1. Dep. of Plant AgricultureUniv. of Guelph Guelph Ontario N1G 2W1 Canada
2. Dep. of Agronomy and Dep. of StatisticsUniv. of Kentucky Lexington KY 40546‐0091
3. Biometrics and Statistics UnitInternational Maize and Wheat Improvement Center (CIMMYT) Lisboa 27, Apdo. Postal 6‐641 06600 Mexico D.F. Mexico