Adjuvants influence weed control and crop tolerance provided by postemergence (POST) herbicides. Adjuvant effects on weed control and corn and soybean tolerance with CGA-248757 and flumiclorac applied alone and soybean tolerance with CGA-248757 and flumiclorac applied in combination with imazethapyr or CGA-277476 were evaluated. Velvetleaf control in the greenhouse and common lambsquarters, redroot pigweed, and common ragweed control in the greenhouse and field with CGA-248757 and flumiclorac were increased by the addition of an adjuvant. However, corn and soybean tolerance to these herbicides was not affected by adding an adjuvant. Laboratory studies suggest enhanced weed control from the addition of an adjuvant resulted from an increase in CGA-248757 and flumiclorac foliar absorption. In greenhouse studies, velvetleaf, common lambsquarters, and redroot pigweed control with CGA-248757 plus imazethapyr; velvetleaf control with flumiclorac plus imazethapyr; redroot pigweed and common ragweed control with CGA-248757 plus CGA-277476; and velvetleaf, redroot pigweed, and common ragweed control with flumiclorac plus CGA-277476 increased by adding an organosilicone adjuvant when compared with adding a nonionic surfactant (NIS). However, in field evaluations of CGA-248757 or flumiclorac plus imazethapyr, the addition of NIS or an organosilicone adjuvant resulted in equivalent soybean injury, common lambsquarters and redroot pigweed control, and soybean yield. Yet in tank mixtures of CGA-248757 or flumiclorac plus CGA-277476, the addition of an organosilicone adjuvant increased redroot pigweed control and soybean yield compared to adding NIS.

Field studies were conducted from 1986 through 1989 to determine the feasibility of using clomazone in a winter wheat-corn-fallow rotation. Clomazone at 1.1 kg ai/ha did not control emerged downy brome in autumn nor did it control redroot pigweed and tumble pigweed that emerged after a spring application of paraquat at 0.6 kg ai/ha. However, clomazone plus atrazine at 0.6 plus 2.2 kg ai/ha provided weed control in winter wheat stubble from October to corn harvest 12 mo later. Corn yields from this treatment were equal to or greater than the conventional practice of applying atrazine plus glyphosate at 2.2 plus 0.4 kg ae/ha in the fall followed by a spring application of metolachlor plus dicamba at 2.8 ai plus 0.3 kg ae/ha. When kochia densities were high, clomazone plus atrazine controlled triazine-resistant kochia better than metolachlor plus dicamba. Atrazine at 2.2 kg/ha applied in autumn followed by clomazone at 0.8 kg/ha in April generally caused more corn injury than when the same rate of clomazone was applied in autumn with the atrazine. The percent of bleached corn plants varied with planter, year, time of application, and rate of clomazone applied. The optimum time to apply clomazone at 0.6 kg/ha was in autumn with atrazine. This treatment caused some corn bleaching but produced the highest corn yield over 3 yr. In a dry autumn and spring, less clomazone injury occurred with a planter equipped with row cleaners than those without. The injury pattern associated with planters was inconsistent when precipitation was average and no injury occurred in a wet autumn and spring.

In west central Minnesota the extent and duration of weed seed shed was measured for two years in corn that received cultivation but no herbicides. Percentage of seed production represented by viable (filled) seeds was about 79% for green foxtail, 68% for wild mustard, 49% for Pennsylvania smartweed, 48% for common lambsquarters, and 35% for redroot pigweed. Percentage viable seeds varied from 11% in 1993 to 59% in 1994 for redroot pigweed, but was more stable for other species. Seed shed commenced in late August in a cool year (1993) and early August in a warm year (1994). Average growing degree days (base 10 C) from corn planting until 25% seed shed was 983 for common lambsquarters, 984 for wild mustard, 1004 for Pennsylvania smartweed, and 1034 for both green foxtail and redroot pigweed. Brief weather events, such as wind storms, dispersed large percentages of total seed production within a single day. More than one-fifth of all viable seeds of green foxtail, redroot pigweed, and common lambsquarters were retained by the seedheads and dispersed by combines at harvest. In contrast, seeds of early-maturing species, such as wild mustard, were completely dispersed before corn harvest in the warmer year, but one-third of seeds were retained by seedheads at harvest in the cooler year. Measurement of seed shed was compared using five seed trap designs. The preferred design consisted of a 10-cm-diam plastic cup, whose bottom was replaced by a brass screen, and the entire unit attached to a small wooden stake for support. This design provided, on average, the highest estimates of seed production, least among-replication variability, highest correlation with weed population density and aboveground dry-weight, lowest assembly cost, and greatest ease for sample access and seed processing.

Sweet corn was grown with a living mulch of white clover, a dead mulch of rye, and without mulch, in both till and no-till conditions. Unplanted controls were also included in the experimental design. Corn yields were highest in clover treatments early in the experiment but lowest in later years. The declining yields in the clover living mulch were related to the strip application of glyphosate which allowed establishment of perennial and biennial weeds, notably dandelion and horseweed. These overwintering weeds apparently prevented effective control of summer annuals, especially redroot pigweed, common lambsquarters and large crabgrass, by atrazine and metolachlor. Presence of a rye mulch decreased weed biomass and had no detrimental effect on corn yield. In general, corn yield was not affected by tillage, although the number of marketable ears was reduced in the no-till treatments during the drought year of 1988. The much greater weed biomass in the unplanted control treatments showed the importance of crop competition for weed control in sweet corn cropping systems.

Six commercial petroleum oil (PO) adjuvants differed in their enhancement of imazethapyr phytotoxicity to green foxtail and kochia but not to redroot pigweed, when imazethapyr was applied postemergence at 17 g ha–1. Imazethapyr at 5 g ha–1 plus 27 combinations of three oils and nine emulsifiers reduced kochia growth by 61 to 88%. Optimum emulsifier concentration in a PO adjuvant ranged from 15 to 20% (v/v). All PO adjuvants tested produced fair to excellent oil-inwater emulsions with imazethapyr; however, emulsion stability did not correlate with weed control.

In 1987 and 1988, effects of full-season interference of individual weeds on cotton yield were measured with area of influence procedures. In 1987, one spurred anoda, common cocklebur, large crabgrass, jimsonweed, common lambsquarters, redroot pigweed, common ragweed, or prickly sida plant per 3 m of row reduced cotton yield between 1% and 7%. Sicklepod did not cause a detectable yield loss. Redroot pigweed, common cocklebur and common ragweed caused 7%, 6% and 5% yield loss, respectively, in 1987. In row weed influence varied from 17 cm (large crabgrass interference) to 86 cm (common cocklebur interference). In 1988, yield losses by individual weeds ranged from 3% to 27%. Common cocklebur, jimsonweed, and common ragweed reduced cotton yields 28%, 15% and 12%, respectively. Spurred anoda and common cocklebur influenced 160 cm and 136 cm of cotton row, respectively. Sicklepod influenced only 47 cm of cotton row.

Field experiments were conducted in 1996, 1997, and 1998 to evaluate weed control and corn tolerance from soil-applied RPA 201772. Treatments alone and in tank mixtures with other corn herbicides were applied preemergence (PRE) at two locations with conventional tillage and at early preplant (EPP) and PRE application timings in no-tillage corn. RPA 201772 was applied alone and in tank mixtures with one-half the typical field rates of other PRE corn herbicides. In conventional tillage experiments in 1996, 1997, and at one location in 1998, all treatments containing RPA 201772 provided > 90% control of common lambsquarters, redroot pigweed, common ragweed, and velvetleaf. In two no-tillage experiments, common lambsquarters and velvetleaf control was > 90%, regardless of application timing. However, control of redroot pigweed and common ragweed varied among years and application timings. Weed control was more variable from herbicide treatments applied EPP compared with the PRE application timing. Giant foxtail control in both tillage systems was rate, timing, and year dependent. RPA 201772 rates higher than 79 g/ha controlled giant foxtail > 85% at three of five locations. At one location, tank mixtures with RPA 201772 increased giant foxtail control. Corn injury occurred in one of two conventional tillage locations and at the no-tillage location in both 1996 and 1997. Injury was most commonly observed in coarse-textured soils with low clay and organic matter and was more severe with higher rates of RPA 201772. Increased corn injury was also observed when RPA 201772 was combined with acetochlor plus dicloramid or BAYFOE 5043 plus metribuzin. Corn injury from RPA 201772 occurred at application rates above the proposed rate for use on corn. In some cases, severe injury to corn reduced corn yield. Injury to corn from RPA 201772 was not unique to any tillage system and was site, year, and rate dependent.

Field studies were conducted to identify herbicides suitable for improved control of hairy nightshade, redroot pigweed, and common lambsquarters in pinto beans. Fomesafen at 0.25 kg ai ha-1 did not adequately control these weeds. Clomazone at 0.5 kg ai ha-1 controlled common lambsquarters but only suppressed the growth of redroot pigweed and hairy nightshade. Ethalfluralin at 0.84 to 1.1 kg ai ha-1 gave excellent initial control of these weeds but did not control later flushes of hairy nightshade. Imazethapyr applied PPI or POST at 50 to 75 g ai ha-1 controlled hairy nightshade, redroot pigweed, and common lambsquarters throughout the growing season. Imazethapyr combined with ethalfluralin gave superior weed control and resulted in greater yields than the most commonly used herbicides in pinto beans in western Canada.

Field research was conducted at Arlington, WI, and Janesville, WI, in 1986 and 1987 to evaluate the effect of conventional-tillage, chisel plow, and no-till systems on the density and control of annual weed species in solid-seeded soybean. Common lambsquarters densities were not greatly influenced by tillage systems, but redroot pigweed densities were generally highest in the chisel plow system. Conventional tillage always had greater velvetleaf densities than no-till and no-till always had greater giant foxtail densities than conventional tillage. Giant foxtail and redroot pigweed became more difficult to control when tillage was reduced, while velvetleaf became less of a problem. This response was not observed with all herbicide treatments evaluated and several herbicide treatments provided excellent weed control. Soybean yield was not affected by tillage systems under weed-free conditions and differences in soybean yield appeared to be due to differences in weed control.

Field experiments were conducted near Scottsbluff, NE, to evaluate the selectivity of sequential POST applications of ethofumesate and triflusulfuron for weed control in sugarbeet. When applied alone ethofumesate and triflusulfuron reduced common lambsquarters, hairy nightshade, and redroot pigweed density while triflusulfuron also reduced kochia density compared with an untreated control. Combining ethofumesate with desmedipham plus phenmedipham increased sugarbeet injury compared with desmedipham plus phenmedipham. Combining triflusulfuron with desmedipham plus phenmedipham increased kochia control but did not increase sugarbeet injury over that obtained with desmedipham plus phenmedipham. Weed control and sugarbeet injury from herbicides generally were unaffected by application in 97 or 182 L/ha of water carrier. However, herbicide control of hairy nightshade in 1993 was greater with 97 than 182 L/ha of water carrier.

Field experiments were conducted in 1988 and 1989 to (i) evaluate annual weed control with nicosulfuron applied at rates of 17 to 70 g ai/ha with several additives and (ii) evaluate annual weed control with nicosulfuron applied at rates of 17 to 105 g/ha at three corn growth stages. In 1988, in the adjuvant study, giant foxtail control increased linearly with no additive or with urea ammonium nitrate as the rate of nicosulfuron increased. Petroleum oil concentrate, nonionic surfactant, and a combination of either petroleum oil concentrate or nonionic surfactant with urea ammonium nitrate applied with nicosulfuron increased giant foxtail control to 90% or greater regardless of rate both years. Nicosulfuron at all rates with no additive and in combination with all additives controlled 93% or more of redroot pigweed and Pennsylvania smartweed both years. Corn grain yield was related to the level of giant foxtail control. In the rate by corn growth stage study, giant foxtail, redroot pigweed, and Pennsylvania smartweed control was 90% or greater regardless of nicosulfuron rate or application stage both years. Corn grain yield was related more to the duration of weed competition than the level of weed control with grain yield 8 to 12% lower with nicosulfuron applied at the V7 growth stage compared with the V3 or V5 growth stage.

Weed control in cole crops and onion by postemergence spraying with liquid ammonium nitrate was studied to determine its feasibility under Fraser Valley, British Columbia conditions. Ammonium nitrate solutions (7.5, 10, 15 and 20% N) were applied at 800 L ha−1 on warm, sunny days. Shepherd's-purse, low cudweed, redroot pigweed and ladysthumb were susceptible to this treatment; common lambsquarters, common purslane and annual bluegrass were tolerant. In order to determine the effect of weed density on weed control, several shepherd's-purse densities were established in broccoli and sprayed with ammonium nitrate solution (20% N; 800 L ha−1). Two days after treatment, weed control was 87.2% for an initial density of 500 shepherd's-purse plants m−2 (1987) and 76.1% for an initial density of 988 plants m−2 (1988). The seedlings that survived the treatment were sufficiently set back that they did not cause any adverse effect on crop yield. In a separate study, cabbage and onion were found to be tolerant to the ammonium nitrate treatment. Initially, cauliflower, broccoli and Brussels sprouts growth was slightly inhibited by the ammonium nitrate treatment but the plants recovered and crop yield was not affected.

A 2-yr field study was conducted to evaluate the efficacy of ICIA-0051 for the control of annual grass and broadleaf weed species in conventional tillage corn. Treatments consisted of postemergence applications of ICIA-0051 alone and in combination with cyanazine or atrazine. ICIA-0051 at 0.6 kg ha-1 alone and in combination with cyanazine or atrazine at 1.1 kg ha-1 controlled large crabgrass, yellow foxtail, fall panicum, common lambsquarters, and redroot pigweed effectively (over 90%). The addition of cyanazine or atrazine at 1.1 kg ha-1 to the lowest rate (0.3 kg ha-1) of ICIA-0051 also improved large crabgrass, yellow foxtail, and fall panicum control by 9, 7, and 26%, respectively. None of the treatments of ICIA-0051 except the highest rate (1.1 kg ha-1) reduced either silage or grain yields of corn.

Weed control decision rules were derived for the application of postemergence herbicides to control pigweed species in soybean. Field experiments were conducted at two locations in 1992 and 1993 to evaluate soybean-pigweed interference. A damage function was determined that related yield loss to time of pigweed emergence, density, and soybean weed-free yield. A control function described pigweed species response to variable doses of imazethapyr and thifensulfuron. The integration of these two functions formed the basis of an economic model used to derive two weed control decision rules, the biologist's “threshold weed density” and the economist's “optimal dose.” Time of weed emergence had a more significant role than weed density in the economic model. Later-emerging pigweed caused less yield loss and therefore, decision rules lead to overuse of herbicides if emergence time is not considered. The selected herbicide dose influenced the outcome of the control function. Depending on the desired level of weed control, a herbicide could be chosen to either eradicate the escaped weed species (label or biologically-effective doses) or reduce the growth of the weed species and thereby offset interference (optimal dose). The development of a biologically-effective dose by weed species matrix was recommended. Decision rules should not be utilized as an exclusive weed management strategy but rather as a component of an integrated weed management program.