1. Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK;
2. CSIC, Estacion Agricola Experimental, Aptdo 788, 24080 Leon;
3. University of Leon, Department of Animal Production, 24071 Leon, Spain; and
4. Biomathmatics and Statistics Scotland, Craigiebuckler, Aberdeen AB15 8QH, UK