1Centro de Investigación en Ciencias Agropecuarias of the Universidad Autónoma del Estado de México, Instituto Literario No 100, C.P. 50000, Toluca, Mexico
2Institute of Ecology and Resource Management, University of Edinburgh, West Mains Road, Edinburgh EH9 3JG, Scotland, UK
3International Livestock Research Institute (ILRI), PO Box 30709, Nairobi, Kenya