1. INRA, Ctr. de Recherche de Toulouse, Agronomie BP27 31326 Castanet‐Tolosan France
2. USDA‐ARS 808 E. Blackland Rd. Temple TX 76502
3. Dep. of Agronomy, Louisiana Agric. Exp. Stn. Louisiana State Univ. Agric. Ctr. Baton Rouge LA 70803
4. USDA‐ARS, North Central Soil Conservation Res. Lab. N. Iowa Ave. Morris MN 56267