1. Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA 50011.
2. Corresponding author. Phone: 515-294-6358. Fax: 515-294-6383. E-mail address: igneb@iastate.edu.
3. Department of Agronomy, Iowa State University, Ames, IA 50011.
4. Research associate, Pioneer Hi-Bred Int’l, Inc., Johnston, IA 50131.