Received for publication 6 June 1988. Journal Paper no. J-13097 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project no. 2231. The use of trade names in this publication does not imply endorsement by the Iowa Agriculture and Home Economics Experiment Station, nor criticism of similar products not mentioned. 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.