1. New Zealand Pastoral Agriculture Research Institute Ltd., Ruakura Agricultural Research Centre, Private Bag 3123, Hamilton, New Zealand; Chemistry Department, The University of Waikato, Private Bag 3105, Hamilton, New Zealand; Institute of Environmental Science and Research Ltd., Kenepuru Science Centre, P.O. Box 50-348, Porirua, New Zealand; and Cawthron Institute, 98 Halifax Street, Private Bag 2, Nelson, New Zealand