The effects of stocking rates on pasture-derived greenhouse gas (GHG) flux are rarely reported for cow-calf production systems but work out of Michigan State University has shed some addition insight in this area.
M.B. Chiavegato, W. Powers, S.A. Utsumi and J. Rowntree have compared GHG fluxes from cow-calf herds grazing under different stocking rates. They reported their results during JAM 2013 in Indianapolis, Ind.
During 2011 and 2012, methane and nitrous oxide fluxes from grazed pastures were quantified. Pastures were grazed with cow-calf herds at high stocking rate (2.5 cow/ha; 520 ± 50.4 kg, 30 day grazing return; HI), low stocking rate (1 cow/ha; 521 ± 40.4 kg, 60–90 day grazing return; LO), or excluded from grazing for several years (control).
Each year, 10 randomly deployed static chambers per paddock (750 m2; n = 3 paddocks per treatment) were used to determine daily methane and nitrous oxide fluxes over 14-day post-grazing in early (May–June) and late (August–September) growing seasons. Soil and ambient temperature and soil water content were monitored.
Gas flux was analyzed using ANOVA for a completely randomized design (α = 0.05) although restrictions to randomization existed due to farm logistics and management, the researchers said.
In early season, negative methane flux (i.e., sink) was detected for Control and HI, however LO was a source of methane. In late season, HI and LO were sources and Control remained a sink. Soil temperature, ambient temperature and soil water content was not found to influence methane fluxes.
Nitrous oxide flux was not found to change from early to late season. Fluxes from HI, LO and Control were not found to be significantly different. Soil temperature, ambient temperature and soil water content influenced nitrous oxide fluxes. The net flux of C-equivalent for HI, LO and Control were not found to be significantly different.
Although grazed pastures are net sources of GHG, the researchers said their results indicate potential opportunities to reduce GHG fluxes by methane consumption in pasture soils grazed at high stocking rates.