Received for publication 14 Aug. 1985. Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the USDA and does not imply its approval to the exclusion of other products or vendors that may also be suitable. I gratefully acknowledge Roxanne Wiseman and Randall Cubbedge for technical assistance and Victor Chew for conducting the statistical analysis. 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.