1. Section of Animal Production Systems, School of Veterinary Medicine, University of Pennsylvania, 382 West Street Road, Kennett Square, Pennsylvania 19348, Institut de Recherche pour le Développement, 213 Rue La Fayette, 75 480 Paris, Cedex 10 France, Cornell University, Ithaca, New York 14853, University of Maryland, College Park, Maryland 20742, and Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061