Received for publication 20 June 1984. Journal Article No. 11444 of the Michigan State Univ. Agr. Expt. Sta. This work was supported by USDA Special Grant 7503G. The authors gratefully acknowledge the assistance of J. Scott Cameron, LuAnn Kent, Jerri Gillett, Kathy Morimoto, John Nelson, and Jan Roueche. 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.