Assessment of grazing management on farm greenhouse gas intensity of beef production systems in the Canadian Prairies using life cycle assessment

Grazing is a common practice in the beef cattle industry and is an integral component of pasture and rangeland management. The objective of this study was to evaluate impacts of grazing management scenarios on greenhouse gas (GHG) intensity [kg carbon dioxide equivalents (CO2e) kg− 1 beef] at the farm-gate for beef production systems in western Canada using life cycle assessment. A life cycle assessment over an 8-year period was conducted on a hypothetical but typical beef farm that managed 120 cows, 4 bulls, and their progeny. Calves were backgrounded (raised) on rangeland and market cattle were finished on grain for an average of 134 ± 11 d. Four grazing management scenarios were examined: i) light continuous grazing (LC) for all cattle, ii) heavy continuous grazing (HC) for all cattle, iii) light continuous grazing for cow-calf pairs and moderate rotational grazing for backgrounded cattle (LCMR), and iv) heavy continuous grazing for cow-calf pairs and moderate rotational grazing for backgrounded cattle (HCMR). Greenhouse gas emissions from various sources within the farm were estimated using the whole-farm model, Holos. Soil organic carbon (C) change due to each grazing management scenario was estimated using the Introductory Carbon Balance Model. Primary model inputs came from short- and long-term grazing management studies. Greenhouse gas intensity of beef varied among grazing management scenarios, ranging from 14.5-16.0 kg CO2e kg− 1 live weight and 24.1-26.6 kg CO2e kg− 1 carcass weight. Greenhouse gas intensity decreased with increasing stocking rate: that of HC grazing management was 9.2% lower than that of LC treatment (14.5 vs 16.0 kg CO2e kg− 1 live weight, respectively). Greenhouse gas intensity was similar (< 3%) between LC and LCMR or between HC and HCMR, indicating that the use of moderate rotational grazing for the backgrounding operation in LCMR and HCMR had no effect on overall intensity estimates. However, LCMR management had 7% higher GHG intensity than HCMR (15.6 vs 14.6 kg CO2e kg− 1 live weight, respectively). Average farm production efficiency (kg beef per unit land area) was 17-25% higher for the HC and HCMR grazing management scenarios than the LC and LCMR scenarios. Regardless of grazing management, methane emission from enteric fermentation was the major source of emissions (67-68% of total), followed by nitrous oxide (14-16% of total) from manure management. The rate of soil C sequestration ranged from 0.01 Mg C ha− 1 yr− 1 for rangeland under HC to 0.46 Mg C ha− 1 yr− 1 for a triticale field used for swath grazing. When soil C sequestration was included in the total emission analysis, GHG intensity estimates decreased by 12-25%, and there was no difference in intensity estimates among the scenarios. The largest reduction in GHG intensity arising from soil C sequestration was observed for LC (22%) and LCMR (25%) because they sequestered more C than HC and HCMR. Overall, results of our study indicated that grazing management impacted GHG intensity of beef production by influencing diet quality, animal performance and soil C change. It also emphasizes the importance of accounting for all emission sources and sinks within a beef production system when estimating its environmental impacts.