1. Dep. of Agronomy and Horticulture Univ. of Nebraska‐Lincoln 202 Keim Hall Lincoln NE 68583 USA
2. North Central Soil Conservation Research Lab. USDA‐Agriculture Research Service Morris MN 56267 USA
3. Dep. of Agronomy, Horticulture and Plant Sciences South Dakota State Univ. P.O. Box 2140C Brookings SD 57007 USA