Pamela L Ruegg1, Christina S Petersson-Wolfe2. 1. Department of Animal Science, Michigan State University, 474 South Shaw Lane, East Lansing, MI 48864, USA. Electronic address: plruegg@msu.edu. 2. Department of Dairy Science, Virginia Tech, 175 West Campus Drive, Blacksburg, VA 24061, USA. Electronic address: milk@vt.edu.