In 1985, when weed densities were low (169 plants/m2 in untreated control), imazaquin applied at 0.07 kg ai/ha early preplant controlled over 90% of all weeds before no-till planting of soybeans. In 1986 and 1987 when weed densities were higher (589 plants/m2 in untreated control), addition of 1.1 kg ai/ha or more of metolachlor to imazaquin (0.07 kg/ha) before soybean planting controlled 95% or more of the grass weeds and 83% or more of the broadleaf weeds. Imazaquin plus metolachlor applied less than 1 day after soybean planting controlled less than 70% of the emerged weeds in 1986 and 1987; common lambsquarters was most tolerant. Early preplant treatments controlled more weeds throughout the growing season than treatments applied after planting. Splitting herbicide treatments among application times generally did not increase weed control compared to single applications. Early preplant applications resulted in higher soybean densities and taller soybeans 30 days after planting in 1986 and 1987 than treatments applied after planting. Soybean yields increased as weed control increased. Weed control and soybean yields were greater with early preplant treatments than paraquat plus alachlor plus metribuzin applied preemergence in 1986 and 1987. No carryover of imazaquin residue was detected through corn bioassay in the field.

Clomazone at 0.7 kg ai/ha or more, applied early preplant, completely controlled weeds before planting of no-till soybeans. Under low weed density (57 plants/m2 in untreated control) in 1985, grass weed control was nearly complete and not affected by clomazone application time. Late-season broadleaf weed control was less with preemergence application of clomazone at 1.1 or 1.4 kg/ha than with an early preplant or early preplant-preemergence split application of the same clomazone rate. Addition of metribuzin at 0.2 kg ai/ha overcame this control deficiency. Under greater weed densities (330 plants/m2 in untreated control) during 1986 and 1987, early preplant-preemergence split applications gave the greatest control of both grass and broadleaf weeds throughout the growing season. Preemergence application of clomazone failed to completely control common lambsquarters emerged at the time of application. Early preplant applications failed to maintain redroot pigweed control throughout the season. Differences in soybean yield were attributed to differences in weed control. No significant carryover of clomazone residue was detected through greenhouse or field bioassays.

Application time did not greatly influence control of velvetleaf (Abutilon theophrasti Medik. # ABUTH) or common lambsquarters (Chenopodium album L. # CHEAL) in no-till corn (Zea mays L. ‘Pioneer 3747’) with fluorochloridone {3-chloro-4-(chloromethyl)-1-[3-(trifluoromethyl) phenyl]-2-pyrrolidinone}. Giant foxtail (Setaria faberi Herrm. # SETFA) control was reduced as much as 25% by 90 days after planting when fluorochloridone was applied early preplant rather than preemergence. Fluorochloridone at 0.8 kg/ha applied preplant or preemergence gave 83% or greater control of common lambsquarters and giant foxtail for the entire growing season. However, velvetleaf control with the same treatments was 61% or less. Fluorochloridone caused minimal corn injury. Greenhouse bioassay indicated that fluorochloridone may carry over and injure soybean [Glycine max (L.) Merr.] the year after application. Imbibition of fluorochloridone by seed of corn and giant foxtail did not reduce germination at concentrations up to 10-3 M. Giant foxtail seedling fresh weight was reduced 80% following imbibition of 10-5 M fluorochloridone. Corn seedling fresh weight was not reduced by imbibition of up to 10-4 M fluorochloridone.

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 research was conducted at Arlington, WI, in 1988 and 1989 to determine the influence of application time on weed control and residue carryover with imazethapyr in no-tillage soybean production. Imazethapyr at ≥ 55 g ai ha−1 applied early preplant controlled > 90% of the common lambsquarters, velvetleaf, and giant foxtail before no-tillage planting of soybean. Early preplant and sequential treatments with an early preplant component controlled ≥ 88% of all weeds for the entire growing season. Delaying the initial imazethapyr application until immediately after soybean planting reduced weed control compared to the early preplant treatments. Low level of weed control with planting time treatments appeared to be due to a lack of control of common lambsquarters emerged at the time of imazethapyr application and dry weather following imazethapyr application. No soybean injury from imazethapyr was observed and differences in soybean yield appeared to be due to differences in weed control. No significant carryover of imazethapyr was detected through a corn bioassay in the field.

Field experiments were conducted in 1986 and 1987 to evaluate the effect of herbicide rate (two-thirds and maximum labeled rate) and timing of application before soybean planting on common cocklebur control. In 1986, chlorimuron plus metribuzin at 50 plus 320 and 80 plus 480 g ai/ha and imazaquin at 90 g ai/ha applied at planting controlled common cocklebur better than when applied 4 weeks before planting (WBP). Imazaquin at 140 g/ha applied at planting controlled common cocklebur better than when applied 6 WBP. All treatments at two-thirds of the labeled rate applied at planting controlled common cocklebur comparable to the full labeled rate applied 2, 4, or 6 WBP. In 1987, all treatments applied 2 WBP controlled common cocklebur better than when applied 4, 6, or 8 WPB.

Lack of consistent weed control has been a major limiting factor in the adoption of no-till soybean production. Field studies were conducted at Waseca, MN from 1987 through 1990 to evaluate the efficacy of imazethapyr applied either alone or in combination with other herbicides for weed control in no-till soybean. Fall applications of imazethapyr did not provide acceptable weed control. Imazethapyr applied 2 to 4 wk before planting provided a weed-free seedbed whereas burndown treatments applied 1 to 3 d before planting failed to do so. Early preplant imazethapyr applied during the second week of April did not control weeds as well as imazethapyr applied during the last week of April. Imazethapyr applied alone PRE failed to control weeds adequately. A split application of early preplant plus PRE imazethapyr resulted in excellent weed control, especially when metribuzin was added with each application. Imazethapyr is a promising herbicide for weed control in no-till soybean production.

Field studies were conducted to evaluate the influence of herbicide application timing on weed control in no-till soybean production. Row spacing generally had no effect on weed control. Herbicide treatments containing chlorimuron plus metribuzin applied as many as 45 days prior to planting in 1988 and 1989 controlled broadleaf weeds throughout the growing season. Imazaquin applied 45 and 30 days prior to planting provided poor control of common cocklebur in 1989. Giant foxtail control was inconsistent with all herbicide treatments. Soybean yields subsequent to early preplant herbicide applications were greater than or equal to those in which applications were made at planting when late-season weed control was adequate. Herbicides applied preemergence did not control high densities of common lambsquarters in 1989.

Fall-applied residual and spring preplant burn-down herbicide applications are typically used to control winter annual weeds and may also provide early-season residual control of summer annual weed species such as giant ragweed. Field experiments were conducted from 2006 to 2008 in southern Illinois to (1) assess the emergence pattern of giant ragweed, (2) evaluate the efficacy of several herbicides commonly used for soil-residual control of giant ragweed, and (3) investigate the optimal application timing of soil-residual herbicides for control of giant ragweed. Six herbicide treatments were applied at four application timings: early fall, late fall, early spring, and late spring. Giant ragweed first emerged in mid- and late-March in 2007 and 2008, respectively. The duration of emergence varied by year, with 95% of emergence complete in late May of 2008, but not until early July in 2007. Giant ragweed emergence occurred more quickly in plots that received a fall application of glyphosate + 2,4-D compared with the nontreated. Fall-applied residual herbicides did not reduce giant ragweed emergence in 2007 when compared with the nontreated, with the exception of chlorimuron + tribenuron applied in late fall. Giant ragweed control from early- and late-spring herbicide applications was variable by year. In 2007, saflufenacil (50 and 100 g ai ha−1) and simazine applied in early spring reduced giant ragweed densities by 95% or greater through mid-May; however, in 2008, early-spring applications failed to reduce giant ragweed emergence in mid-April. The only treatments that reduced giant ragweed densities by > 80% through early July were late-spring applications of chlorimuron + tribenuron or saflufenacil at 100 g ha−1. Thus, the emergence patterns of giant ragweed in southern Illinois dictates that best management with herbicides would include late-spring applications of soil-residual herbicides just before crop planting and most likely requires subsequent control with foliar or soil-residual herbicides after crop emergence.

Field studies were conducted from 1996 to 1998 to evaluate grass control in no-till corn (Zea mays) with herbicides applied early preplant (EPP), preemergence (PRE), and postemergence (POST) at the Belleville Research Center at Belleville, IL. Grass control was affected by application timing rather than herbicide. The herbicides applied PRE provided more consistent giant foxtail (Setaria faberi) and barnyardgrass (Echinochloa crus-galli) control (90 to 98%) than the same herbicides applied EPP (0 to 92%). There also was no difference in giant foxtail and barnyardgrass control between the emulsifiable concentrate (EC) formulation and microencapsulated (ME) formulation of acetochlor. Rimsulfuron plus thifensulfuron applied POST provided 90 to 97% control of giant foxtail and barnyardgrass. Metolachlor, EC-acetochlor, SAN 582H, and rimsulfuron plus thifensulfuron provided 85 to 92% control of yellow nutsedge (Cyperus esculentus) compared with 63 to 74% control for BAY FOE 5043 plus metribuzin and ME-acetochlor. Corn grain yield was greater with herbicides applied either PRE or POST than applied EPP. Grass control and grain yield were greater with herbicides applied either PRE or POST compared with EPP.