In current and next-generation weed control technologies, sequential applications of contact and systemic herbicides for postemergence control of troublesome weeds are needed to mitigate the evolution of herbicide resistance. A clear understanding of the impact auxin herbicide symptomology has on Palmer amaranth groundcover will aid optimization of sequential herbicide applications. Field and greenhouse experiments were conducted in Fayetteville, AR, and a laboratory experiment was conducted in Lonoke, AR, in 2020 to evaluate changes in Palmer amaranth groundcover following an application of 2,4-D and dicamba with various nozzles, droplet sizes, and velocities. Field experiments utilized three nozzles: Extended Range (XR), Air Induction Extended Range (AIXR), and Turbo TeeJet® Induction (TTI), to assess the effect of spray droplet size on changes in Palmer amaranth groundcover. Nozzle did not affect Palmer amaranth groundcover when dicamba was applied. However, nozzle selection did impact groundcover when 2,4-D was applied; the following nozzle order XR > AIXR > TTI reduced Palmer amaranth groundcover the most in both site-years of the field experiment. This result (XR > AIXR > TTI) matches percent spray coverage data for 2,4-D and is inversely related to spray droplet size data. Rapid reductions of Palmer amaranth groundcover from 100% at time zero to 39.4% to 64.1% and 60.0% to 85.8% were observed 180 min after application in greenhouse and field experiments, respectively, regardless of herbicide or nozzle. In one site-year of the greenhouse and field experiments, regrowth of Palmer amaranth occurred 10,080 min (14 d) after an application of either 2,4-D or dicamba to larger than labeled weeds. In all experiments, complete reduction of live Palmer amaranth tissue was not observed 21 d after application with any herbicide or nozzle combination. Control of Palmer amaranth escapes with reduced groundcover may potentially lead to increased selection pressure on sequentially applied herbicides due to a reduction in spray solution contact with the targeted pest.

Glyphosate, alone and in combination with other herbicides or additives, was evaluated as a spot or shielded treatment for the control of silverleaf nightshade in cotton in three experiments over four years. Glyphosate used at 10.8 g ae/L in 1985 on dryland cotton controlled 81 to 98% of the silverleaf nightshade, but cotton lint yields were reduced by all treatments compared to the weed-free check. Glyphosate used at 7.2 g ae/L in 1986 on irrigated cotton controlled 6 to 83% of the silverleaf nightshade, and lint yields were reduced only when multiple spot applications were made. Spot applications of glyphosate were effective for silverleaf nightshade control when applied 7 wk after crop emergence in 1988. Weed control and cotton injury were lower with shielded applications than spot applications. In 1989, retreatments during a season or the next year resulted in 95% or greater silverleaf nightshade control. Cotton yield increases from herbicide treatment were not observed in 1988; however, yield increases were observed in 1989 when spot treatments were applied to the more successful 1988 plots.

A surface-roller wiper that rotates by direct contact with the ground was developed to apply herbicides to broadleaf weeds in turf. The free-wheeling roller 1.8 m wide consists of a 30-cm diam PVC pipe covered with a 1.3-cm thick carpet The herbicide is delivered by gravity to the carpet through a pipe manifold. Broadleaf weeds in turf were effectively controlled with 2,4-D roller applied once as a 2% solution (2.2 kg ae/ha) or twice as a 1% solution (1.1 kg/ha). Herbicide drift was avoided with the roller applicator as indicated by lack of injury to tomato plants downwind from 2,4-D roller applied as a 5% (5.6 kg/ha) solution.

A small -plot, pneumatic applicator was developed to apply either granular herbicide formulations or liquid herbicides impregnated on commercial dry fertilizer to field research plots. Pre-weighed herbicide samples poured into a stainless steel, motor-driven belt, cone seeder are automatically sput into four subsamples in a four-outlet rota seed distributor run by a 12-volt electric motor, and delivered to four plastic venturi chambers. A gasoline engine-powered leaf blower forces air through a 10-cm diam polyvinylchloride (PVC) pipe to the input sides of the venturi chambers. The forced air stream carries the herbicide samples through 2.5-cm diam flexible plastic tubing to four deflector shields mounted 61 cm apart on a fiberglass boom. The applicator uniformly delivers granules up to 400 kg/ha in a 2.5-m wide swath.

A rotary wiper was developed to apply herbicides selectively to weeds beneath horticultural crops such as fruit trees and ornamentals. Herbicide is delivered internally through a circular pipe manifold imbedded in a rotary polyurethane foam disk, thus avoiding herbicide drift. A free-wheeling bumper wheel, larger than the wiper disk, is mounted just above the disk causing it to move around a tree trunk or shrub on contact. The rotary wiper effectively applied herbicides to weeds within the row of fruit trees.

A new machine consisting of a detachable hopper of containerized granules with a preselected circular orifice to regulate flow of granules, and a revolving hollow-cone spreader turned by an electric motor, was tested. Flow of herbicide granules at typical use rates was linearly correlated with orifice diameter. Coefficients of determination for flow rates of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine], and norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3 (2H)-pyridazinone] were 0.96 or greater. Based on patterns of distribution of commercial trifluralin granules parallel and perpendicular to a linear path, the machine could be used to apply herbicide granules in row crops. Control of johnsongrass [Sorghum halepense (L.) Pers. # SORHA] with alachlor plus oryzalin [4-(dipropylamino)-3,5-dinitrobenzenesulfonamide] applied preemergence (pre) as sprays and granules did not differ significantly in soybean [Glycine max (L.) Merr. ‘Forrest′]. Trifluralin granules applied preplant incorporated (ppi) at 1.1 kg ai/ha controlled 80 to 95% of the johnsongrass and doubled soybean yield compared to the cultivated control plot. Norflurazon plus alachlor pre at 1.7 to 2.2 plus 2.2 to 3.0 kg ai/ha controlled annual grasses and prickly sida (Sida spinosa L. # SIDSP) equally with sprays and granules.