The adsorption of the sulfonylurea herbicide rimsulfuron, [N-((4,6-dimethoxypyrimidin-2-yl)aminocarbonyl)-3-(ethylsulfonyl)-2-pyridinesulfonamide], on clay minerals with different saturating cations was studied. Three smectites with different lattice charge distribution (hectorite, montmorillonite and nontronite) were selected and made homoionic to Ca2+, Cu2+ and Al3+. Because of the instability of rimsulfuron in water, the experiments were carried out in chloroform solution. The interaction mechanism depends on the nature of the saturating cation and the tetrahedral layer charge of the silicate. Among the exchangeable ions studied, only Al3+ is able to produce degradation of the herbicide to N-(4,6-dimethoxypyrimidin-2-yl)-N-[(3-(ethylsulfonyl)-2-pyridinyl]urea. In this case, the lower the tetrahedral charge, the more active the degradation. The Ca2+-saturated clays are ineffective in the degradation. In contrast, the formation of a stable chelate complex with the saturating ion permits rimsulfuron to be adsorbed to a rather high extent into Cu(II)-clays and to be stable against degradation.

Herbicides are often used to terminate cover crops. Producers would like to use herbicides that work quickly, are effective, and do not increase the risk of selecting herbicide-resistant weeds. Eight experiments were conducted to determine whether mixing glyphosate (900 g a.e. ha−1) with rimsulfuron (15 g a.i. ha−1), mesotrione (100 g a.i. ha−1), or rimsulfuron + mesotrione enhances winter rye control and to ascertain whether using urea ammonium nitrate (UAN) as the herbicide carrier improves and accelerates herbicide efficacy. Winter rye control was assessed 1, 2, 3, and 4 wk after application (WAA) and biomass was measured 4 WAA. The addition of rimsulfuron, mesotrione, or rimsulfuron + mesotrione to glyphosate did not enhance winter rye control. Similarly, using UAN as the herbicide carrier did not improve or accelerate herbicide efficacy. Glyphosate alone provided the greatest level of winter rye control. The addition of rimsulfuron, mesotrione, or rimsulfuron + mesotrione to glyphosate did not increase the level or speed of control. However, mixing glyphosate with rimsulfuron, mesotrione, or rimsulfuron + mesotrione adds other modes of action without compromising winter rye control.

Field studies were conducted to determine hazelnut tolerance to quinclorac and clopyralid and control efficacy of Canada thistle and field bindweed at three commercial orchards in western Oregon. Hazelnut cultivars evaluated included ‘Jefferson’, ‘Wepster’, and ‘McDonald’. Clopyralid at 278, 547, and 1,090 g ae ha−1, and quinclorac at 420, 840, and 1,680 g ai ha−1 were applied once a year as basal-directed applications to trees that were 1, 2, and 5 yr old. Treatments were imposed in the early spring of 2019 and reapplied in 2020. In both years, treatments covered hazelnut suckers. Hazelnut injury from clopyralid and quinclorac was consistently between 0% and 13% and not different from nontreated control plants (P > 0.05) between 14 d and 455 d after initial treatment. Similarly, there was no treatment effect on plant canopy index, leaf chlorophyll content, trunk cross-sectional area, internode length, or yield among treatments, even at the highest rates of clopyralid and quinclorac. In separate efficacy studies, clopyralid (278 g ae ha−1) resulted in 68% Canada thistle control and did not differ when clopyralid was mixed with carfentrazone (278 + 35 g ai ha−1) or glufosinate (278 + 1,148 g ai ha−1). Clopyralid-containing herbicide treatments suppressed field bindweed growth but did not kill plants even when mixed with carfentrazone or glufosinate. Quinclorac (420 g ha−1) alone provided 80% control of field bindweed and 93% and 98% control when combined with rimsulfuron (35 g ai ha−1) or carfentrazone (35 g ai ha−1), respectively. Still, all herbicide treatments resulted in similar field bindweed biomass. Results indicate that clopyralid and quinclorac are effective tools to help manage Canada thistle and field bindweed and that hazelnut can tolerate clopyralid and quinclorac at rates equivalent to 4-fold commercial-use rates not affecting plant growth and yield.

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.

Field bindweed is a perennial vining weed with vigorous growth, and is commonly found in highbush blueberry fields of Oregon. It requires and integrated strategy using multiple applications of postemergence herbicides and hand weeding for adequate control. Quinclorac is a herbicide that has been shown to control field bindweed, but no information is available indicating the tolerance of blueberry to quinclorac. The objective of this study was to evaluate the response of blueberry to quinclorac and to evaluate field bindweed control with quinclorac in different mixtures. Three groups of field studies were designed to assess 1) single application control of field bindweed, 2) use of sequential treatments to control field bindweed, and 3) long-term impact of quinclorac on field bindweed. In the single application control studies, a single application of quinclorac at 210 or 420 g ai ha−1 alone or in a mixture with rimsulfuron (35 g ai ha−1) or carfentrazone (35 g ai ha−1), controlled field bindweed by 69% to 76% while reducing its biomass between 22% and 44% compared to the nontreated control (61 g m−2). In a sequential treatment study, a single application of quinclorac (420 g ai ha−1) provided 83% to 100% control of field bindweed, outperforming three sequential applications of carfentrazone. In the long-term study, a single application of quinclorac reduced field bindweed biomass by 50% to 82% in 2019 and 62% to 87% in 2020. These results indicate that quinclorac can be safely applied to highbush blueberry plants. Early spring applications of quinclorac to field bindweed will reduce or eliminate the need for subsequent applications later in the season.

Italian ryegrass has become a problematic weed in hazelnut orchards of Oregon because of the presence of herbicide-resistant populations. Resistant and multiple-resistant Italian ryegrass populations are now the predominant biotypes in Oregon; there is no information on which herbicides effectively control Italian ryegrass in hazelnut orchards. Six field studies were conducted in commercial orchards to evaluate Italian ryegrass control with POST herbicides. Treatments included flazasulfuron, glufosinate, glyphosate, paraquat, rimsulfuron, and sethoxydim applied alone or in selected mixtures during early spring when plants were in the vegetative stage. Treatment efficacy was dependent on the experimental site. The observed range of weed control 28 d after treatment was 13% to 76% for glyphosate, 1% to 72% for paraquat, 58% to 88% for glufosinate, 16% to 97% for flazasulfuron, 8% to 94% for rimsulfuron, and 25% to 91% for sethoxydim. Herbicides in mixtures improved control of Italian ryegrass compared to single active ingredients based on contrast analysis. Herbicides in mixture increased control by 27% compared to glyphosate, 18% to rimsulfuron, 15% to flazasulfuron, 19% to sethoxydim, and 12% compared to glufosinate when averaged across all sites, but mixture did not always improve reduction of ground coverage or of biomass. This complex site-specific response highlights the importance of record-keeping for efficient herbicide use. Glufosinate is an effective option to manage Italian ryegrass. However, the glufosinate-resistant biotypes documented in Oregon may jeopardize this practice. Nonchemical weed control options are needed for sustainable weed management in hazelnuts.

Striking a balance between the weed control capacity of living mulches and their competition with the main crop is complex. At rates that avoid severe injury to living mulch, herbicides may reduce their vigor while simultaneously contributing to weed control. In a 2-yr field study carried out in Freeville, NY, we evaluated the effects of various combinations consisting of two herbicides, applied sequentially at reduced rates, on the growth of a sunn hemp living mulch and weeds (including common lambsquarters, common purslane, hairy galinsoga, and Powell amaranth). When a herbicide with primarily POST activity (Type 1; e.g., rimsulfuron, 0.005 to 0.007 kg ai ha−1) was applied first, performance of sunn hemp (1700 to 3900 kg ha−1 dry biomass; 10% to 88% groundcover) was poor and weed growth (25% to 62% groundcover) was high, likely because sunn hemp was severely injured at a young growth stage and was outcompeted by weeds. A follow-up application (approximately 2 wk later) of a herbicide with primarily PRE and residual activities (Type 2; e.g., metribuzin, 0.05 to 0.15 kg ai ha−1), with a surfactant to enhance its POST activity, had little effect on established weeds. However, because sunn hemp was already 20 cm tall at weed emergence, applying a Type 2 herbicide first did not cause severe injury to sunn hemp and reduced weed pressure, thereby also enhancing sunn hemp performance (3,800 to 6,100 kg ha−1 dry biomass; 85% to 94% groundcover). Moreover, the follow-up application of a Type 1 herbicide affected the smaller weeds more (4% to 21% groundcover) than the better-established sunn hemp. Our results demonstrate that an appropriate sequence of herbicides at reduced rates may be important to control weeds while maintaining a healthy living mulch stand.

Hairy fleabane and horseweed are pervasive weed species in agriculture. Glyphosate-resistant (GR) and glyphosate/paraquat–resistant (GPR) biotypes challenge current management strategies. These GR and GPR biotypes have non–target site resistance, which can confer resistance to herbicides with different sites of action (SOAs). This study’s objective was to characterize the response of GR, GPR, and glyphosate/paraquat–susceptible (GPS) biotypes of both weed species to herbicides with a different SOA. Whole-plant dose–response bioassays indicated a similar response among tested biotypes of both weed species to rimsulfuron, dicamba, hexazinone, glufosinate, flumioxazin, saflufenacil, or mesotrione. The hairy fleabane GR and GPR biotypes were 2.7- and 2.9-fold resistant to 2,4-D relative to the GPS biotype (GR50 766.7 g ai ha–1), confirming 2,4-D resistance in hairy fleabane for the first time in California. The GR and GPR biotypes were not cross-resistant to dicamba. No differences in response to 2,4-D were observed among horseweed biotypes with a GR50 ranging from 150.2 to 277.4 g ai ha–1. The GPR biotypes of both species were cross-resistant to diquat, with a 44.0-fold resistance in hairy fleabane (GR50 863.7 g ai ha–1) and 15.6-fold resistance in horseweed (GR50 563.1 g ai ha–1). The confirmation of multiple resistances to glyphosate, paraquat, and 2,4-D in hairy fleabane curtails herbicide SOA alternatives and jeopardizes resistance management strategies based on herbicide rotation and tank mixtures, underscoring the critical need for nonchemical weed control alternatives.

POST weed control atop the bed during strawberry production is limited to hand weeding, clopyralid, and acetyl CoA carboxylase inhibitors. Identification of additional modes of action is desirable to increase available options for producers and alleviate herbicide resistance concerns. The study objective was to screen sulfonylurea herbicides for safety of strawberry coordinated with efficacy against Carolina geranium. Herbicide treatments included metsulfuron-methyl, flazasulfuron, foramsulfuron, thifensulfuron-methyl, trifloxysulfuron-sodium, and rimsulfuron. Strawberry plants were heavily damaged by all herbicides apart from foramsulfuron. Although the strawberry plant was dramatically affected by the evaluated herbicides, demonstrating strong epinasty, there were no differences in resultant biomass at 31 d after treatment (DAT) compared to controls. Carolina geranium was severely injured by metsulfuron-methyl, flazasulfuron, and thifensulfuron-methyl, and moderately injured by foramsulfuron. There were consistent reductions in biomass by 31 DAT by metsulfuron-methyl and flazasulfuron. Overall, metsulfuron-methyl and flazasulfuron are suitable candidates for Carolina geranium control in row-middles. Foramsulfuron is a suitable candidate for additional field-based screening for utility in POST use in strawberry production atop the bed. Consideration toward doses, surfactants, timings, and cultivar tolerance may be necessary to minimize injury as observed in the greenhouse (15% to 20%).

Cover cropping is limited by seasonal constraints following corn harvest in the Upper Midwest of the United States. Grass, clover, and brassica cover crops can be interseeded in corn; however, this is problematic because cover crops must tolerate herbicide applications to manage weeds. The objective of this research was to determine the tolerance of broadcast interseeded annual ryegrass, oilseed radish, and crimson clover to PRE and POST residual herbicide applications in corn. From 2016 to 2018 field trials were conducted in Michigan to determine the tolerance of annual ryegrass, oilseed radish, and crimson clover to 13 PRE and 14 POST (applied to V2 corn) herbicides. Cover crops were interseeded into corn at the V3 and V6 stages. Greenhouse experiments to evaluate these species were also conducted from 2016 to 2018; PRE and POST herbicides were applied at 1×, 0.5×, and 0.25× (0.25× was PRE only) of field-application rates. Based on these results, annual ryegrass can be interseeded into V3 or V6 corn following a PRE application of atrazine, clopyralid, saflufenacil, bicyclopyrone, isoxaflutole, or mesotrione, or a POST application of atrazine, bromoxynil, or mesotrione. Oilseed radish can be interseeded into V3 or V6 corn following a PRE application of clopyralid, atrazine, S-metolachlor, bicyclopyrone, or isoxaflutole or at V6 following application of acetochlor, dimethenamid-P, or mesotrione. Oilseed radish can also be interseeded following POST application of atrazine (571 g ai ha−1), bromoxynil, fluthiacet, acetochlor, mesotrione, dicamba + diflufenzopyr, or dimethenamid-P + topramezone. In greenhouse trials, crimson clover was tolerant to rimsulfuron, saflufenacil, and pyroxasulfone applied PRE. Annual ryegrass and oilseed radish can be interseeded into corn at the V3 and V6 stages, but special attention must be given to cover crop species selection and herbicide label restrictions when following herbicide applications in corn.

Field studies were conducted at the Pontotoc Ridge–Flatwoods Branch Experiment Station in Pontotoc, MS, in 2016 and 2017 to determine sweetpotato crop response to saflufenacil and rimsulfuron/thifensulfuron-methyl. Saflufenacil treatments consisted of a factorial of two rates (25 or 50 g ai ha–1) by three application timings [0, 3, or 6 wk before transplanting (WBP)]. Rimsulfuron/thifensulfuron-methyl treatments consisted of a factorial of two rates (18/18 or 35/35 g ai ha–1) by two application timings (3 or 6 WBP). A nontreated check was included for comparison. Saflufenacil resulted in as much as 20% stunting injury in 2016, but ≤4% crop injury in 2017. Compared to the nontreated check, saflufenacil did not reduce yield of any sweetpotato grade regardless of application rate or timing. Findings from this trial indicate that saflufenacil applied in pre-transplanting burndown and field preparation procedures did not have a negative impact on the subsequent sweetpotato crop and that the current plant-back interval (4 to 5 mo) may be excessive. Applications of rimsulfuron/thifensulfuron-methyl at 35/35 g ha–1 made 3 WBP resulted in significant crop injury but did not reduce yield of any sweetpotato grade. Findings from this trial suggest that rimsulfuron/thifensulfuron-methyl applications up to 35/35 g ha–1 applied at least 6 WBP and 18/18 g ha–1 applied at least 3 WBP had little impact on sweetpotato crop growth and may be a safe preplant burndown option.

Living mulches can provide many sustainability benefits. However, living mulch–cash crop competition and unreliable weed control are major challenges in living mulch systems. In this study, we evaluated the potential of herbicides used at reduced rates in combination with living mulch to suppress weeds, while simultaneously reducing living mulch vigor. Herbicide treatments were a combination of two POST applications, each consisting of a single, different herbicide. Field trials were conducted in Freeville, NY, USA, using: fresh market field tomato as cash crop; sesbania and sunn hemp as living mulch; and the herbicides fomesafen, halosulfuron, metribuzin, and rimsulfuron. In 2015, when water was not limiting, tomato yield and living mulch biomass were positively correlated. This relationship was negative in 2016, likely because of drought during the growing season. Compared with the untreated living mulch check, using the herbicide treatments in combination with living mulch reduced tomato yield losses by up to 71% in 2015 and 51% in 2016. In these herbicide plus living mulch plots, weed biomass was reduced by up to 97%, compared with the weedy check. Living mulch in herbicide treatments generated up to 2500 kg ha−1 of dry matter during both 2015 and 2016, with an average ground cover of 63% in 2015 and 85% in 2016. A predominantly PRE herbicide with residual soil activity (metribuzin), followed by a herbicide with greater POST activity (halosulfuron/rimsulfuron) was the most effective herbicide application sequence. Results from our study indicate that well-designed herbicide applications may enhance the practicability of living mulch systems.

Invasive winter annual grass infestations on rangeland accumulate large quantities of litter on the soil surface, as plants senesce yearly and decompose slowly. It has been speculated that winter annual grass litter can adsorb soil-active herbicides and reduce overall performance. Three experiments were conducted from 2017 to 2018 at the Colorado State University Weed Research Laboratory to evaluate interception and subsequent desorption of herbicides applied to litter from three invasive winter annual grass species with simulated rainfall. Imazapic, rimsulfuron, and indaziflam were applied to medusahead [Taeniatherum caput-medusae (L.) Nevski], ventenata [Ventenata dubia (Leers) Coss.], and downy brome (Bromus tectorum L.) litter at two amounts (equivalent to 1,300 and 2,600 kg ha−1). Rainfall was simulated at 3, 6, 12, and 24 mm at 0, 1, and 7 d after herbicide application. Herbicide concentration from the collected rainfall was measured using liquid chromatography–tandem mass spectrometry. At 2,600 kg ha−1, B. tectorum herbicide interception was 84.3%, while V. dubia and T. caput-medusae averaged 76% herbicide interception. There were no differences in desorption among the three litter types. Simulated rainfall at 0 d after application recovered 100% of the intercepted rimsulfuron and imazapic from B. tectorum litter, while recovery decreased to 65% with rainfall at 1 or 7 d after application. Only 54% of indaziflam could be recovered at 0 d, and recovery decreased to 33% when rainfall was applied at 1 or 7 d after application. Applying soil-active herbicides before forecasted rain or tank mixing with a POST herbicide to provide initial control could potentially increase the amount of herbicide reaching the soil and provide more consistent invasive winter annual grass control.

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.

In the mid-Atlantic region, there is increasing interest in the use of intercropping strategies to establish cover crops in corn cropping systems. However, intercropping may be limited by potential injury to cover crops from residual herbicide programs. Field experiments were conducted from 2013 to 2015 at Pennsylvania, Maryland, and New York locations (n=8) to evaluate the effect of common residual corn herbicides on interseeded red clover and annual ryegrass. Cover crop establishment and response to herbicide treatments varied across sites and years. S-metolachlor, pyroxasulfone, pendimethalin, and dimethenamid-P reduced annual ryegrass biomass relative to the nontreated check, whereas annual ryegrass biomass in acetochlor treatments was no different compared with the nontreated check. The rank order of observed annual ryegrass biomass reduction among chloroacetamide herbicides was S-metolachlor>pyroxasulfone>dimethenamid-P>acetochlor. Annual ryegrass biomass was not reduced by any of the broadleaf control herbicides. Mesotrione reduced red clover biomass 80% compared to the nontreated check. No differences in red clover biomass were observed between saflufenacil, rimsulfuron and atrazine treatments compared to the nontreated check. Red clover was not reduced by any of the grass control herbicides. This research suggests that annual ryegrass and red clover can be successfully interseeded in silt loam soils of Pennsylvania following use of several shorter-lived residual corn herbicides, but further research is needed in areas with soil types other than silt loam or outside of the mid-Atlantic cropping region.

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 antidotes dichlormid, MON-4660, CGA-154281, R-29148, and MON-13900 were tested in the greenhouse to protect Zea mays L. (corn) against RPA 201772 injury. High rates of RPA 201772 injured four Z. mays hybrids > 30%. R-29148 and MON-13900 were the most effective of the five antidotes evaluated. R-29148 applied at rates ⩾ 45 g ha−1 provided excellent protection against RPA 201772 injury and also prevented injury to Z. mays from diketonitrile, the active metabolite of RPA 201772. In laboratory studies, R-29148 did not alter absorption of 14C-RPA 201772 from soil; however, R-29148 significantly enhanced the rate of RPA 201772 metabolism and inactivation in Z. mays. The mixed function oxidase inhibitor piperonyl butoxide (PBO) increased RPA 201772 injury on all hybrids. These results demonstrate that Z. mays tolerance to RPA 201772 can be enhanced with the use of antidotes such as R-29148 and MON-13900, that R-29148 protects Z. mays from RPA 201772 and diketonitrile by the enhancement of metabolism, and that oxidative reactions may be involved in the metabolism of RPA 201772 in Z. mays.

A time course of Egyptian broomrape development on tomato roots grown in pots in the greenhouse revealed an increase in the number of parasitic attachments from 2.6 at 4 weeks after planting (WAP) to 139 when rated 2 wk later. Rimsulfuron sprayed on tomato foliage as single and split applications (12.5 g ai/ha) 3, 4, 5, 6, and 7 WAP did not decrease Egyptian broomrape damage to tomato plants, as expressed by reduced tomato plant height and fresh weight, and the total number of parasitic attachments on the roots was not significantly different compared to the nontreated control. A single foliar application of rimsulfuron (25 g ai/ha) to tomato at 10 days after planting (DAP) and split applications at 10 and 20 and 10, 20, and 30 DAP significantly reduced the number of Egyptian broomrape inflorescences to 2.2, 0.2, and 0, respectively, 9 WAP compared with 19.6 inflorescences in the nontreated control. Application of the herbicide under the same conditions to tomato plants in pots covered with activated charcoal did not significantly influence the number of Egyptian broomrape inflorescences. Chlorsulfuron and triasulfuron applied directly to the soil at rates ≥ 3: 3.75 g ai/ha completely controlled Egyptian broomrape. Primisulfuron and chlorimuron, each at 22.5 g ai/ha, reduced the number of inflorescences to 0.6 and 0.4, respectively. Chlorimuron did not significantly reduce the total number of parasitic attachments on the roots, but primisulfuron at the same concentration significantly increased the total number of parasitic attachments on the root system to 50 as compared to 23 in the nontreated control. Tribenuron had no effect on the parasitism process, as reflected in the number of inflorescences and parasitic attachments on the roots.

Economic decision rules for postemergence herbicide control of barnyardgrass in corn were developed. Damage and control functions that formed the basis of an economic model were estimated. Barnyardgrass density and time of emergence relative to the crop were fundamental to calculate the damage function. The control function described barnyardgrass dry weight response to variable doses of two herbicides. Both the biologist's and economist's weed control decision rules, derived from the economic model, were influenced by time of weed emergence relative to the crop, corn yield, and price. Inclusion of time of weed emergence relative to the crop improved our interpretative ability of derived decision rules. The biologist's threshold weed density was more sensitive to changes in parameter values than the economist's optimal herbicide dose strategy. Herbicide use with recommended label dose was greater than either the economically optimal or the biologically effective doses. Use of the biologically effective dose for postemergence weed control decisions was cost efficient and could be of practical significance to corn growers.

Nine corn inbreds were selected from a test on 250 inbreds based on distinctly different response to herbicides. These inbreds showed up to 50,000-fold differences, which varied by inbred and herbicide. Only the inbred GA209 was sensitive to bentazon and the four sulfonylurea herbicides. GA209 and B90 were the most sensitive inbreds to the sulfonylurea herbicides. F2R was generally the most tolerant inbred. The pattern of herbicide tolerance varied greatly. For example, the relative tolerance of F2 to thifensulfuron was 12-fold more than to rimsulfuron while Mo17 was 56-fold more tolerant to rimsulfuron than thifensulfuron. F2 was fivefold more tolerant of chlorsulfuron than chlorimuron, whereas B80 was 100-fold the reverse. These inbreds provide a small subset of varieties to evaluate the large and inherent differences in herbicide tolerance of corn.