Carbon Release From Steers and Fossil Fuels In Integrated Crop-Livestock Systems.

The use of fossil fuels and fossil fuel-derived products in U.S. agriculture increases productivity but also increases expenses, relies on non-renewable foreign energy, and releases CO₂ to the atmosphere.  Quantifying and understanding agricultural fossil fuel use is important to improve efficiency.  We evaluated fossil fuel use and associated C emissions from three integrated crop-livestock systems in the Texas High Plains, ranging from low to moderate stocking rates and inputs ha^-1.  All systems were grazed by stocker steers.  Systems were evaluated for 3 to 5 years in a randomized block design.  As stocking rate and quantity of inputs ha^-1 increased, CO₂ emissions associated with fossil fuel use increased from 752 to 2838 kg ha^-1, primarily as a result of energy required to pump irrigation water (0 to 29% of total energy used), N fertilizer (13 to 25%), and the off-site cost of maintaining a cow herd to produce stocker steers (25 to 46%).  Efficiency of live weight gain did not differ (p > 0.05) between the most (49.9 MJ kg^-1) and least (41.0 MJ kg^-1) intensive systems.  Non-irrigated cotton emitted more CO₂ (715 kg ha^-1) than native grasses (110) or foxtail millet (528), but less than irrigated bluestem (979) and bermudagrass (1599).  Estimates of CH₄ and CO₂ from enteric fermentation and respiration by the steers, expressed as CO₂ equivalents ha^-1, were slightly greater in magnitude than CO₂ emissions associated with fossil fuel use, ranged from 866 to 4825 kg ha^-1, and increased with stocking rate.  Measurement of soil C indicated C is being sequestered most rapidly in the soils of the intensive system, but this may not compensate for greater rates of CO₂, CH₄, and NO₂ release from fossil fuels, steers, and soils.  Improvements in cow-calf production and forage water- and N-use efficiency would decrease CO₂ release associated with fossil fuels.