Field experiments were conducted in 1983 and 1984 at two locations to determine the influence of various densities and durations of kochia interference in sunflower. Sunflower achene yield and dry weight were reduced by all densities of kochia, averaged over locations and years. Season-long competition by kochia densities of 0.3, 1, 3, and 6 plants/m of row decreased sunflower achene yield 7, 10, 20, and 27%, respectively. Sunflower achene yield and sunflower dry weight decreased as weeks of kochia competition increased. Only 2 weeks of kochia competition after sunflower emergence decreased sunflower achene yield 6%. Sunflower achene yield loss increased as the duration of kochia competition increased. Sunflower 200 achene weight, oil content, and plant height were not influenced by various densities or durations of kochia competition.

A biotype of kochia resistant to chlorsulfuron has been isolated from a Kansas wheat field that had been treated with chlorsulfuron for five consecutive years. In greenhouse tests, the biotype also was resistant to both preemergence and postemergence herbicides with the same mode of action including imazapyr, CGA-131036, the methyl ester of metsulfuron, DPX-M6316, DPX-L5300, and the methyl ester of sulfometuron. The degree of cross resistance varied among the herbicides tested. Herbicides with different modes of action, including atrazine, bromoxynil, MCPA, and diuron, effectively controlled the resistant biotype in the greenhouse.