Associate Professor. Present address: University of Nebraska-North Platte Station, Route 4, Box 46A, North Platte, NE 69101.
Received for publication 5 Mar. 1984. Published as Paper Number 7427, Journal Series, Nebraska Agricultural Experiment Station. 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.