Field experiments were conducted near Hays, KS in 2007 and 2008 to evaluate the effects of single and sequential postemergent applications of tribenuron on broadleaf weed control and crop response in tribenuron-resistant sunflower. Weeds were acetolactate-synthase–susceptible biotypes of kochia, puncturevine, Russian thistle, and tumble pigweed in 2007 and puncturevine, redroot pigweed, and tumble pigweed in 2008. Tribenuron at 18 g ai ha−1 applied early POST with methylated seed oil (MSO) provided > 96% control of all species in 2007 and 92 and 99% control of redroot pigweed and puncturevine, respectively, but only 69% control of tumble pigweed in 2008. Early-POST tribenuron at 9 g ha−1 and late-POST tribenuron at 18 g ha−1 generally provided less weed control compared to early-POST tribenuron at 18 g ha−1. Sequential applications slightly improved redroot pigweed and tumble pigweed control in 2008 compared to single applications of tribenuron. Some tribenuron treatments caused transitory crop injury, but imazamox at 35 g ha−1 caused 24 to 44% crop injury at 7 d after treatment and permanent crop stunting in 2007. Significant yield losses occurred with imazamox and single treatments of tribenuron in 2008. Collectively, tribenuron at 18 g ha−1 alone can provide satisfactory control of the evaluated broadleaf weed species when applied to appropriate weed sizes, and this rate does not cause significant injury to tribenuron-resistant sunflower, regardless of the crop size.
Nomenclature: Imazamox; tribenuron; kochia, Kochia scoparia (L.) Schrad.; puncturevine, Tribulus terrestris L.; redroot pigweed, Amaranthus retroflexus L.; Russian thistle, Salsola iberica (Sennen & Pau) Botch. ex; tumble pigweed, Amaranthus albus L.; sunflower, Helianthus annuus L. ‘Pioneer 63N81 or 63N82’.