The influence of cow-calf grazing systems on carbon flux

Grazing management has been identified as the most suitable means to increase forage production and reduce GHG emissions from the cow-calf sector, while potentially increasing carbon sequestration. The grazing management applied to pastures determines factors such as forage growth, residuals accumulation, manure distribution, and soil properties, which create favorable or unfavorable conditions for microbial populations to develop. Methane, N2O and CO2 production and consumption in soils are microbial processes. Forage maturity determines forage quality and CH4 production in the rumen. This study applied a system-based approach to assess net GHG exchange, in terms of C equivalent, and soil organic C accumulation in pastures grazed with cow-calf herds under different stocking rate and densities, and non-grazed pasture sites. Data were collected post-grazing and at a farm-scale and the variability associated to GHG emissions from pasture soils was very large. CO2 emissions did not differ between systems. Soil and ambient temperature and soil water content had effects on CO2 emissions. The effect of grazing was not conclusively observed on CH4 and N2O emissions. In addition, soil and ambient temperature and soil water content did not conclusively explain CH4 and N2O emissions. Other soil properties might be better predictors of CH4 and N2O, such as water filled pore space (WFPP) or soil O 2 content. Further research is needed to confirm the effect of WFPS and O2 content on GHG emissions. We did not observe any clear trade-offs between GHG; generally GHG emissions increased from 2011 to 2013, which was likely associated to weather changes. Our results indicate that grazing management did not affect daily enteric CH4 emissions from lactating beef cows. Additionally, CH4 emissions tended to be lower than reported values for lactating beef cows. The selective grazing allowed cows managed with different grazing strategies to eat forage with similar qualities that met nutritional requirements with reduced CH4 emissions. Results indicate forage quality might be a better predictor to daily CH4 emissions than DMI. Grazing systems resulted in higher C equivalent flux than non-grazed pasture sites, which was a result mainly of enteric CH4 emissions. However, the effect of greater enteric CH4 contribution from high stocking rate systems was offset by GHG exchange from the soil, and C equivalent flux was not different between grazing systems. High stocking rate, low stocking density system potentially increased total SOC stock, the addition of SOC to deeper layers and SOM. However, low stocking rate, high stocking density systems accrue litter on top of the soil. SOM decomposition rate was slower on the low stocking rate, high stocking density system, which could allow for greater resilience to adverse conditions. Long-term research is needed to confirm SOC sequestration potential of these systems and SOM decomposition rates. Grazing management should be adaptive and farm management decisions are inherent to management. Both grazing systems have opportunities to improve ecosystems services at the farm level, including animal production and food provisioning.