Received for publication 25 Sept. 1986. Contribution paper 87-87-J of the Agricultural Experiment Station, Kansas State Univ., Manhattan, KS 66506. We thank Eileen Schofield for her assitance in editing this manuscript. 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.