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