Glyphosate-resistant (GR) horseweed is a problematic weed for Michigan soybean growers. Additionally, rosette- and upright-horseweed growth types have been observed co-emerging during mid- to late summer in several Michigan fields. In the greenhouse, shade levels from 35% to 92% reduced rosette- and upright-horseweed biomass 31% to 99% compared with the upright growth type grown under 0% shade. Greater reductions in biomass occurred under 69% and 92% shade. Thus, increased shading by planting in narrow rows and/or planting green into cereal rye may improve horseweed suppression. A field experiment conducted over 3 site-years compared the effect of fall-planted cereal rye terminated with glyphosate 1 wk after planting (WAP; planting green) with a preemergence residual herbicide program (glyphosate + 2,4-D + flumioxazin + metribuzin) on horseweed control in soybean planted in three row widths (19, 38, and 76 cm). Planting green or applying a residual herbicide program across all row widths reduced horseweed biomass 86% to 91% and 95% to 99%, respectively, compared with soybean planted with no cover in 76-cm rows, 4 to 6 WAP. At soybean harvest, when a noneffective postemergence herbicide (glyphosate) was applied, horseweed biomass was 42% and 81% lower by planting green or applying a residual-herbicide program compared with no cover, respectively. Similarly, planting soybean in 19-cm rows reduced horseweed biomass compared with 38- and 76-cm rows. When an effective postemergence program was applied, similar horseweed biomass reductions were observed by planting green or applying a residual herbicide across all row widths. Additionally, soybean yield and economic returns were similar between planting green and applying a residual herbicide in 1 of 2 site-years. Integrating planting green and an effective postemergence herbicide program offers an alternative horseweed management strategy to applying a residual preemergence herbicide program.

Horseweed [Conyza canadensis (L.) Cronquist] grows in one of two distinct growth phenotypes, “rosette” and “upright” growth types, and they have recently been observed co-occurring in Michigan fields. Previous research found that upright plants from two glyphosate-resistant populations were 3- and 4-fold less sensitive to glyphosate than their rosette siblings. Further experiments were conducted to investigate whether differential glyphosate sensitivity of the growth types was due to glyphosate retention, absorption, or translocation. The total amount of glyphosate retained on the C. canadensis leaf surface was similar for both growth types; however, on a per-weight and per-area bases, the upright growth type retained 21% and 18% less glyphosate, respectively. Glyphosate absorption was up to 85% at 168 h after treatment (HAT), and was not different between the rosette and upright growth types or between the susceptible (S) and resistant (R) biotypes. Additionally, there was no difference in translocation between the two growth types within each biotype at any time point. Interestingly, at 168 HAT, [14C]glyphosate translocation was higher in the S rosette compared with the two growth types from the R biotype; however, the S upright type was similar to both R growth types. Thus, glyphosate resistance in the R biotype may be due to an alternative mechanism rather than impaired translocation, which has been cited as the primary mechanism of glyphosate resistance in C. canadensis. These results suggest that reduced glyphosate retention on a per-weight and per-area bases of the upright growth type may contribute to increased glyphosate tolerance due to a diluted concentration of glyphosate in the plant. However, another factor is likely related to the mechanism of resistance within the R biotype, which is contributing to a 3-fold difference in glyphosate sensitivity between the two growth types, such as alterations in EPSPS gene expression or changes in undescribed metabolism genes.

Alternative strategies are needed for management of glyphosate-resistant (GR) horseweed in soybean. Integrating a cereal rye cover crop with soybean planted in narrow rows may improve control and reduce herbicide selection pressure for herbicide-resistant horseweed biotypes. Four site-years of experiments were conducted in Michigan to determine whether fall-planted cereal rye terminated with glyphosate 1 wk prior to (early termination) or 1 wk after (planting green) planting in combination with narrow-row soybean improved GR horseweed management. At postemergence (POST) herbicide application, horseweed biomass was reduced by 71% to 90% when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow-row soybean suppressed horseweed through soybean harvest. When glyphosate was applied POST (noneffective), horseweed biomass was 36% to 46% lower when planting green compared with early terminated cereal rye and no cover. Similarly, planting soybean in 19- and 38-cm rows reduced horseweed biomass by 48% and 28%, respectively, compared with 76-cm rows. Cereal rye did not affect soybean yield pooled over 3 of 4 site-years; however, narrow row soybean yielded 11% to 18% higher than 76-cm rows. Soybean yield was 11% higher when an effective POST herbicide was applied. In conclusion, fall-seeded cereal rye or narrow-row soybean suppressed horseweed compared with no cover and 76-cm rows; however, the effects of early termination did not last throughout the growing season in most cases. Delaying cover crop termination by planting green reduced horseweed biomass and density through soybean harvest, but reduced yield in 1 site-year due to an increased incidence of white mold. These cultural practices have a positive influence on suppressing horseweed that should be part of an overall horseweed management strategy; however, the use of an effective POST herbicide is still needed for complete season-long horseweed management.

Horseweed [Conyza canadensis (L.) Cronquist] is a facultative winter annual weed that can emerge from March to November in Michigan. Fall-emerging C. canadensis overwinters as a rosette, while spring-emerging C. canadensis skips the rosette stage and immediately grows upright upon emergence. In Michigan, primary emergence recently shifted from fall to spring/summer and therefore from a rosette to an upright growth type. Growth chamber experiments were conducted to determine (1) whether both C. canadensis growth types could originate from a single parent and (2) whether common environmental cues can influence growth type. Variations in temperature, photoperiod, competition, shading, and soil moisture only resulted in the rosette growth type in four C. canadensis populations originating from seed collected from a single parent of the upright growth type. However, a vernalization period of 4 wk following water imbibition, but before germination, resulted in the upright growth type. Dose–response experiments were conducted to determine whether glyphosate sensitivity differed between C. canadensis growth types generated from a single parent of the upright growth type. Upright-type C. canadensis from known glyphosate-resistant populations ISB-18 and MSU-18 were 4- and 3-fold less sensitive to glyphosate than their rosette siblings, respectively. Interestingly, differences in glyphosate sensitivity were not observed between growth types from the susceptible population. These results suggest that while C. canadensis populations shift from winter to summer annual life cycles, concurrent increases in glyphosate resistance could occur.

Glyphosate-resistant horseweed is difficult to manage in no-tillage crop production fields and new strategies are needed. Cover crops may provide an additional management tool but narrow establishment windows and colder growing conditions in northern climates may limit the cover crop biomass required to suppress horseweed. Field experiments were conducted in 3 site-years in Michigan to investigate the effects of two fall-planted cover crops, cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, to suppress horseweed when integrated with three preplant herbicide strategies in no-tillage soybean. The preplant strategies were control (glyphosate only), preplant herbicide without residuals (glyphosate + 2,4-D), and preplant herbicide with residuals (glyphosate + 2,4-D + flumioxazin + metribuzin). Cereal rye produced 79% more biomass and provided 12% more ground cover than winter wheat in 2 site-years. Increasing seeding rate provided 41% more cover biomass in 1 site-year. Cover crops reduced horseweed density 47% to 96% and horseweed biomass by 59% to 70% compared with no cover at the time of cover crop termination. Cover crops provided no additional horseweed suppression 5 wk after soybean planting if a preplant herbicide with or without residuals was applied, but reduced horseweed biomass greater than 33% in the absence of preplant herbicides. Cover crops did not affect horseweed suppression at the time of soybean harvest or influence soybean yield. Preplant herbicide with residuals and without residuals provided at least 52% and 20% greater soybean yield compared with the control at 2 site-years, respectively. Cereal rye and winter wheat provided early-season horseweed suppression at biomass levels below 1,500 kg ha−1, lower than previously reported. This could give growers in northern climates an effective strategy for suppressing horseweed through the time of POST herbicide application while reducing selection pressure for horseweed that is resistant to more herbicide sites of action.

Integrated strategies for management of glyphosate-resistant (GR) horseweed are needed to reduce reliance on herbicides. Planting a cover crop after corn or soybean harvest in the Upper Midwest may reduce horseweed establishment and growth. Experiments were conducted in Michigan to determine if cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, and terminated with glyphosate at 1.27 kg ae ha−1 1 wk before planting (early termination) or 1 wk after soybean planting (planting green) would suppress establishment and growth of GR horseweed. Cover-crop biomass was 212% to 272% higher when termination was delayed by planting green compared with early termination. At the time of termination, cover crops reduced GR horseweed biomass 41% to 89% compared with no cover. Planting green increased the C:N ratio of cover-crop residue, which improved residue persistence and GR horseweed suppression at the time of POST herbicide application, approximately 5 wk after planting. Planting green reduced GR horseweed biomass 46% to 93% compared with no cover at the time of POST herbicide application; early termination provided less consistent suppression. Cover crops alone did not suppress GR horseweed through soybean harvest. Soybean yield was 30% to 108% greater when planting green compared with early termination at 2 site-years. Cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, provided early-season GR horseweed suppression. Results from this research indicate that the practice of planting green may improve GR horseweed suppression through the time of POST herbicide application.

Six experiments were conducted in 2018 on field sites located in Arkansas, Indiana, Michigan, Nebraska, Ontario, and Wisconsin to evaluate the off-target movement (OTM) of dicamba under field-scale conditions. The highest estimated percentages of dicamba injury in non–dicamba-resistant (DR) soybean were 55%, 44%, 39%, 67%, 15%, and 44% injury for noncovered areas and 55%, 5%, 13%, 42%, 0%, and 41% injury for covered areas during dicamba application in Arkansas, Indiana, Michigan, Nebraska, Ontario, and Wisconsin, respectively. The level of injury generally decreased as the downwind distance increased under covered and noncovered areas at all sites. There was an estimated 10% injury in non-DR soybean at 113, 8, 11, 8, and 8 m; and estimated 1% injury at 293, 28, 71, 15, and 19 m from the edge of treated fields downwind when plants were not covered during dicamba application in Arkansas, Indiana, Michigan, Ontario, and Wisconsin, respectively. Assessment of filter-paper collectors placed from 4 to 137 m downwind from the edge of the sprayed area suggested the dicamba deposition reduced exponentially with distance. The greatest injury to non-DR soybean from dicamba OTM occurred at Nebraska and Arkansas (as far as 250 m). Non-DR soybean injury was greatest adjacent to the dicamba sprayed area, but injury decreased with no injury beyond 20 m downwind or in any other direction from the dicamba sprayed area in Indiana, Michigan, Ontario, and Wisconsin. The presence of soybean injury under covered and noncovered areas during the spray period for primary drift suggests that secondary movement of dicamba was evident at five sites. Additional research is needed to determine the exact forms of secondary movement of dicamba under different environmental conditions.

The occurrence of herbicide tank contamination with dicamba or 2,4-D will likely increase with the recent commercialization of dicamba- and 2,4-D-resistant soybean. High-value sensitive crops, including dry bean, will be at higher risks for exposure. In 2017 and 2018, two separate field experiments were conducted in Michigan to understand how multiple factors may influence dry bean response to dicamba and 2,4-D herbicides, including 1) the interaction between herbicides applied POST to dry bean and dicamba or 2,4-D, and 2) the impact of low rates of glyphosate with dicamba or 2,4-D. Dry bean injury was 20% and 2% from POST applications of dicamba (5.6 h ae ha−1) and 2,4-D (11.2 g ae ha−1), respectively, 14 days after treatment (DAT). The addition of glyphosate (8.4 g ae ha−1) did not increase dry bean injury from dicamba or 2,4-D. Over 2 site-years the addition of dry bean herbicides to dicamba or dicamba + glyphosate (8.4 g ae ha−1) increased dry bean injury and reduced yield by 6% to 10% more than when dicamba or dicamba + glyphosate was applied alone. The interaction between 2,4-D (11.2 g ae ha−1) and dry bean herbicides was determined to be synergistic. However, 2,4-D (11.2 g ae ha−1) had little effect on dry bean with or without the addition of a dry bean herbicide program. These studies document that synergy also occurs between dicamba and dicamba + glyphosate and both common dry bean herbicide programs tested: 1) imazamox (35 g ha−1) + bentazon (560 g ha−1), and 2) fomesafen (280 g ha−1). The synergy between dry bean herbicide and dicamba and dicamba + glyphosate can increase plant injury, delay maturity, and reduce yield to a greater extent than dicamba or dicamba + glyphosate alone. This work emphasizes the need to properly clean out sprayers after applications of dicamba to reduce the risk of exposure to other crops.

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.

Dicamba and 2,4-D exposure to sensitive crops, such as dry bean, is of great concern with the recent registrations of dicamba- and 2,4-D–resistant soybean. In 2017 and 2018, field experiments were conducted at two Michigan locations to understand how multiple factors, including dry bean market class, herbicide rate, and application timing, influence dry bean response to dicamba and 2,4-D. Dicamba and 2,4-D at rates of 0.1%, 1%, and 10% of the field use rate for dicamba and 2,4-D choline were applied to V2 and V8 black and navy bean. Field-use rates for dicamba and 2,4-D choline were 560 and 1,120 g ae ha−1, respectively. There were few differences between market classes or application timings when dry bean was exposed to dicamba or 2,4-D. Estimated rates to cause 20% dry bean injury 14 d after treatment were 4.5 and 107.5 g ae ha−1 for dicamba and 2,4-D, respectively. When dicamba was applied at 56 g ae ha−1, light interception was reduced up to 51% and maturity was delayed up to 16 d. Although both herbicides caused high levels of injury to dry bean, yield reductions were not consistently observed. At four site-years, 2,4-D did not reduce dry bean yield or seed weight with any rate tested. However, when averaged over site-years, dicamba rates of 3.7, 9.8 and 17.9 g ae ha−1 were estimated to cause 5%, 10%, and 15% yield loss, respectively. Dicamba also reduced seed weight by 10% when 56 g ae ha−1 was applied. However, the germination of harvested seed was not affected by dicamba or 2,4-D. Long delays in dry bean maturity from dicamba injury can also indirectly increase losses in yield and quality due to harvestability issues. This work further stresses the need for caution when using dicamba or 2,4-D herbicides near sensitive crops.

The objective of this WSSA Weed Loss Committee report is to provide quantitative data on the potential yield loss in sugar beet due to weed interference from the major sugar beet growing areas of the United States and Canada. Researchers and extension specialists who conducted research on weed control in sugar beet in the United States and Canada provided quantitative data on sugar beet yield loss due to weed interference in their regions. Specifically, data were requested from weed control studies in sugar beet from up to 10 individual studies per calendar year over a 15-yr period between 2002 and 2017. Data collected indicated that if weeds are left uncontrolled under optimal agronomic practices, growers in Idaho, Michigan, Minnesota, Montana, Nebraska, North Dakota, Ontario, Oregon, and Wyoming would potentially lose an average of 79%, 61%, 66%, 68%, 63%, 75%, 83%, 78%, and 77% of the sugar beet yield. The corresponding monetary loss would be approximately US$234, US$122, US$369, US$43, US$40, US$211, US$12, US$14, and US$32 million, respectively. The average yield loss due to weed interference for the primary sugar beet growing areas of North America was estimated to be 70%. Thus, if weeds are not controlled, growers in the United States would lose approximately 22.4 million tonnes of sugar beet yield valued at approximately US$1.25 billion, and growers in Canada would lose approximately 0.5 million tonnes of sugar beet yield valued at approximately US$25 million. The high return on investment in weed management highlights the importance of continued weed science research for sustaining high crop yield and profitability of sugar beet production in North America.

Seven half-day regional listening sessions were held between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide-resistance management. The objective of the listening sessions was to connect with stakeholders and hear their challenges and recommendations for addressing herbicide resistance. The coordinating team hired Strategic Conservation Solutions, LLC, to facilitate all the sessions. They and the coordinating team used in-person meetings, teleconferences, and email to communicate and coordinate the activities leading up to each regional listening session. The agenda was the same across all sessions and included small-group discussions followed by reporting to the full group for discussion. The planning process was the same across all the sessions, although the selection of venue, time of day, and stakeholder participants differed to accommodate the differences among regions. The listening-session format required a great deal of work and flexibility on the part of the coordinating team and regional coordinators. Overall, the participant evaluations from the sessions were positive, with participants expressing appreciation that they were asked for their thoughts on the subject of herbicide resistance. This paper details the methods and processes used to conduct these regional listening sessions and provides an assessment of the strengths and limitations of those processes.

Herbicide resistance is ‘wicked’ in nature; therefore, results of the many educational efforts to encourage diversification of weed control practices in the United States have been mixed. It is clear that we do not sufficiently understand the totality of the grassroots obstacles, concerns, challenges, and specific solutions needed for varied crop production systems. Weed management issues and solutions vary with such variables as management styles, regions, cropping systems, and available or affordable technologies. Therefore, to help the weed science community better understand the needs and ideas of those directly dealing with herbicide resistance, seven half-day regional listening sessions were held across the United States between December 2016 and April 2017 with groups of diverse stakeholders on the issues and potential solutions for herbicide resistance management. The major goals of the sessions were to gain an understanding of stakeholders and their goals and concerns related to herbicide resistance management, to become familiar with regional differences, and to identify decision maker needs to address herbicide resistance. The messages shared by listening-session participants could be summarized by six themes: we need new herbicides; there is no need for more regulation; there is a need for more education, especially for others who were not present; diversity is hard; the agricultural economy makes it difficult to make changes; and we are aware of herbicide resistance but are managing it. The authors concluded that more work is needed to bring a community-wide, interdisciplinary approach to understanding the complexity of managing weeds within the context of the whole farm operation and for communicating the need to address herbicide resistance.

Earlier reports have summarized crop yield losses throughout various North American regions if weeds were left uncontrolled. Offered here is a report from the current WSSA Weed Loss Committee on potential yield losses due to weeds based on data collected from various regions of the United States and Canada. Dry bean yield loss estimates were made by comparing dry bean yield in the weedy control with plots that had >95% weed control from research studies conducted in dry bean growing regions of the United States and Canada over a 10-year period (2007 to 2016). Results from these field studies showed that dry bean growers in Idaho, Michigan, Montana, Nebraska, North Dakota, South Dakota, Wyoming, Ontario, and Manitoba would potentially lose an average of 50%, 31%, 36%, 59%, 94%, 31%, 71%, 56%, and 71% of their dry bean yield, respectively. This equates to a monetary loss of US $36, 40, 6, 56, 421, 2, 18, 44, and 44 million, respectively, if the best agronomic practices are used without any weed management tactics. Based on 2016 census data, at an average yield loss of 71.4% for North America due to uncontrolled weeds, dry bean production in the United States and Canada would be reduced by 941,000,000 and 184,000,000 kg, valued at approximately US $622 and US $100 million, respectively. This study documents the dramatic yield and monetary losses in dry beans due to weed interference and the importance of continued funding for weed management research to minimize dry bean yield losses.

Control of multiple-resistant Palmer amaranth populations in corn will rely heavily on the use of POST 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides. Therefore, field and greenhouse experiments were conducted to: (1) evaluate Palmer amaranth control with four HPPD inhibitors alone and in combination with atrazine at two application timings and (2) investigate the joint activity of HPPD-inhibiting herbicides and atrazine in atrazine-resistant (AR) and atrazine-susceptible (AS) Palmer amaranth populations. Control of the AR Palmer amaranth population varied among the HPPD-inhibiting herbicides with tolpyralate>tembotrione=topramezone>mesotrione based on GR50 values in the greenhouse. In the field, Palmer amaranth control was lower when the HPPD-inhibiting herbicides, with the exception of tolpyralate, were applied to 15- vs. 8-cm-tall Palmer amaranth. Tolpyralate controlled Palmer amaranth ≥95% at both application timings. The addition of atrazine (560 g ai ha−1) improved Palmer amaranth control with mesotrione and topramezone at the 8-cm application timing and with mesotrione and tembotrione at the 15-cm application timing. In the greenhouse, joint activity of mesotrione and atrazine and tembotrione and atrazine was synergistic with both the AR and AS Palmer amaranth populations. In the AR population, an additional 980 g ai ha−1 of atrazine (8X) was needed to cause a synergistic response compared with the AS population. Synergistic responses with mesotrione were detected with all atrazine rates for the AS population and for atrazine rates ranging from 280 to 2,240 g ai ha−1 for the AR population. Only additive responses were observed when atrazine was applied with tolpyralate and topramezone, indicating that joint activity in the form of synergism occurs more readily with the triketones compared with the benzopyrazoles. When faced with an AR Palmer amaranth population, the addition of atrazine to HPPD inhibitors may increase the overall success of weed management due to joint activity.

Three field experiments were conducted from 2013 to 2015 in Barry County, MI to evaluate the effectiveness of PRE, POST, and one- (EPOS) and two-pass (PRE followed by POST) herbicide programs for management of multiple-resistant Palmer amaranth in field corn. The Palmer amaranth population at this location has demonstrated resistance to glyphosate (Group 9), ALS-inhibiting herbicides (Group 2), and atrazine (Group 5). In the PRE only experiment, the only herbicide treatments that consistently provided ~80% or greater control were pyroxasulfone and the combination of mesotrione + S-metolachlor. However, none of these treatments provided season-long Palmer amaranth control. Only topramezone provided >85% Palmer amaranth control 14 DAT, in the POST only experiment. Of the 19 herbicide programs studied all but three programs provided ≥88% Palmer amaranth control at corn harvest. Herbicide programs that did not control Palmer amaranth relied on only one effective herbicide site of action and in one case did not include a residual herbicide POST for late-season Palmer amaranth control. Some of the EPOS treatments were effective for season-long Palmer amaranth control; however, application timing and the inclusion of a residual herbicide component will be critical for controlling Palmer amaranth. The programs that consistently provided the highest levels of season-long Palmer amaranth control were PRE followed by POST herbicide programs that relied on a minimum of two effective herbicide sites of action and usually included a residual herbicide for late-season control.

Greenhouse and laboratory studies were conducted to examine certain characteristics of RPA 201772 and how they may affect its utility. 14C-RPA 201772 was used to determine the stability of RPA 201772 in various pH spray solutions over time. After 24 h, degradation of RPA 201772 was 20, 36, and 93% in spray solutions adjusted to pH 4.0, 7.0, and 10.0, respectively. The major metabolite was diketonitrile (DKN), which is herbicidally active. In addition, 9 and 15% of the RPA 201772 had degraded into an inactive benzoic acid derivative at pH 7.0 and 10.0, respectively. The differences in herbicidal activity of RPA 201772 and its metabolite DKN were also evaluated. Corn seeds and velvetleaf seeds readily imbibed RPA 201772, but only low levels of DKN were imbibed. Preemergence (applications of RPA 201772 and DKN were equally active on corn and velvetleaf. Further studies determined that the principal sites of uptake of RPA 201772 by corn was roots and seeds for four different corn hybrids. Another study determined that light was required for RPA 201772 activity. Corn shoots from seeds germinated under 14 h of light exhibited rate-dependent injury, while corn germinated and grown in the darkness was not injured.

Five biotypes of common cocklebur that were not controlled with acetolactate synthase (ALS)-inhibiting herbicides were tested in greenhouse and laboratory studies to determine the magnitude of resistance and cross-resistance to four ALS-inhibiting herbicides. In vivo inhibition of ALS was also evaluated. Based on phytotoxicity, all five ALS-resistant biotypes of common cocklebur were > 390 times more resistant than the susceptible biotype to imazethapyr. However, only four of these biotypes were also resistant to another imidazolinone, imazaquin. Two biotypes were cross-resistant to the sulfonylurea, chlorimuron, and the triazolopyrimidine sulfonanilide, NAF-75. One biotype demonstrated intermediate susceptibility to imazaquin, chlorimuron, and NAF-75. In all cases, the resistance exhibited at the whole plant level was associated with an insensitive ALS.

Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of four Zea mays L. hybrids to RPA 201772. Differences in Zea mays tolerance were quantified by determining the herbicide rate required to injure and reduce Z. mays height 50% (GR50). GR50 values indicated that the Z. mays hybrids ‘Pioneer 3751’ and ‘Pioneer 3737’ were less tolerant to RPA 201772 than the hybrids ‘Pioneer 3394’ and ‘Pioneer 3963.’ Experiments using 14C-RPA 201772 were conducted to determine if hybrid sensitivity was due to differential uptake, translocation, or metabolism of the herbicide. Differences in hybrid tolerance were primarily due to differential herbicide metabolism rates. The time required for 50% inactivation (T½) of RPA 201772 was 42 and 52 h for the more tolerant hybrids and 66 and 93 h for the more sensitive hybrids. Increased uptake of RPA 201772 was also a contributing factor to the sensitivity of one of the hybrids.

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.