1. Department of Animal and Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg 24060
2. Institute of Biological, Environmental, and Rural Sciences, Aberystwyth University, Aberystwyth SY23 3FG, UK
3. ADAS Rosemaund, Preston Wynne, HR1 3PG, UK
4. Sustainable Livestock Systems Group, Scottish Agricultural College, Edinburgh EH9 3JG, UK