Herbicide efficacy, coverage, and retention were evaluated for spray applied through Drift Guard, Turbo TeeJet, AI TeeJet, and TurboDrop drift-reducing nozzles compared to a conventional flat-fan nozzle. Percentage spray coverage detected on water-sensitive cards was greater for conventional and Drift Guard nozzles than for Turbo TeeJet, AI TeeJet, and TurboDrop nozzles. Spray without adjuvants was retained better by redroot pigweed for treatments applied with conventional and Drift Guard nozzles than Turbo TeeJet, AI TeeJet, and TurboDrop nozzles. However, spray with adjuvants was retained similarly for all nozzle types when averaged over spray adjuvant and two weed species. The efficiency of spray retention was greater for spray applied in 47 than in 190 L/ha spray volume for all nozzles. Paraquat and glyphosate, representing contact and translocated herbicides, respectively, provided similar grass species control for all nozzle types, regardless of spray volume. Paraquat and glyphosate were also equally or more effective in 47 compared to 190 L/ha spray volume.

Field experiments were conducted to examine the influence of spray volume on glyphosate efficacy in relation to glyphosate rate, formulation, ammonium sulfate addition, and type of sprayer nozzle. Using several grass species it was shown that glyphosate efficacy increased as spray volume decreased from 190 to 23 L/ha. To obtain equal efficacy, glyphosate rates can be reduced by at least one-third when glyphosate is applied in 23 or 47 L/ha spray volume compared with 94 or 190 L/ha. The amount of surfactant in formulated glyphosate at 35 to 140 g ae/ha rates was insufficient when glyphosate was applied in 94 or 190 L/ha spray volumes. Additional surfactant enhanced glyphosate efficacy at these rates when applied in 94 or 190 L/ha spray volume, but efficacy was still less than when applied in 23 or 47 L/ha without additional surfactant. Thus, low spray volumes maximized glyphosate efficacy primarily through high herbicide concentration in the spray deposit and reduced salts from the carrier to antagonize efficacy. Glyphosate applied in 23 L/ha spray volume with drift-reducing nozzles provided control equal to that provided by glyphosate applied with standard flat-fan nozzles. Grass control also was equal from several glyphosate formulations that contained surfactants, regardless of spray volume.

The effects of surfactants on retention, absorption, and efficacy were determined for bromoxynil, 2,4-D amine, and glyphosate on kochia and Russian thistle. Bromoxynil, 2,4-D amine, and glyphosate retention were similar for both species. Surfactants improved spray retention on kochia and Russian thistle compared with spray mixtures without surfactant. Herbicides mixed with allinol 810-60 surfactant were generally retained better than with MON 0818, oxysorbic 20, or R-11 surfactant. Bromoxynil phytotoxicity was not affected by surfactants, and all surfactants equally enhanced 2,4-D amine phytotoxicity. Glyphosate phytotoxicity to kochia was enhanced only by MON 0818 and oxysorbic 20, and phytotoxicity to Russian thistle was enhanced only by MON 0818. Bromoxynil, with or without surfactants, was absorbed similarly by kochia and Russian thistle. 2,4-D amine and glyphosate absorption were greater with surfactants than without. Kochia and Russian thistle leaves had visibly similar crystalline epicuticular wax structure when plants were grown at ≤ 40% or ≥ 80% relative humidity, which did influence absorption of these herbicides.

Field experiments were conducted to determine the influence of nozzle type, spray volume, and adjuvants on herbicide efficacy. Carfentrazone and imazamox, representing contact and translocated herbicides, respectively, were equally or more effective when applied with drift-reducing TurboDrop or Drift Guard nozzles than the conventional flat-fan nozzle in 95% of the comparisons. Imazamox was equally effective when applied in 47, 94, or 190 L/ha spray volume, regardless of nozzle type or adjuvant. Carfentrazone applied in 47 L/ha spray volume controlled flax and sunflower as well as in 94 or 190 L/ha, provided urea ammonium nitrate fertilizer and either nonionic surfactant or methylated vegetable oil were included in the spray mixture. Conversely, carfentrazone with only urea ammonium nitrate was less effective when applied in 47 than in 94 or 190 L/ha spray volume. Carfentrazone and imazamox applied in low spray volumes (47 L/ha) gave consistent control when an effective adjuvant was included in the spray mixture.

Field experiments were conducted in 1999 and 2000 to evaluate reduced-rate split-applied treatments of imazamethabenz and ICIA 0604 for wild oat control in hard red spring wheat. Single herbicide treatments were applied at standard timing to wild oat at the two- to four-leaf stage. The first split-applied treatments were made when most of the wild oat had emerged but prior to the two-leaf stage. The second split-applied treatments were made after the surviving wild oat had visibly recovered and prior to the two-leaf stage of late-emerging wild oat. Split applications of imazamethabenz or ICIA 0604 allowed the herbicide rate to be reduced by half while maintaining or improving wild oat control. Both imazamethabenz and ICIA 0604 were most effective with methylated vegetable oil adjuvants. Wheat yield and net returns were greatest for half-rate split applications of imazamethabenz and ICIA 0604 in both years, likely because of less early-season wild oat competition and reduced wheat injury caused by ICIA 0604.