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Impacts of livestock forage and pasture use on carbon sequestration and greenhouse gas emissions

Representative models of U.S. dairy (WI, CA) and beef (AL, TX, UT, VA, WI) production systems were constructed to sum the land use requirements, greenhouse gas (GHG) emissions and product outputs. Total GHG emissions expressed, as CO2 equivalents per unit of milk were 1.4 and 1.5 kg/kg, while the production of beef as live weight at the farm gate resulted in from 13.8 to 18.2 kg/kg. The 2 dairy systems utilized 0.6 (CA) and 1.1(WI) ha of land per mature cow unit. Beef systems’, including stocker and feedlot, land use was projected to range from 1 to 3.7 ha per mature cow unit. Best management practices applied to all land used in these operations is expected to store .12 and .22 Mg/cow unit for dairy systems and from .2 to .7 Mg of carbon/beef cow unit, anywhere from 4 to 50% of total system GHGs. Twenty-yr simulations of soil C were run using the Century model for 7 to 10 soil types in 3 locations assuming normal tillage operations. Changing from pasture to either hay or various grain cropping regimens generally decreased soil-C (av. -0.25 Mg/ha/yr), however, the changes were quite variable by soil type. The reverse simulation, change from grains or hay to pasture, at the third location showed wide variations also but averaged +0.28 Mg/ha/yr. Thus changing to increased pasture usage could yield additive C sequestrations above those realized from BMP’s. Potential grazing land C-sequestration in the U.S. has been estimated at 4 fold the total agricultural grazing land GHG emissions. Caution must be exercised since tradeoffs, as will be discussed, can occur e.g., if reduced rate of production increases CH4/product or excess forage N enhances N2O emissions. Also, long term measurements of soil C in intensively managed pastures can show C losses as noted for a location in NZ.