Received for publication 28 Sept. 1988. Paper no. 11830 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the NCARS or USDA and does not imply its approval to the exclusion of other products or vendors that may also be suitable. We gratefully acknowledge the technical assistance of R.R. Horton, Jr. and J. Mather. 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.