1. Rothamsted Research; North Wyke Okehampton EX20 2SB Devon UK
2. College of Life and Environmental Sciences; University of Exeter; Exeter EX4 4RJ Devon UK
3. Rothamsted Research, Sustainable Soils and Grassland Systems; Harpenden AL5 2JQ Hertfordshire UK
4. School of Veterinary Sciences; University of Bristol; Langford BS40 5DU Somerset UK
5. AgResearch, Ruakura Research Centre, Farm Systems North; Private Bag 3123 Hamilton 3214 New Zealand