Received for publication 4 June 1987. Journal paper no. A6-111-87 of the Lincoln Univ. Agricultural Experiment Station. Trade names are mentioned with the understanding that no discrimination is intended and no endorsement by the authors or Lincoln Univ. is implied. Research supported in part by USDA funds allocated to the Univ. Agriculture Experiment Station. 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.