1. Agroecosystem Management Research Unit, United States Department of Agriculture-Agricultural Research Service (USDA-ARS), 137 Keim Hall, University of Nebraska-Lincoln, Lincoln, NE 68583-0937, USA
2. Grassland, Soil and Water Research Laboratory, USDA-ARS, 808 East Blackland Road, Temple, TX 76502-6712, USA
3. Resource Assessment Division, USDA Natural Resources Conservation Service, 808 East Blackland Road, Temple, TX 76502-6712, USA