Present address: Research Station, Agriculture Canada, Harrow, Ont., Canada NOR 1G0.
Received for publication November 26, 1982. The authors gratefully acknowledge the statistical advice of I. McMillan, University of Guelph and V. A. Dirks, Agriculture Canada. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.