1. Graduate research assistant, professor, and assistant professor, respectively, South Dakota State University, Department of Agricultural and Biosystems Engineering, 1400 North Campus Drive, Brookings, SD 57007.
2. Iowa State University, Department of Agricultural and Biosystems Engineering, 3167 NSRIC Building, Ames, IA 50011.
3. Corresponding author. Phone: (515) 294-4019. Fax: (515) 294-6633. E-mail: karosent@iastate.edu