Animal factors that affect enteric methane production measured using the GreenFeed monitoring system in grazing dairy cows

Similar to all dairy systems internationally, pasturebased dairy systems are under increasing pressure to reduce their greenhouse gas (GHG) emissions. Ireland and New Zealand are 2 countries operating predominantly pasture -based dairy production systems where enteric CH 4 contributes 23% and 36% of total national emissions, respectively. Ireland currently has a national commitment to reduce 51% of total GHG emissions by 2030 and 25% from agriculture by 2030, as well as striving to achieve climate neutrality by 2050. New Zealand's national commitment is to reduce 10% of methane emissions by 2030 and between 24% and 47% reduction in methane emissions by 2050. To achieve these reductions, factors that affect enteric methane (CH 4 ) production in a pasture -based system need to be investigated. The objective of this study was to assess the relationship between enteric CH 4 and other animal traits (feed intake, metabolic liveweight, energy corrected milk yield, milk urea concentration, and body condition score [BCS]) in a grazing dairy system. Enteric CH 4 emissions were measured on 45 late lactation (213.8 +/- 29 d after calving) grazing Holstein -Friesian and Holstein -Friesian x Jersey crossbred cows (lactation number 3.01 +/- 1.65, 538.64 +/- 59.37 kg live weight, and 3.14 +/- 0.26 BCS) using GreenFeed monitoring equipment for 10 wk. There was a training period for the cows to use the GreenFeed of 3 wk before the 10wk study period. The average enteric CH 4 produced in the study was 352 g +/- 45.7 g per day with an animal to animal coefficient of variation of 13%. Dry matter intake averaged 16.6 kg +/- 2.23 kg per day, while milk solids (fat plus protein) averaged 1.62 kg +/- 0.29 kg per day. A multiple linear regression model indicated that each one unit increase in energy corrected milk yield, metabolic liveweight and milk urea concentration, resulted in an increase in enteric CH 4 production per day by 3.9, 1.74, and 1.38 g, respectively. Although each one unit increase in BCS resulted in a decrease in 39.03 g CH 4 produced per day. When combined, these factors explained 47% of the variation in CH 4 production, indicating that there is a large proportion of variation not included in the model. The repeatability of the CH 4 measurements was 0.66 indicating that cows are relatively consistently exhibiting the same level of CH 4 throughout the study. Therefore, enteric CH 4 production is suitable for phenotyping.