Paper No. 9431 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina, USA. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture or North Carolina Agricultural Research Service, nor does it imply approval to the exclusion of other products that may be suitable. This paper was prepared by US Government employees as part of their official duties and legally cannot be copyrighted.