Atrazine detection in drinking water has raised questions about how to reduce levels of this herbicide. Field experiments were conducted near Ottawa, KS, to determine atrazine levels in runoff from grain sorghum grown under three tillage systems in three growing seasons. Atrazine at 1.12 kg ai/ha was applied several weeks before planting to areas where no-till (NT), ridge-till (RT), and chisel-disk (CD) systems were used, followed by an additional 0.56 kg ai/ha applied at planting. Surface-water runoff was collected from enclosed metal frames within each tillage system, and total water runoff and atrazine concentration were determined and used to compute total atrazine loss. Atrazine loss was less from CD than from the other tillage systems in years when atrazine was soil incorporated. Atrazine loss was highly variable among years and tillage systems. This suggests that other factors besides till systems influenced atrazine runoff.

Wild carrot is becoming a serious weed problem in Michigan continuous no-tillage crop production. Greenhouse and field research was conducted to identify effective management strategies for wild carrot control in no-tillage cropping systems. In the greenhouse, PRE applications of acetochlor plus dichlormid (5.8:1 w:w), cyanazine, linuron plus chlorimuron (18:1), and metribuzin plus chlorimuron (10:1) and POST applications of bentazon, CGA-152005, clopyralid, cyanazine, and MON 12000 provided the greatest control of wild carrot seedlings. In the field, PRE and POST treatments containing chlorimuron consistently controlled overwintered wild carrot greater than 71% at 30 d after the POST application in no-tillage soybean. Atrazine, MON 12000, nicosulfuron, and primisulfuron applied POST consistently controlled overwintered wild carrot greater than 78% at 30 DAT in no-tillage corn. Glyphosate at 0.84 or 1.68 kg ae/ha applied in October to established wild carrot provided greater than 74% control the following spring. Early preplant (EPP) applications of glyphosate at 0.84 kg/ha in no-tillage soybean gave 95 and 24% control of overwintered wild carrot in St. Clair and Lenawee Counties, MI, respectively, at 70 DAT. PRE applications of glyphosate at 0.84 kg/ha controlled overwintered wild carrot less than 69% at 58 DAT in no-tillage corn and soybean. Fall applications of 2,4-D ester at 1.12 kg ae/ha provided 18 and 88% control of wild carrot the following spring in Clinton and Lenawee Counties, respectively. EPP applications of 2,4-D ester at 1.12 kg/ha provided 7 and 72% control of overwintered wild carrot in St. Clair and Lenawee Counties, respectively.

Field studies were conducted at two locations in 1997 and 1998 to evaluate crop injury, weed control, yield, and net economic returns of single and sequential postemergence applications of labeled and reduced rates of glyphosate to no-till, glyphosate-resistant soybean planted in narrow rows. Sequential applications provided at least 91% control of giant foxtail, while single applications provided at least 86% control with labeled rates and 68–93% control with reduced rates. Common waterhemp control was slightly higher with sequential vs. single treatments and with labeled rates vs. reduced rates. Velvetleaf control was greater than 96% with all treatments. Common cocklebur control was 90% or higher with all treatments except a single application of glyphosate at 210 g/ha. Lower control of giant foxtail and common waterhemp with single-application, reduced-rate treatments in two of the four trials resulted in lower yields. Overall, sequential applications, regardless of rate, provided greater weed control, yield, and net income and lower coefficients of variation (C.V.s) of net income than reduced-rate single applications. Single-application treatments showed a trend of decreased weed control, yield, and net income and higher C.V.s of net income with reduced rates of glyphosate.

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 trials were conducted in no-till field corn in northern Missouri to compare weed control and crop response of metolachlor plus atrazine applied under five preplant herbicide management practices. The practices consisted of (1) applying the entire dose of metolachlor plus atrazine 15, 30, or 45 d early preplant (EPP), (2) applying a split application of an EPP (67%) followed by (fb) a PRE (33%), (3) applying the entire dose PRE, (4) applying metolachlor PRE fb dicamba early post (EPOST), or (5) applying atrazine alone EPOST. Weed control at 7 wk after planting was more variable with EPP treatments than with EPP fb PRE or PRE treatments. Giant foxtail and fall panicum control was greatest with treatments that included metolachlor PRE. Velvetleaf and common cocklebur control was greatest with atrazine or dicamba EPOST and was generally unacceptable (< 80%) with any soil-applied herbicide treatment. Common lambsquarters control was greater than 90% with all treatments that included metolachlor, atrazine, or dicamba, regardless of timing. Corn yields were slightly higher with EPP fb PRE and PRE fb EPOST dicamba than with EPP alone. The results suggest that herbicide applications made nearer to the date of planting provide less variable weed control and corn yield than treatments made more than 15 d EPP.

Field studies conducted in 1991 to 1993 evaluated the efficacy of clomazone applied at various timings for weed control in soybean. Clomazone applied 45, 30, 15, or 0 days prior to planting (DPP) provided season-long control of velvetleaf and giant foxtail. In 1991 and 1992 clomazone alone 30 and 45 DPP failed to control redroot pigweed. Clomazone alone 45 DPP failed to control common lambsquarters. In 1991 and 1992 clomazone at 0.84 kg/ha plus metribuzin applied 45 DPP failed to control redroot pigweed. The addition of metribuzin plus chlorimuron to the above clomazone treatments resolved these weed control deficiencies. Weed control in 1993 was nearly complete across all clomazone treatments. In 1993 clomazone treatments which included metribuzin or metribuzin plus chlorimuron applied PPI or PRE reduced yield. Herbicide injury is the likely cause of this reduction because most treatments in 1993 provided 99% control of all weed species.

Quackgrass is a serious weed problem in the northern United States and southern Canada. Field experiments were conducted in 1990 and 1991 to examine the effect of herbicide rate, application timing, quackgrass growth stage, and cultivation on quackgrass control with nicosulfuron and primisulfuron in corn. Nicosulfuron (35 g ai/ha) and primisulfuron (40 g ai/ha) each provided 84% or greater control of two-leaf plants and 93% or greater control of four-leaf quackgrass plants. Sequential applications of both nicosulfuron and primisulfuron provided greater season-long control than single applications of the same rate on two-leaf but not four-leaf quackgrass. Cultivation 10 d after nicosulfuron or primisulfuron application often increased early-season control, but by corn harvest few differences were observed. Postemergence (POST) applications of nicosulfuron and primisulfuron provided at least 88% control, compared with 66 to 86% from an early preplant application of glyphosate. Corn yields were similar regardless of the quackgrass control program.

The premix atrazine + S-metolachlor is commonly used to control a wide range of weeds in corn, but it is weak on velvetleaf and several other broadleaf species. RPA 201772, used at reduced rates in combination with atrazine + S-metolachlor, may improve the weed control spectrum. In field studies at Urbana and Dekalb in 1998 and 1999, RPA 201772 was combined with a premix of atrazine at 1,820 g ai/ha and S-metolachlor at 1,408 g ai/ha to compare RPA 201772 rate effects on corn injury, weed control, and yield. RPA 201772 was applied at 0, 26, 53, 78, 105, and 132 g ai/ha early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE). Compared with atrazine + S-metolachlor alone, RPA 201772 combinations did not improve giant foxtail control. However, RPA 201772 at 26 or 53 g/ha increased control of redroot pigweed, common lambsquarters, ivyleaf morningglory, common ragweed, giant ragweed, and velvetleaf in some environments. At Urbana in both years, RPA 201772 at 26 g/ha in combination with atrazine + S-metolachlor increased velvetleaf control 15% compared with the premix alone. RPA 201722 at 26 to 78 g/ha decreased velvetleaf densities with all application methods. Greater weed control and higher corn yield occurred in treatments applied PRE or PPI compared with EPP applications, except where conditions were dry after PRE applications. This research demonstrates the benefit of RPA 201772 at low rates for broadleaf weed control with less potential for crop injury compared with high RPA 201772 rates.

The persistence and efficacy of acetamide herbicides at application timings from fall to preemergence (PRE) were studied in 1998 and 1999 on mollisols (1.1 to 2.8% organic carbon). Metolachlor, s-metolachlor, acetochlor (as an emulsifiable concentrate [EC] formulation and two encapsulated formulations, capsule suspension [CS] and microencapsulated [ME]), and the combination of flufenacet + metribuzin were evaluated at five application times including late fall, 60 and 30 d early preplant (EPP), preplant incorporated, and PRE. Soil bioassays 180 d after application indicated flufenacet + metribuzin, metolachlor, s-metolachlor, and the acetochlor CS had 62 to 74% giant foxtail control, whereas acetochlor EC and ME had 43 to 46% control. Applications at 60 EPP of metolachlor, s-metolachlor, and acetochlor CS provided 70 to 75% giant foxtail control in greenhouse bioassays, whereas flufenacet + metribuzin, acetochlor ME, and acetochlor EC provided 38 to 57% control. At the 30 EPP timing, metolachlor and acetochlor CS had 80 to 82% control, whereas acetochlor EC provided 46% control, and acetochlor ME, flufenacet + metribuzin, and s-metolachlor had 65 to 74% control. Quantitative soil analysis (0 to 6 cm) 10 d after planting (DAP) indicated metolachlor, s-metolachlor, and acetochlor CS concentrations ranged from 12 to 16% and 32 to 47% of applied herbicide for the fall and PRE application timings, respectively, whereas acetochlor (ME and EC) were from 1 to 3% and 16 to 21% of applied for the fall and PRE application timings, respectively. Bioassay reduction was correlated (R2 = 0.68) with soil-herbicide concentrations at 10 DAP.

Field studies were conducted to evaluate corn vigor reduction, weed control, corn yield, and economic returns in a no-till system with various herbicide strategies using full and reduced rates of acetochlor and atrazine with glyphosate, glufosinate, or imazethapyr + imazapyr in their respective type of herbicide-resistant, no-tillage corn. Crop vigor reduction due to herbicide injury was 10% or less with all treatments. A burndown plus a full label rate of a residual herbicide applied early preplant (EPP) generally provided less than 80% control of giant foxtail, common waterhemp, and common cocklebur but usually greater than 85% control of common ragweed and common lambsquarters. Two-pass strategies generally provided greater than 85% control of all species evaluated. Early postemergence, mid-postemergence (MPOST), and late postemergence strategies generally provided inconsistent and poor overall weed control. EPP–MPOST strategies generally provided lower weed control than strategies using acetochlor or atrazine EPP followed by a postemergence application. Corn yield and net economic returns followed a similar trend as weed control, with strategies that provided greater than 80% weed control showing minimal crop vigor reduction and high grain yields. Two-pass strategies with residual herbicides generally provided the highest yields, economic returns, and low coefficients of variation (CV) of net income. Although EPP strategies provided similar economic returns as some of the two-pass strategies, they had higher CVs, implying greater risk to economic return.

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.

Metolachlor, dimethenamid, acetochlor, and a commercial premixture of flufenacet plus metribuzin were applied 60, 45, 30, and 15 d before planting (DBP) and at planting (preemergence [PRE]) at Dekalb and Urbana, IL in 1995 and 1996. The soil types were a Drummer silty clay loam (fine-silty, mixed, superactive, mesic Typic Endoaquoll) with 2.8% organic carbon and a pH of 6.5 at Dekalb and a Flanagan silt loam (Fine, smectitic, mesic Aquic Argiudoll) with 2.0% organic carbon and a pH of 6.2 at Urbana. Herbicide and application timing affected giant foxtail control and densities. Neither herbicide nor application timing affected crop injury or grain yield. Test of the main effects showed that metolachlor and flufenacet plus metribuzin were more effective than acetochlor or dimethenamid in controlling giant foxtail at 30 and 60 d after planting (DAP) and in reducing foxtail density at 60 DAP. Giant foxtail control 60 DAP was greater than 80% for both metolachlor and flufenacet plus metribuzin for all application timings. Dimethenamid and acetochlor applied at 60 and 45 DBP provided less giant foxtail control when compared with metolachlor and flufenacet plus metribuzin applied at the same time. All herbicides provided greater than 90% giant foxtail control at the PRE application timing. Giant foxtail control provided by metolachlor and flufenacet plus metribuzin was insensitive to application timing from 60 DBP to PRE, whereas both dimethenamid and acetochlor efficacy was lowered when applied between 30 and 60 DBP.