded from 453 mixed-breed cattle across four trial sites utilizing GreenFeed units (GFU) (C-Lock, USA). Data was generated and combined from several sources, including GFU, weather data, forage samples, and estimated forage biomass. Using a linear mixed model, the significance and variance of each factor on methane production was evaluated. Factors identified as explaining most of the variation for methane production included GFU (P < 0.001), total visitation (P < 0.001), daily visitation (P < 0.001), trial (P < 0.001), hour of visitation (P = 0.03), air temperature (P < 0.001), initial weight (P < 0.001), visit duration (P < 0.001), and forage biomass (P < 0.001). Four factors were tested and found to be not significant in explaining variation in methane production, including Bos taurus indicus (BI) content (P = 0.99), sex (P = 0.06), rain (P = 0.07), and temperature heat index (P = 0.2). A moderate coefficient of variation (CV) of 23.2% was observed in the raw observations of methane production. This study highlights important factors to consider when utilizing GFU for methane measurement of grazing beef cattle. This study provides a foundation for future studies aiming to quantify individual animal methane emissions for use in genetic evaluation programs to reduce industry emissions and improve the sustainability of beef production in tropical and subtropical regions.