1. AgResearch Ltd., Ruakura Research Centre, Private Bag 3123, Hamilton 3240, New Zealand; Cawthron Institute, Private Bag 2, Nelson, New Zealand; HortResearch Ltd., Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand; Chemistry Department, The University of Waikato, Private Bag 3105, Hamilton, New Zealand; and National Veterinary Institute, P.B. 8156 Dep., NO-0033 Oslo, Norway