Fort Ellis Research and Extension Center Study for Greenhouse gas Reduction through Agricultural Carbon Enhancement network in Sidney, Montana

Sheep (Ovis aries L.) grazing is an inexpensive method of weed control in dryland cropping systems but little is known about its effect on net greenhouse gas (GHG) emissions. We evaluated the effect of sheep grazing compared to herbicide application for weed control on GHG (CO2, N2O, and CH4) emissions from May to October, 2010 and 2011, net global warming potential (GWP), and greenhouse gas intensity (GHGI) in a silt loam under dryland cropping systems in western Montana. Treatments were two fallow management practices (sheep grazing [GRAZ] and herbicide application [CHEM]) and three cropping sequences (continuous alfalfa [Medicago sativa L.] [CA], continuous spring wheat [Triticum aestivum L.] [CSW], and spring wheat-pea [Pisum sativum L.] /barley [Hordeum vulgaris L.] hay-fallow [W-P/B-F]). Gas samples were collected at 3 to 14 d intervals with a vented, static chamber. Regardless of treatments, GHG fluxes peaked immediately following substantial precipitation (>12 mm) and/or N fertilization mostly from May to August. Total CO2 flux from May to October was greater in GRAZ with CA, but total N2O flux was greater in CHEM and GRAZ with CSW than in other treatments. Total CH4 flux was greater in CA than in W-P/B-F. Net GWP and GHGI were greater in GRAZ with W-P/B-F than in most other treatments. Greater CH4 flux due to increased enteric fermentation as a result of longer duration of grazing during fallow, followed by reduced crop residue returned to the soil and/or C sequestration rate, probably increased net GHG flux in GRAZ with W-P/B-F. Sheep grazing on cropping sequence containing fallow may not reduce net GHG emissions compared to herbicide application for weed control on continuous crops.