Effect of Frame Size on Enteric Methane (CH4) and Carbon Dioxide (CO2) Production by Lactating Beef Cows Grazing Native Tall-Grass Prairie Pasture in Central Oklahoma, USA, 1: Summer Season

Methane (CH4 ) gas has been suggested as a major contributor to global warming. Increased rumen fermentation CH4 is associated with high fiber diets and its production is inversely related to energy availability for the host animal. Within the US cow-calf sector, cattle diet ingredients are primarily high fiber feeds (e.g. pasture forages and hay) which cannot be utilized as human food. It is suggested the primary U.S. beef production model has led to the selection of an animal type which is efficient within the feedlot finishing sector, but not within the cow-calf sector due to increased cow size. We examined the relationship between cow frame size (FS), enteric CH4 production, and cow productivity within a tall-grass prairie grazing system. Twenty-eight lactating Angus cows (BW: 545 ± 49 kg) of either medium FS (n=14) or large FS (n=14) were utilized in this study. Animals grazed simultaneously within a pasture in central Oklahoma, USA during the summer season (SS). Daily estimates of individual animal enteric CH4 and carbon dioxide (CO2 ) production were taken via commercially available breath analyzers deployed within the pasture. Medium-frame cows had a lower body weight (BW) and frame score than large FS (481 versus 609 kg, and 4.6 versus 6.8, respectively). Large-frame cows produced greater daily CH4 (315 versus 270 g/day) compared to medium-frame cows. There was no difference in SS enteric CH4 per unit of 205d adjusted calf weaning weight (WW; 0.11 kg/kg).When expressed as CH4 per unit of cow BW for the total SS, large frame cows produced less CH4 (48.3 versus 52.6 g/kg BW). Based on estimated dry matter intake (DMI), large-frame cows required less herbage dry matter per unit of 205 day WW than medium (8.4 versus 9.4 kg DM/kg WW), and produced greater CH4 per unit of DMI (13.6 versus 12.2 g/kg).Our results indicate large-frame cows may have increased rumen DM digestion, which results in greater methane production per unit of DMI, and less DM required per unit of calf production. Therefore, within the southern Great Plains, large-frame cows appear to be more efficient at converting native prairie into saleable product, and cow efficiency has not been sacrificed due to increased cow size. The sector's methane footprint may best be improved through rumen fermentation manipulation.