Livestock Issues Research Unit, Agriculture Research Service, USDA, Lubbock, TX, USA. sdowd@lbk.ars.usda.gov|Livestock Issues Research Unit, Agriculture Research Service, USDA, Lubbock, TX, USA. sdowd@lbk.ars.usda.gov|Livestock Issues Research Unit, Agriculture Research Service, USDA, Lubbock, TX, USA. sdowd@lbk.ars.usda.gov|Livestock Issues Research Unit, Agriculture Research Service, USDA, Lubbock, TX, USA. sdowd@lbk.ars.usda.gov|Livestock Issues Research Unit, Agriculture Research Service, USDA, Lubbock, TX, USA. sdowd@lbk.ars.usda.gov