1. Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA
2. Middleburg Agricultural Research and Extension Center, 5527 Sullivans Mill Road, Middleburg, Virginia 20117, USA
3. Department of Animal Sciences, Biometrics Program, University of Maryland, College Park, Maryland 20502, USA
4. Equine Studies Group, WALTHAM Centre for Pet Nutrition, Melton Mowbray, UK.