1. Division of Medicinal and Natural Products Chemistry, and Center for Biocatalysis and Bioprocessing, College of Pharmacy, University of Iowa, Iowa City, Iowa 52242, USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Route 1, Box 2021, Miles City, Montana 59302, and Department of Animal Science, Kildee Hall, Iowa State University, Ames, Iowa 50011