1. Dep. of Crop Sciences Univ. of Illinois Urbana‐Champaign IL USA
2. Current address: Dep. of Agronomy Purdue Univ. 915 West State St. West Lafayette IN 47907 USA
3. Dep. of Plant Sciences Univ. of California Davis CA USA
4. Dep. of Agricultural and Biological Engineering Univ. of Illinois Urbana‐Champaign IL USA