The efficacy of herbicides applied broadcast by hand with a soil carrier or sprayed with water carrier were compared for Italian ryegrass control in winter wheat in central Oklahoma. Italian ryegrass was controlled 77 to 100% with BAY SMY 1500, chlorsulfuron, and CGA-131036 applied with both carriers. Metribuzin applied POST with 187 L ha–1 water was less phytotoxic to Italian ryegrass and wheat than with 200 kg ha–1 soil. All herbicides applied with 200 kg ha–1 silt loam soil as carrier at two locations controlled Italian ryegrass 96 to 100%. Increasing herbicide rate, soil carrier quantity, or broadcasting soil-carried herbicide in two directions rather than one did not significantly improve herbicide efficacy over that of conventional spraying.

High herbicide costs and uncertainty about annual weed control at planting have limited adoption of no-till spring wheat production systems in the northern Great Plains. Chlorsulfuron, metsulfuron, and CGA-131036 at 10 to 20 g ai ha–1 plus nonionic surfactant generally controlled both emerged kochia and wild mustard equally well (>80%) whether or not combined with glyphosate at 250 g ha–1 plus nonionic surfactant. In two of three trials persistent phytotoxic residues of these sulfonylurea herbicides in soil controlled both weeds better in midseason and early summer 1 yr after treatment than did glyphosate, which has only foliar activity. While the absolute net returns of different treatments varied among herbicides, relative net returns were insensitive to changes in either herbicide or wheat price. Herbicide use tended to boost net returns for no-till spring wheat in years with good weather but depressed net returns in a drought year. Chlorsulfuron at 10 and 20 g ha–1 increased net returns in all three trials. Metsulfuron and combinations of either metsulfuron or chlorsulfuron with glyphosate had variable effects on net returns.

The concentration of three sulfonylurea herbicides in soil was determined by a modified petri dish bioassay procedure. The method involved planting pregerminated seed of selected species in petri dishes containing 65 to 100 g of treated soil and measuring the radicle lengths after 24 h. Chlorimuron was detected in two soils at 0.002 μg g–1 using either corn, sorghum, or sicklepod as the assay species. Chlorsulfuron and CGA-131036 were detected in soils at 0.001 μg g–1 using corn, and chlorsulfuron was detected at the same level using sicklepod. In contrast to other methods, the bioassay procedure described could be completed in 48 h, including pregermination of the seed, growth of the plants on treated soil, and plant measurements.

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.

Field experiments were established to evaluate preemergence and postemergence herbicides for control of triazine-resistant smooth pigweed and common lambsquarters in no-till corn. When applied preemergence, alachlor in the microencapsulated formulation controlled smooth pigweed better than the emulsifiable concentrate formulation and better than either metolachlor or pendimethalin. These herbicides applied preemergence did not control common lambsquarters consistently. Pendimethalin controlled both triazine-resistant species when applied as a sequential treatment of a preemergence and an early postemergence application. Control of triazine-resistant smooth pigweed and common lambsquarters was excellent when dicamba was applied early postemergence in treatments containing alachlor, metolachlor, or pendimethalin applied preemergence and/or early postemergence. Thiameturon and CGA-131036 controlled triazine-resistant smooth pigweed with acceptable crop tolerance. Thiameturon also controlled common lambsquarters, but control was unacceptable with CGA-131036.

Chlorsulfuron or metsulfuron applied POST at 10 g ai ha–1 at the four-leaf stage prevented yellow foxtail from forming seed, whereas seed production per pot was 53 to 77% of untreated checks following POST tribenuron or CGA-131036 treatments in greenhouse experiments. Chlorsulfuron applied POST at the two- and four-leaf stage severely reduced total seed production of yellow foxtail primarily by increasing plant mortality, rather than only reducing seed production per plant. Chlorsulfuron at 10 g ha–1 applied at the six- and eight-leaf stages decreased total seed production less than at the two earlier stages, by reducing both plant mortality and seed production per surviving plant. Yellow foxtail was much more susceptible than green foxtail to either chlorsulfuron or metsulfuron.