Yellow and knotroot foxtail are two common weed species infesting turfgrass and pastures in the southeastern region of the United States. Yellow and knotroot foxtail share morphological similarities and are frequently misidentified by weed managers, thus leading to confusion in herbicide selection. Greenhouse research was conducted to evaluate the response of yellow and knotroot foxtail to several turfgrass herbicides: pinoxaden (35 and 70 g ai ha−1), sethoxydim (316 and 520 g ai ha−1), thiencarbazone + dicamba + iodosulfuron (230 g ai ha−1), nicosulfuron + rimsulfuron (562.8 g ai ha−1), metribuzin (395 g ha−1), sulfentrazone (330 g ai ha−1), sulfentrazone + imazethapyr (504 g ai ha−1), and imazaquin (550 g ai ha−1). All treatments controlled yellow foxtail >87% with more than 90% reduction of the biomass. By comparison, only sulfentrazone alone controlled knotroot foxtail 90% and completely reduced aboveground biomass. Sethoxydim (520 g ai ha−1), metribuzin, and imazaquin controlled knotroot foxtail >70% at 28 d after application. In a rate response evaluation, nonlinear regression showed that yellow foxtail was approximately 8 times more susceptible to pinoxaden and 2 times more susceptible to sethoxydim than knotroot foxtail based on log (WR50) values, which were 50% reduction in fresh weight. Our research indicates that knotroot foxtail is more difficult to control across a range of herbicides, making differentiation of these two species important before herbicides are applied.

Khakiweed is a perennial broadleaf weed that is difficult to control because of its multiple means of reproduction, vigorous growth, and deep taproot. Khakiweed reduces the performance of pasture, pecan, and turf areas by choking out desirable grass and legume species. Because information on the effectiveness of contact and residual herbicides for control in pecan and pasture areas is limited, greenhouse studies were conducted to determine the effect of application timing, mode of action, and rate on khakiweed control. Preemergence and postemergence herbicides were applied to mature khakiweed plants at 0.25X, 0.5X, 1X, or 2X the label recommended rate for general broadleaf control. Biomass was collected 3 wk after application. Plants regrew from roots in the greenhouse until a second biomass harvest was collected at 6 wk after treatment (WAT). Metsulfuron-methyl, indaziflam, or pendimethalin was applied preemergence to the soil surface. All rates of preemergence herbicides provided high-efficacy control of regrowth (>85%) compared to the nontreated control. The efficacies of postemergence-applied metsulfuron-methyl, metsulfuron-methyl + nicosulfuron, indaziflam, aminopyralid + florpyrauxifen-benzyl, 2,4-D amine, and 2,4-D amine + florpyrauxifen-benzyl were also examined. All postemergence herbicide treatments exhibited control compared to the nontreated plants at both sample timings (3 and 6 WAT) and increased with herbicide application rate. No herbicide provided high-efficacy control during the initial postspray period (0 to 3 WAT). During the regrowth period (3 to 6 WAT), metsulfuron-methyl alone and in combination gave >85% control of khakiweed biomass, indicating that the sulfonylurea herbicides used in this study are well suited to controlling khakiweed.

Knotroot foxtail has become more prevalent and problematic in pastures and hayfields in the southeastern United States. Gaps exist in our knowledge of which herbicide practices are best for managing this species in bermudagrass forage production. This study was conducted to determine the efficacy of various ways to control knotroot foxtail in bermudagrass with herbicide applications in autumn, postemergence (POST), with and without also applying a herbicide in preemergence (PRE), in spring. The study was a randomized complete block with a factorial arrangement of treatments and included a nontreated control for both fall and spring timings. Glyphosate at two rates (0.35 or 0.7 kg ae ha−1), nicosulfuron (0.07 kg ai ha−1) + metsulfuron (0.012 kg ai ha−1), and hexazinone (1.3 kg ai ha−1) were applied alone in the fall or followed by indaziflam (0.067 kg ai ha−1) or pendimethalin (4.46 kg ai ha−1) in the spring. Three harvests were conducted throughout the growing season to evaluate weed species (knotroot foxtail, large crabgrass, and horsenettle) and bermudagrass biomass as well as overall species composition. The combination of fall and spring treatments did not affect weed species or bermudagrass biomass. Therefore, treatment main effects were analyzed by fall or spring application timing. A spring application of either pendimethalin or indaziflam increased bermudagrass biomass compared with that of the nontreated control. However, neither PRE herbicide effectively reduced knotroot foxtail biomass compared with the nontreated control, although pendimethalin did reduce season-long knotroot foxtail composition. Spring PRE herbicides are an effective tool for forage producers, but further research is needed to identify effective herbicides and additional approaches for the control of knotroot foxtail.

The sorption of two sulfonylurea herbicides (SU), metsulfuron methyl and nicosulfuron, on pure clays and organoclays was investigated. Three clays (Arizona smectite, SAz-1, Wyoming smectite, SWy-2, and hectorite, SHCa-1), were treated with amounts of octadecylammonium (ODA) or dioctadecyldimethylammonium (DODMA) cations equal to ∼50 and 100% of the clays’ cation exchange capacity (CEC). Sorption isotherms were fitted to the Freundlich equation. While no measurable sorption was found on the pure clays (Kf = 0), organoclays prepared using both primary and quaternary amines were effective as SU sorbents. The metsulfuron methyl Kf values ranged between 196 and 1498 µmol1−1/n kg−1 L1/n, and Kf values for nicosulfuron, which were lower than those of metsulfuron methyl, ranged from 35 to 198 umol1−1/n kg−1 L1/n. As shown by sorption coefficients, Kd and KOC, SWy-2 treated with DODMA at ∼100% of the CEC was the most effective sorbent for metsulfuron, Kd = 684 L kg−1 and KOC = 2138 L kg−1. For nicosulfuron the most effective sorbent was SAz-1 with ODA at ∼ 50% of the CEC (Kd = 147 L kg−1 and KOC = 1233 L kg−1). In contrast to other weak-acid herbicides, such as phenoxy and picolinic acids, no clear relationships were found between sorption and layer charge, organic carbon content, and basal spacing of the organoclays for both sulfonylurea herbicides. Sorption of both herbicides on organoclays was assumed to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations was a very important factor.

Weed control in corn is a major challenge due to increasing problems with highly dominant weed species and herbicide resistance evolution. Common lambsquarters and johnsongrass constitute up to 80% to 90% of the weed population in many spring crops, such as soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and corn, in Serbia. Currently, acetolactate synthase–inhibiting herbicides, such as the systemic selective sulfonylurea nicosulfuron, are most commonly used for chemical weed control of those species. A better understanding of the impact of nozzle type and adjuvant use on nicosulfuron efficacy can help to improve control of common lambsquarters and johnsongrass and minimize herbicide resistance development. Field trials were conducted in Serbia from 2020 to 2022 to evaluate the impact of two adjuvants (a non-ionic surfactant [NIS] and a mineral fertilizer ammonium sulfate [AMS]) and two nozzle types (drift-reducing nozzles and flat-fan nozzles) on common lambsquarters and johnsongrass control using nicosulfuron. Satisfactory biomass reduction of common lambsquarters (83% to 87%) and johnsongrass (83% to 97%) was achieved after nicosulfuron application. Adding a NIS adjuvant increased the biomass reduction for common lambsquarters (94% to 98%) and johnsongrass (90% to 100%) independently of the nozzle type used. Selection of nozzle type did not show consistent effects on common lambsquarters and johnsongrass control. Nicosulfuron efficacy was increased with NIS adjuvant for both nozzle types compared to nicosulfuron solo for both species, and Extended Range (XR) TeeJet® nozzles on average resulted in a higher efficacy for common lambsquarters compared to Turbo TeeJet® induction. Adding a mineral AMS adjuvant resulted in lower biomass reduction for both nozzle types and weed species (65% to 78% and 61% to 91% for common lambsquarters and johnsongrass, respectively). Corn grain yield was predominantly influenced by annual meteorological conditions and adjuvant type added to nicosulfuron. This research suggests that addition of the non-ionic adjuvant is an essential factor for successful control of common lambsquarters and johnsongrass in corn and enables use of drift-reducing nozzles.

Five johnsongrass populations collected from corn grown in northern Greece were studied to elucidate the levels and mechanisms of resistance to acetolactate synthase (ALS)- and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides. Whole-plant response assays indicated that two populations were highly cross-resistant to all ALS inhibitors tested (foramsulfuron, nicosulfuron, rimsulfuron, and imazamox) but were effectively controlled by the recommended rate of the ACCase-inhibiting herbicides propaquizafop and clethodim. The ALS gene sequence revealed a point mutation that resulted in the substitution of Trp574 by Leu in the ALS enzyme, suggesting that the resistance mechanism is target-site mediated. These findings highlight a serious threat against the sustainable use of the ALS-inhibiting herbicides in controlling johnsongrass and other grass weeds in cornfields, suggesting rotational use of herbicides with different modes of action, along with the use of nonchemical methods, for viable Johnsongrass management.

Owing to the lack of effective POST herbicide options, producers typically rely on nicosulfuron as the main POST grass herbicide in sweet corn production systems. In 2019, a Wisconsin sweet corn producer reported fall panicum control escapes after spraying nicosulfuron. Seeds from mature plants were collected to (1) measure fall panicum response to acetolactate synthase (ALS)-inhibiting herbicides, (2) elucidate the resistance mechanism, and (3) evaluate its response to alternative POST herbicides. Greenhouse and laboratory investigations were conducted to assess fall panicum response to ALS-inhibiting herbicides and elucidate the resistance mechanism. Dose–response results showed that fall panicum was highly resistant to nicosulfuron with a resistance ratio of >12.9-fold (survived rates >254 g ai ha−1, or 8× the field label rate). Molecular and genetic studies indicated that there are multiple ALS gene copies in fall panicum and that resistance was due to a mutation in one copy, resulting in an Asp-376-Glu amino acid substitution. Additional greenhouse experiments indicate that clethodim (105 g ai ha−1), quizalofop-p-ethyl (70 g ae ha−1), glyphosate (864 g ae ha−1), and glufosinate (650 g ai ha−1) are effective POST options to manage the ALS-resistant fall panicum (>90.0% control and 96.8% biomass reduction) in rotational years. The 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides isoxaflutole (105 g ai ha−1), mesotrione (105 g ai ha−1), tembotrione (92 g ai ha−1), and tolpyralate (39 g ai ha−1) did not provide effective POST fall panicum control. Because these herbicides are commonly used for POST weed control in sweet corn, more investigations are required to evaluate combinations of HPPD-inhibiting herbicides with herbicides from other sites of action for POST fall panicum control. Herein we confirm the first case of herbicide resistance in fall panicum in the United States.

Previous research has shown that glufosinate and nicosulfuron at low rates can cause yield loss to grain sorghum. However, research has not been conducted to pinpoint the growth stage at which these herbicides are most injurious to grain sorghum. Therefore, field tests were conducted in 2016 and 2017 to determine the most sensitive growth stage for grain sorghum exposure to both glufosinate and nicosulfuron. Field test were designed with factor A being the herbicide applied (glufosinate or nicosulfuron). Factor B consisted of timing of herbicide application including V3, V8, flagleaf, heading, and soft dough stages. Factor C was glufosinate or nicosulfuron rate where a proportional rate of 656 g ai ha−1 of glufosinate and 35 g ai ha−1 of nicosulfuron was applied at 1/10×, 1/50×, and 1/250×. Visible injury, crop canopy heights (cm), and yield were reported as a percent of the nontreated. At the V3 growth stage visible injury of 32% from the 1/10× rate of glufosinate and 51% from the 1/10× rate of nicosulfuron was observed. This injury was reduced by 4 wk after application (WAA) and no yield loss occurred. Nicosulfuron was more injurious than glufosinate at a 1/10× and 1/50× rate when applied at the V8 and flagleaf growth stages resulting in death of the shoot, reduced heading, and yield. Yield losses from the 1/10× rate of nicosulfuron were observed from V8 through early heading and ranged from 41% to 96%. Yield losses from the 1/50× rate of nicosulfuron were 14% to 16% at the flagleaf and V8 growth stages respectively. The 1/10× rate of glufosinate caused 36% visible injury 2 WAA when applied at the flagleaf stage, which resulted in a 16% yield reduction. By 4 WAA visible injury from either herbicide at less than the 1/10× rate was not greater than 4%. Results indicate that injury can occur, but yield losses are more probable from low rates of nicosulfuron at V8 and flagleaf growth stages.

A non-GMO trait called Inzen™ was recently commercialized in grain sorghum to combat weedy grasses, allowing the use of nicosulfuron POST in the crop. Inzen™ grain sorghum carries a double mutation in the acetolactate synthase (ALS) gene Val560Ile and Trp574Leu, which potentially results in cross-resistance to a wide assortment of ALS-inhibiting herbicides. To evaluate the scope of cross-resistance to Weed Science Society of America Group 2 herbicides in addition to nicosulfuron, tests were conducted in 2016 and 2017 at the Lon Mann Cotton Research Station near Marianna, AR, the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, and in 2016 at the Pine Tree Research Station near Colt, AR. The tests included ALS-inhibiting herbicides from all five families: sulfonylureas, imidazolinones, pyrimidinylthiobenzoics, triazolinones, and triazolopyrimidines. Treatments were made PRE or POST to grain sorghum at a 1× rate for crops in which each herbicide is labeled. Grain sorghum planted in the PRE trial were Inzen™ and a conventional cultivar. Visible estimates of injury and sorghum heights were recorded at 2 and 4 wk after herbicide application, and yield data were collected at crop maturity. In the PRE trial, no visible injury, sorghum height reduction, or yield loss were observed in plots containing the Inzen™ cultivar. Applications made POST to the Inzen™ grain sorghum caused visible injury, sorghum height reduction, and yield loss of 20%, 13%, and 35%, respectively, only in plots where bispyribac-Na was applied. There was no impact on the crop from other POST-applied ALS-inhibiting herbicides. These results demonstrate that the Inzen™ trait confers cross-resistance to most ALS-inhibiting herbicides and could offer promising new alternatives for weed control and protection from carryover of residual ALS-inhibiting herbicides in grain sorghum.

Velvetleaf is an economically important weed in popcorn production fields in Nebraska. Many PRE herbicides in popcorn have limited residual activity or provide partial velvetleaf control. There are a limited number of herbicides applied POST in popcorn compared with field corn, necessitating the evaluation of POST herbicides for control of velvetleaf. The objectives of this study were to (1) evaluate the efficacy and crop safety of labeled POST herbicides for controlling velvetleaf that survived S-metolachlor/atrazine applied PRE and (2) determine the effect of velvetleaf height on POST herbicide efficacy, popcorn injury, and yield. Field experiments were conducted in 2018 and 2019 near Clay Center, Nebraska. The experiments were arranged in a split-plot design with four replications. The main plot treatments were velvetleaf height (≤15 cm and ≤30 cm) and subplot treatments included a no-POST herbicide control, and 11 POST herbicide programs. Fluthiacet-methyl, fluthiacet-methyl/mesotrione, carfentrazone-ethyl, dicamba, and dicamba/diflufenzopyr provided greater than 96% velvetleaf control 28 d after treatment (DAT), reduced velvetleaf density to fewer than 7 plants m−2, achieved 99% to 100% biomass reduction, and had no effect on popcorn yield. Herbicide programs tested in this study provided greater than 98% control of velvetleaf 28 DAT in 2019. Most POST herbicide programs in this study provided greater than 90% control of up to 15 cm and up to 30 cm velvetleaf and no differences between velvetleaf heights in density, biomass reduction, or popcorn yield were observed, except with topramezone and nicosulfuron/mesotrione 28 DAT in 2018. On the basis of contrast analysis, herbicide programs with fluthiacet-methyl or dicamba provided better control than herbicide programs without them at 28 DAT in 2018. It is concluded that POST herbicides are available for control of velvetleaf up to 30-cm tall in popcorn production fields.

Reductions in MSMA use for weed control in turfgrass systems may have led to increased common carpetgrass infestations. The objective of our research was to identify alternative POST herbicides for control of common carpetgrass using field and controlled-environment experiments. Field applications of MSMA (2.2 kg ai ha−1) and thiencarbazone + iodosulfuron + dicamba (TID) (0.171 kg ai ha−1) resulted in the greatest common carpetgrass control 8 wk after initial treatment (WAIT): 94% and 91%, respectively. Thiencarbazone + foramsulfuron + halosulfuron (TFH) (0.127 kg ai ha−1) applied in the field resulted in 77% control 8 WAIT, whereas all other treatments were ≤19% effective at 8 WAIT. All treatments resulted in greater common carpetgrass control when applied in the greenhouse. Applications of MSMA, TFH, and TID resulted in the highest common carpetgrass control in the greenhouse 8 WAIT: 94%, 94%, and 91%, respectively. Control with nicosulfuron (0.035 kg ai ha−1) and trifloxysulfuron (0.028 kg ai ha−1) (81% and 75%, respectively) was greater in the greenhouse than observed in the field 8 WAIT. Sequential applications of foramsulfuron (0.058 kg ai ha−1) resulted in only ≤11% common carpetgrass control 8 WAIT, regardless of application site. All herbicide treatments in the greenhouse resulted in reduced aboveground common carpetgrass biomass 8 WAIT compared to the nontreated control (12.9 g). Aboveground biomasses of common carpetgrass in response to MSMA, TID, TFH, nicosulfuron, and trifloxysulfuron were 1.6 to 2.1 g, regardless of treatment. Reduced efficacy of foramsulfuron was reflected in greater biomass (4.7 g) in response to treatments. Thiencarbazone + iodosulfuron + dicamba may be an alternative to MSMA for common carpetgrass control; however, long-term assessment may be warranted to evaluate treatment effectiveness. Further investigation into application timing may be necessary to enhance the efficacy of TFH for the control of common carpetgrass.

Field studies were conducted to determine the possible rate and timing of nicosulfuron to suppress annual ryegrass (ARG) seeded as a cover crop at the time of corn planting without affecting corn performance near Ridgetown, ON, Canada, in 2016 and 2017. Nicosulfuron was applied at rates from 0.8 to 50 g ai ha–1 when the ARG was at the two- to three- or four- to five-leaf stages, or approximately 3 or 4 wk after emergence of both corn and ARG. There were no differences between the two application timings in grain yield responses or ARG suppression. As the rate of nicosulfuron increased from 0.8 to 50 g ai ha–1, ARG was suppressed 6% to 76% and 5% to 96%, at 1 and 4 wk after application (WAA), respectively. At 4 WAA, ARG biomass decreased from 29 to 1 g m–2 as the rate of nicosulfuron increased from 0.8 to 50 g ai ha–1, compared to 36 g m–2 in the untreated control. Where nicosulfuron was not applied to ARG, grain corn yield was reduced by 6% compared to the ARG-free control; similar effects on corn yield were observed with nicosulfuron at the lowest rate applied at 0.8 g ai ha–1. Grain corn yield was reduced by 2.5% with the application of nicosulfuron at 25 g ai ha–1 (label rate for corn) compared to no ARG control, but this was not statistically significant. This study identified rates of nicosulfuron that suppressed ARG when emerged approximately the same day as corn, but there was evidence that grain corn yields were lowered because of interference, possibly during the critical weed control period. Based on this study, an ARG cover crop should not be seeded at the same time as corn unless one is willing to accept a risk for corn grain yield losses for the sake of the cover crop.

Purple nutsedge is difficult to control in vegetable plasticulture due to its ability to penetrate the plastic mulch. Experiments were conducted in Spring 2011 and 2012 at the Plant Science Research and Education Center in Citra, Florida, and in Spring and Fall 2013 at the Gulf Coast Research and Education Center in Balm, Florida. The objective was to determine tomato (cv. Amelia, Charger, and Florida 47) tolerance and purple nutsedge response to herbicides and herbicide tank-mixes applied POST-directed to base of tomato. Chlorimuron-ethyl, flazasulfuron, fomesafen, halosulfuron, imazosulfuron, rimsulfuron, nicosulfuron, and trifloxysulfuron applied POST-directed to the base of the crop did not cause crop damage. Halosulfuron or tank-mixes that contained halosulfuron tended to provide the greatest nutsedge suppression in all experiments. Halosulfuron or nicosulfuron+rimsulfuron applications when tomato (cv. Charger) were flowering reduced marketable yields by 22-28% compared to the nontreated control. No yield reductions occurred with Florida 47 or Amelia cultivars. Flazasulfuron provided similar purple nutsedge suppression to halosulfuron and did not damage tomato. Tank-mixes that contained halosulfuron tended not to provide any added benefit over halosulfuron alone. This research identified herbicides that are safe for use as a POST-directed application in tomato. Additional research is needed to evaluate efficacy of these herbicides on broadleaf weeds.

Field experiments were conducted to evaluate the hypothesis that johnsongrass control in corn causes increased maize dwarf mosaic virus (MDMV) and maize chlorotic dwarf virus (MCDV) disease severity because of increased movement of insect vectors from dying johnsongrass to the corn crop. Johnsongrass control treatments included 1) broadcast POST nicosulfuron, 2) directed POST imazethapyr, 3) mechanical control, and 4) no treatment. Disease severity in both a virus-susceptible and a virus-tolerant corn hybrid was evaluated. With the virus-susceptible hybrid, greater disease severity was observed where johnsongrass was controlled in the experimental area than where johnsongrass was not controlled. Increases in disease severity were independent of the method of johnsongrass control. Corollary studies conducted on the same site verified a double infection of corn with MDMV and MCDV and documented movement of blackfaced leafhoppers, the insect vector of MCDV, subsequent to treatment of johnsongrass.

The effect of DPX-79406 was evaluated on eight corn hybrids in 1993 and on 18 hybrids in 1994. Experiments were conducted each year at Charlottetown, Prince Edward Island, and Stewiacke, Nova Scotia. Corn tolerance was similiar between sites but varied from year to year with certain hybrids. Generally, tolerance was hybrid dependent. Most hybrids displayed chlorosis and stunting 7 days after treatment (DAT) but recovered quickly. Injury at 7 DAT ranged from 8 to 64%. Grain yield indicated that ‘Pickseed 2444,’ ‘Pioneer 3921,’ ‘Pioneer 3979,’ and ‘DK 302’ were tolerant to 25 g ai/ha of DPX-79406. Pioneer 3979 and Pickseed 2444 grain yields were reduced with DPX-79406 at 50 g/ha in 1 yr. ‘DK 291’ and ‘DK 235’ were severely injured both years and did not recover appreciably from early injury. Responses of ‘Pioneer 3984’ and ‘Pickseed 2477’ varied from year to year, and they were considered sensitive to DPX-79406. ‘Pioneer 3947,’ ‘Pioneer 3962,’ ‘Pickseed 2290,’ ‘Pickseed 2299,’ ‘DK 220,’ ‘G-4011,’ and ‘N0565’ were tested only 1 yr and were tolerant to both rates of DPX-79406.

The effects of chlorimuron, imazaquin, imazethapyr, nicosulfuron, primisulfuron, and thifensulfuron were evaluated on a population of smooth pigweed in Painter, VA with no history of treatment with acetolactate synthase (ALS)-inhibitor herbicides. Imazethapyr and nicosulfuron gave the greatest smooth pigweed control, and subsequently were used in field and greenhouse studies to investigate susceptibility of smooth pigweed and livid amaranth populations to ALS-inhibitor herbicides. Approximately 5 million smooth pigweed plants from Painter were treated with imazethapyr or nicosulfuron from 1992 to 1994 and no ALS-inhibitor-resistant plants were identified. In the greenhouse, the response of smooth pigweed from Painter, VA, Marion, MD, and Oak Hall, VA and livid amaranth from Warren County, NJ to imazaquin or imazethapyr and nicosulfuron was investigated. Smooth pigweed from Marion and Oak Hall and livid amaranth from NJ had histories of treatment with ALS-inhibitors. Painter smooth pigweed control was 81 to 97% by imazethapyr and nicosulfuron while control of the Marion and Oak Hall populations was 3 and 18% by imazaquin at 560 and 1120 g ai/ha, respectively, and control by nicosulfuron at 35 g ai/ha was 50 to 73%. Control of livid amaranth from Warren County, NJ was 8 to 15% by imazethapyr at 560 g ai/ha, and was 30 to 58% by nicosulfuron at 35 g/ha.

Field studies were conducted in 1992 and 1993 to evaluate AC 263,222 applied postemergence (POST) alone and as a mixture with atrazine or bentazon for weed control in imidazolinone-resistant corn. Nicosulfuron alone and nicosulfuron plus atrazine were also evaluated. Herbicide treatments were applied following surface-banded applications of two insecticides, carbofuran or terbufos at planting. Crop sensitivity to POST herbicides, corn yield, and weed control was not affected by insecticide treatments. AC 263,222 at 36 and 72 g ai/ha controlled rhizomatous johnsongrass 88 and 99%, respectively, which was equivalent to nicosulfuron applied alone or with atrazine. AC 263,222 at 72 g/ha controlled large crabgrass 99% and redroot pigweed 100%, and this level of control exceeded that obtained with nicosulfuron alone. AC 263,222 at 72 g/ha controlled sicklepod and morningglory species 99 and 98%, respectively. Nicosulfuron alone or with atrazine controlled these two species less than AC 263,222 at 72 g/ha. Addition of bentazon or atrazine to AC 263,222 did not improve control of any species compared with the higher rate of AC 263,222 at 72 g/ha applied alone. Corn yield increased over the untreated control when POST herbicide(s) were applied, but there were no differences in yield among herbicide treatments.

Extensive field and greenhouse studies were done to characterize varietal response of three recently commercialized sulfonylurea corn herbicides: nicosulfuron, primisulfuron, and thifensulfuron. Most of the 94 varieties tested were highly tolerant to these herbicides. The 37 inbreds represented all major inbred families now used in hybrid seed production as well as several sensitive experimentals. Twenty-one defined hybrids from these inbreds as well as 36 commercially coded hybrids were also tested. Sensitive inbreds produced tolerant hybrids when crossed with tolerant inbreds. Sensitive hybrids occurred when both parents were sensitive. Genetic analysis of sensitive by tolerant crosses showed that sensitivity is controlled by a single recessive gene. Nicosulfuron had the widest corn safety margin and fewest sensitive varieties. Dose response analysis showed varieties can vary more than 40 000-fold in sensitivity. Only corn varieties with the AHAS-modified XA-17 gene showed any change in enzyme sensitivity. This gene overcame sensitivity to sulfonylureas, even when the organophosphate insecticide terbufos was present. Thus, breeders have three options to eliminate sulfonylurea sensitivity: backcross sensitive inbreds with tolerant, always use at least one tolerant hybrid parent, or use the XA-17 gene.

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.

Control and regrowth of hemp dogbane, wild blackberry, and triazine-resistant common lambsquarters (TR-CHEAL) were studied in no-till corn from 1992 to 1994. Hemp dogbane, wild blackberry, and TR-CHEAL population increased 10, 123, and 177%, respectively, between 1992 and 1994 in plots treated with PRE applications of paraquat, atrazine, and metolachlor (weedy checks). POST applications of tank mixtures of 35 g ai/ha nicosulfuron or 20 g/ha primisulfuron with 280 g/ha 2,4-D or 140 g/ha dicamba, and 560 g/ha dicamba applied alone controlled hemp dogbane, wild blackberry, and TR-CHEAL 67 to 98%. These treatments reduced the population or prevented expansion of these weeds the year following treatment. In 1992, corn yield response to weed control was inconsistent. In 1993 and 1994, all plots treated with POST herbicides yielded higher than the weedy check. Corn yield of plots treated with combinations of nicosulfuron or primisulfuron with 2,4-D or dicamba and 560 g/ha dicamba applied alone were 102 to 149% and 124 to 153% higher than the weedy check in 1993 and 1994, respectively.