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A false cleavers population that survived treatment with triasulfuron/bromoxynil in 1996 was identified in central Alberta, Canada, in a field that had been treated with acetolactate synthase (ALS) inhibitors in 3 of the previous 6 yr. In greenhouse studies, this biotype was highly resistant to the ALS inhibitors triasulfuron, thifensulfuron/tribenuron, and sulfometuron and moderately resistant to imazethapyr; GR50, values were > 16, > 5, > 1.0, and 9.9, respectively. In addition, cross-resistance was identified to the auxin-type herbicide quinclorac (GR50 value > 6.7) but not to fluroxypyr (GR50 value 1) or MCPA/mecoprop/dicamba. Quinclorac had not been used previously in this field. Analysis of ALS extracted from the resistant biotype and a susceptible biotype from a nearby location indicated that resistance to ALS inhibitors was due to an altered target site with reduced sensitivity to a broad range of ALS inhibitors. The ALS I50 values for triasulfuron, metsulfuron, chlorsulfuron, thifensulfuron, and imazethapyr were 36, 34, 92, 96, and 14 times higher, respectively, for the resistant compared to the susceptible biotype. The mechanism of resistance to quinclorac is unknown. This is the first report of high-level herbicide resistance in this weed species.

Greenhouse and field experiments were conducted to assess the effectiveness of herbicide tank mixtures for control of sulfonylurea (SU)-resistant and -susceptible kochia biotypes. Kochia control at three growth stages was evaluated for fluroxypyr, dicamba, 2,4-D, bromoxynil, and bromoxynil plus MCPA. Control decreased as kochia size increased, especially at less than full labeled herbicide rates. Treatments containing bromoxynil provided best overall control in the greenhouse but were less effective on larger kochia plants in the field. Dicamba and fluroxypyr provided similar, effective kochia control both in greenhouse and field experiments. 2,4-D did not provide acceptable kochia control regardless of formulation, rate, or application timing. All kochia biotypes responded similarly to non-SU herbicides. In the field, inclusion of an SU herbicide in the treatment generally resulted in no increased R-kochia control above that provided by the non-SU herbicide. R-kochia population dynamics can affect selection of alternative control strategies.

Greenhouse and field experiments were conducted to evaluate herbicidal efficacy on tropical soda apple and bahiagrass. Acifluorfen, clopyralid, dicamba, fluroxypyr, picloram, and triclopyr were the most effective postemergence herbicides, providing > 90% control of tropical soda apple plants with no injury to bahiagrass 145 days after treatment (DAT). Glyphosate and imazapyr resulted in effective (> 90%) control of both seedling and mature tropical soda apple plants. However, these herbicides caused severe (> 90%) damage to bahiagrass. Control of tropical soda apple with 2,4-D, AC-263,222, diuron, fomesafen, lactofen, MSMA, sulfometuron, and triasulfuron was unacceptable (< 90%).