Received for publication 18 Mar. 1985. Journal article no. TA 20422 of the Texas Agr. Expt. Sta. Mention of a trade name or proprietary production does not constitute a guarantee or a warranty of the product by Texas A&M Univ. and does not imply its approval to the exclusion of other products that may also be suitable. 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.