1. a Pasture Systems and Watershed Management Research Unit , USDA-ARS , University Park , PA , 16802-3702 , U.S.A.
2. b Department of Plant Sciences , University of Delaware , Newark , DE , 19717 , U.S.A.
3. c Department of Environmental Science and Engineering , Oregon Graduate Institute , 19600 N.W. von Neumann Dr., Beaverton , OR , 97006-1999 , U.S.A.
4. d Dept. Crop and Soil Sciences , Washington State University , Puyallup , WA , 98371-4998 , U.S.A.
5. e Agric-Economic Division , IFDC , P.O. Box 2040, Muscle Shoals , AL , 35662 , U.S.A.
6. f Agronomy Department , 2004 Throckmorton Soil and Plant Sciences Center , Kansas State University , Manhattan , KS , 66506-5501 , U.S.A.
7. g Department of Soil and Water Sciences , University of Florida , 106 Newell Hall, Gainesville , FL , 32611-0510 , U.S.A.
8. h UNIBEST, Inc. , 3535 Stucky Road, Bozeman , MT , 59718 , U.S.A.