Effects of stocking rate on methane and carbon dioxide emissions from grazing cattle

Pastoral farming contributes significantly to total agricultural emissions of greenhouse gases, and stocking rate is the simplest grassland management decision. A study was conducted during the 2002 and 2003 grazing seasons on a semi-natural grassland in the French Massif Central in order to measure enteric methane (CH4) and total carbon dioxide (CO2) emissions from Holstein-Friesian heifers (initial liveweight (LW) 455 ± 29 and 451 ± 28 kg in 2002 and 2003, respectively) managed at low (LSR) and high (HSR) stocking rates (1.1 LU ha−1 versus 2.2 LU ha−1, respectively) under a continuous grazing system. Measurements took place in late spring, mid summer, late summer and early autumn. Daily CH4 and CO2 emissions by individual heifers were measured during 7 consecutive days in each period using the sulphur hexafluoride (SF6) tracer technique. In both grazing seasons, the herbage in the LSR system had higher mass (HM) than in the HSR system, especially in mid and late summer. In both grazing seasons, herbages offered in the LSR system were of lower quality than those in the HSR system, and consequently feed organic matter (OM) digestibilities (OMD) and intakes (OMI) in the LSR system were lower (P < 0.01) than in the HSR system. In both grazing seasons, heifer LW increased as the seasons progressed (P < 0.001), and heifers in the HSR system tended to be heavier than those in the LSR system. Systems did not differ (P > 0.05) in mean absolute CH4 emission (223 g d−1 versus 242 g d−1 and 203 g d−1 versus 200 g d−1 for LSR and HSR in the 2002 and 2003 seasons, respectively), but as the seasons progressed, CH4 emission per unit of digestible feed intake was higher (P < 0.05) in the LSR than in the HSR treatment. Both absolute CH4 emission (g d−1) and CH4 yield (CH4 energy loss as a percentage of gross energy intake, % of GEI) were consistently related (P < 0.05) to OM intake (both gross and digestible). The SF6 tracer technique appeared to overestimate total CO2 emission from heifers. Nevertheless, significant (P < 0.01) coefficients of correlation were found between measured CO2 emissions and LW (r = 0.68 and 0.41 in season 2002 and 2003, respectively), and these correlations were stronger for HSR-managed heifers than LSR-managed heifers. This study found no effect of SR system on enteric CH4 emissions from heifers. The study also found that the SF6 tracer technique seemed to overestimate CO2 emission.