1. Wageningen Institute of Animal Sciences, Wageningen Agricultural University, 6700 AH, Wageningen, The Netherlands;
2. Institute of Grassland and Environmental Research, North Wyke, Okehampton, EX20 2SB, UK; and
3. T.N.O. Nutrition and Food Research Institute, Department of Animal Nutrition and Meat Technology (ILOB), 6700 AA, Wageningen, The Netherlands