Does diverse grazing behavior of suckler cows have an impact on predicted methane emissions?1

A modeling study based on a dataset from a large-scale grazing study was used to identify the potential impact of grazing behavior and performance of diverse cow genotypes on predicted methane (CH 4 ) emissions. Lactating cows grazing extensive seminatu- ral grassland and heath vegetation were monitored with Global Positioning System collars and activity sensors. The diet selected by cows of 3 different genotypes, Aber- deen Angus cross Limousin (AxL), Charolais (CHA), and Luing (LUI), was simulated by matching their locations during active periods with hill vegetation maps. Mea- sured performance and activity were used to predict ener- gy requirements, DMI, and CH 4 output. The cumulative effect of actual performance, diet selection, and actual physical activity on potential CH 4 output and yield was estimated. Sensitivity analyses were performed for the digestibility of intake, energy cost of activity, proportion of milk consumed by calves, and reproductive efficien- cy. Although with a better performance (P < 0.05), LUI required less total energy than the other genotypes (P < 0.001) as the other 2 spent more ener gy for maintenance (P < 0.001) and activity (P < 0.001). By selecting a better quality diet (P < 0.03), estimated CH 4 of CHA cow-calf pairs was lower than AxL (P = 0.001) and slightly lower than LUI (P = 0.08). Energy lost as CH 4 was 0.17 and 0.58% lower for LUI than AxL and CHA (P < 0.002). This study suggests for the first time that measured activ- ity has a major impact on estimated CH 4 outputs. A 15% difference of the cow-calf pair CH 4 was estimated when using different coefficients to convert actual activity into energy. Predicted CH 4 was highly sensitive to small changes in diet quality, suggesting the relative importance of diet selection on heterogeneous rangelands. Extend- ing these results to a farm systems scale, CH 4 outputs were also highly sensitive to reductions in weaning rates, illustrating the impact on CH 4 at the farm-system level of using poorly adapted genotypes on habitats where their performances may be compromised. This paper demon- strates that variations in grazing behavior and grazing choice have a potentially large impact on CH 4 emissions, illustrating the importance of including these factors in calculating realistic national and global estimates.