Received for publication 31 Aug. 1984. Paper No. 9489 of the Journal Series of the North Carolina Agr. Res. Serv., Raleigh, N.C. The use of trade names in this publication does not imply endorsement by the NCARS of products named, not criticism of similar ones not mentioned. The authors wish to acknowledge the technical assistance of Steve M. Worthington and William Newall. 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.