Imidazolinone-resistant Research Articles

Gene flow and spontaneous mutations are responsible for imidazolinone herbicide-resistant weedy rice (Oryza sativa L.)

For more than two decades, weedy rice (Oryza sativa L.) has been controlled in rice fields by using imidazolinone (IMI) herbicide-resistant rice technology (Clearfield®). Outcrossing in weedy rice populations and spontaneous mutations are potential problems with herbicide-resistant crop management technologies, such as the IMI-resistant rice. The aim of this study was to characterize the mechanism of IMI herbicide resistance in weedy rice through dose-response bioassay study and evaluating amino acid substitutions in acetolactate synthase (ALS) protein. A total of 118 suspected IMI-resistant weedy rice samples, which survived in the field after an IMI herbicide application, were collected at harvest time from Türkiye in 2020 and 2021. Single-dose imazamox application experiment revealed that 38 plants survived herbicide treatment. The imazamox resistance of the surviving plants was confirmed by dose-response experiment. ALS gene region underwent a sanger DNA partial sequencing. No substitution was found in 10 samples, however, amino acid substitutions were found in 26 samples with S563N, one sample with S653T, and one sample with E630D. The S653N point is the same substitution point that serves as the origin of resistance for the Clearfield® rice varieties that are commonly cultivated in the region. It has been hypothesized that the gene flow from IMI-resistant rice may be the cause of resistance in the IMI resistant weedy rice samples with S653N. The other substitution, S653T, were considered spontaneous mutation to IMI resistance. Interestingly, the S653T mutation was detected for the first time in weedy rice. The mechanism of resistance of 10 resistant weedy rice was not confirmed in this study, however, it may be a non-target resistance or another mutation point in target site, but evidently, they did not acquire resistance by gene flow from IMI-resistant rice. It has been concluded that the effectiveness of IMI-resistant rice technology in controlling weedy rice has drastically decreased due to possible gene flow, spontaneous mutation and non-target resistance. In addition to cultural controls like clean seed, clean machinery and crop rotation, other herbicide-tolerant rice systems such as Provisia® and Roxy-RPS® rice are needed to create a diverse weedy rice management ensemble available for rice production and move towards sustainable rice farming.

Effective Striga control and yield intensification on maize farms in western Kenya with N fertilizer and herbicide-resistant variety.

Maize production in western Kenya is limited by the spread of parasitic weed Striga hermonthica and depletion of soil nutrient stocks. Nitrogen (N) fertilizer and imidazolinone resistant (IR) maize are key elements in the agronomic toolbox to control infestations and enhance yields. The circumstances under which their use, individually or combined, is most effective on farmer fields have not been well documented. Inappropriate management decisions and low returns on investments arise from this knowledge gap, causing hunger and poverty in smallholder communities to persist. Experiments were carried out on 60 fields in three different agroecosystems of western Kenya using full-factorial treatments with non-herbicide treated maize (DH) and herbicide treated maize (IR), and N fertilizer omission and application. Trials were stratified on a field with low and high soil fertility within individual farms and repeated over two seasons. Cultivating IR maize instead of DH maize decreased the emergence of Striga with 13 shoots m-2 on average while applying N fertilizer on DH maize led to a reduction of 5 shoots m-2 on average. Decreases of Striga by use of IR maize and N fertilizer were between 6 and 23 shoots m-2 larger at the site with high levels of infestation than at the sites with medium or low emergence. Input of N fertilizer increased grain harvests by 0.59 ton ha-1 on average while use of IR maize enhanced the productivity with 0.33 ton ha-1 on average. Use of N fertilizer had similar yield effects in all three sites, whereas use of IR maize at the site with high Striga emergence increased maize production by 0.26-0.39ton ha-1 more than at the sites with medium or low emergence. The greater Striga responses to IR maize and the greater yield responses to N fertilizer demonstrate their use could be optimized according to field conditions and management goals. Combining IR maize and N fertilizer has larger added yield benefits where their individual effects on grain productivity are smaller. Findings from this study indicate that farmers in western Kenya require guidance on how to align the use of herbicide resistant maize and inorganic N inputs with the level of Striga infestation and maize yield on their fields for effectively controlling the pernicious weed and enhancing food production.

Interference and management of herbicide-resistant crop volunteers

AbstractSince the commercialization of herbicide-resistant (HR) crops, primarily glyphosate-resistant crops, their adoption has increased rapidly. Multiple herbicide resistance traits in crops such as canola (Brassica napusL.), corn (Zea maysL.), cotton (Gossypium hirsutumL.), and soybean [Glycine max(L.) Merr.] have become available in recent years, and management of their volunteers needs attention to prevent interference and yield loss in rotational crops. The objectives of this review were to summarize HR crop traits in barley (Hordeum vulgareL.), canola, corn, cotton, rice (Oryza sativaL.), soybean, sugarbeet (Beta vulgarisL.), and wheat (Triticum aestivumL.); assess their potential for volunteerism; and review existing literature on the interference of HR crop volunteers, yield loss, and their management in rotational crops. HR crop volunteers are problem weeds in agronomic cropping systems, and the impact of volunteerism depends on several factors, such as crop grown in rotation, the density of volunteers, management practices, and microclimate. Interference of imidazolinone-resistant (IR) barley or wheat volunteers can be a problem in rotational crops, particularly when IR crops such as canola or wheat are grown. HR canola volunteers are abundant in the Northern Great Plains due to high fecundity, seed loss before or during harvest, and secondary seed dormancy, and they can interfere in crops grown in rotation such as flax (Linum usitatissimumL.), field peas (Pisum sativumL.), and soybean. HR corn volunteers are competitive in crops grown in rotation such as corn, cotton, soybean, and sugarbeet, with yield loss depending on the density of HR corn volunteers. Volunteers of HR cotton, rice, soybean, and sugarbeet are not major concerns and can be controlled with existing herbicides. Herbicide options would be limited if the crop volunteers are multiple HR; therefore, recording the cultivar planted the previous year and selecting the appropriate herbicide are important. The increasing use of 2,4-D, dicamba, glufosinate, and glyphosate in North American cropping systems requires research on herbicide interactions and alternative herbicides or methods for controlling multiple HR crop volunteers.

Response of acetyl-CoA carboxylase-resistant rice cultivars and advanced lines to florpyrauxifen-benzyl

AbstractFlorpyrauxifen-benzyl and quizalofop were available for POST applications in 2018; however, little is known about the response of acetyl-CoA carboxylase (ACCase)–resistant rice cultivars and advanced lines to POST herbicides. A field study was conducted in 2017 and 2018 at Stoneville, MS, to characterize the response of ACCase-resistant rice cultivars and advanced lines to POST applications of florpyrauxifen-benzyl. The imidazolinone-resistant (IR) rice cultivars ‘CL163’ and ‘CLXL 745’, and ACCase-resistant rice cultivars ‘PVL01’, ‘PVL013’, ‘PVL024-B’, ‘PVL038’, ‘PVL080’, and ‘PVL081’were treated with florpyrauxifen-benzyl at 0 (nontreated control for each cultivar) and 58 g ai ha–1 at the four-leaf to one-tiller (LPOST) growth stage. At 14 d after treatment (DAT), PVL01 was injured 5% to 6% greater than CLXL 745, PVL013, and PVL081; however, injury was ≤10% at that evaluation for all cultivars. Similarly, injury was ≤13% for all cultivars 28 DAT. Mature heights were reduced for all cultivars except PVL013 and PVL081. Rough rice yield was ≥100% of the control for all cultivars except PVL081, PVL013, and CL163. Results suggest that florpyrauxifen-benzyl can safely be applied POST to rice cultivars grown in Mississippi as well as ACCase-resistant cultivars that are currently under development.

Stress response and detoxification mechanisms involved in non‐target‐site herbicide resistance in sunflower

AbstractThe nature of non‐target‐site herbicide resistance (NTSR) to imidazolinone (IMI) in HA425 sunflower (Helianthus annuus L.) has not yet been fully characterized but could be related to xenobiotic metabolism. The objective of this study was to evaluate the role of cytochrome P450 monooxygenases (P450s) and other detoxification‐related proteins in NTSR in sunflower. Two sunflower inbred lines were used: HA 425, which is IMI resistant (Imisun), and HA 89, which is IMI susceptible. The growth response to the IMI herbicide imazethapyr in combination with the P450 inhibitors 1‐aminobenzotriazole (ABT) or piperonyl butoxide (PBO) was evaluated in 15‐d‐old sunflower plantlets. Roots were collected, and label‐free quantitation (LFQ) proteomic analysis was carried out to characterize the NTSR mechanisms involved in the IMI resistance trait in sunflower. The increased phytotoxicity of imazethapyr observed in the resistant line when ABT or PBO were present agrees with the hypothesis that NTSR mechanisms may contribute to herbicide resistance in sunflower. The herbicide treatment also led to changes in the levels of biotic and abiotic stress‐related proteins, glutathione S‐transferases, and cytochrome P450s, among others. Plant growth and root protein expression response to IMI herbicides in sunflower would be a combination of stress‐related and detoxification mechanisms. Understanding the basis of NTSR becomes helpful to exploit this trait in sunflower crop and to develop xenobiotic‐resistant, soil‐remediating cultivars.

Imazamox detoxification and recovery of plants after application of imazamox to an imidazolinone resistant sunflower hybrid

Imidazolinone herbicides combined with imidazolinone resistant (IMI-R) crops provide a tool for solving the important problem of the occurrence of weeds during the early growth stages of sunflower. These herbicides inhibit the synthesis of branched chain amino acids by interrupting the key enzyme acetohydroxyacid synthase (AHAS). We studied the imazamox detoxification in an IMI-R sunflower hybrid together with plant growth and photosynthetic performance. Inhibition of photosynthesis and growth were observed as initial effects of imazamox application. A slight decrease in AHAS activity was also noticed. These effects disappeared within two weeks after application. A fast and well-functioning detoxification mechanism for the herbicide, of which the content decreased for about 90 % at 14 d after application, seems to be responsible for this. The activity of the xenobiotic detoxifying enzyme glutathione S-transferases (GSTs) significantly increased after imazamox application. Our results suggest that the metabolite glutathione serves as an auxiliary tool for imazamox detoxification through conjugation reactions realized by the GSTs, thereby taking part in the non-target mechanisms of resistance in IMI-R sunflower hybrids.

Influence of a thiamethoxam seed treatment on acetolactate synthase–inhibiting herbicide–induced injury to inbred and hybrid imidazolinone-resistant rice

AbstractThe increased use of insecticide seed treatments in rice has raised many questions about the potential benefits of these products. In 2014 and 2015, a field experiment was conducted near Stuttgart and Lonoke, AR, to evaluate whether an insecticide seed treatment could possibly lessen injury from acetolactate synthase (ALS)–inhibiting herbicides in imidazolinone-resistant (IR) rice. Two IR cultivars were tested (a hybrid, ‘CLXL745’, and an inbred, ‘CL152’), with and without an insecticide seed treatment (thiamethoxam). Four different herbicide combinations were evaluated: a nontreated control, two applications of bispyribac-sodium (hereafter bispyribac), two applications of imazethapyr, and two applications of imazethapyr plus bispyribac. The first herbicide application was to two- to three-leaf rice, and the second immediately prior to flooding (one- to two-tiller). At both 2 and 4 wk after final treatment (WAFT), the sequential applications of imazethapyr or bispyribac plus imazethapyr were more injurious to CLXL745 than CL152. This increased injury led to decreased groundcover 3 WAFT. Rice treated with thiamethoxam was less injured than nontreated rice and had improved groundcover and greater canopy heights. Even with up to 32% injury, the rice plants recovered by the end of the growing season, and yields within a cultivar were similar with and without a thiamethoxam seed treatment across all herbicide treatments. Based on these results, thiamethoxam can partially protect rice from injury caused by ALS-inhibiting herbicides as well as increase groundcover and canopy height; that is, the injury to rice never negatively affected yield.

Imazethapyr Co-Application Interactions in Imidazolinone-Resistant Rice

A study was conducted to evaluate interactions of imazethapyr co-application with propanil, thiobencarb, or a prepackaged mixture of propanil plus thiobencarb. At 7, 14, 21, and 49 d after treatment (DAT), synergism occurred for red rice treated with imazethapyr at 70 g ai ha−1mixed with propanil plus thiobencarb at 1,680 and 3,360 g ai ha−1. A synergistic response was also observed with imazethapyr at 70 g ha−1mixed with propanil at 1,680 g ha−1; however, all other co-applications evaluated resulted in a neutral response with no antagonism observed. At 7 DAT, antagonism was observed for barnyardgrass when treated with imazethapyr at 70 g ha−1mixed with propanil plus thiobencarb at 1,680 g ha−1, propanil at 840 g ha−1, or thiobencarb at 840 and 1,680 g ha−1; however, a synergistic response occurred for barnyardgrass control at 14, 21, and 49 DAT with imazethapyr at 70 g ha−1mixed with propanil plus thiobencarb at 3,360 g ha−1. The synergistic response observed for red rice and barnyardgrass control with a mixture of imazethapyr plus propanil plus thiobencarb can benefit producers by increasing control of red rice and barnyardgrass, and this mixture contains three herbicides with three different modes of action, which can be part of an overall resistance-management strategy in imidazolinone-resistant (IR) rice.

Effect of Lablab purpureus L. cover crop and imidazolinone resistant (IR) maize on weeds in drought prone areas, Kenya

Weeds compete for nutrients and soil moisture resulting to low maize yields in dry lands. A three year field study was initiated in 2009 at Kenya Agricultural and Livestock Research Organization, Kiboko to evaluate the effect of dolichos bean (Lablab purpureus L.) and open pollinated imazapyr herbicide coated imidazolinone-resistant (IR)1 maize on weed species composition, density, and maize yield. Initially, weed species were identified, and then controlled using glyphosate at 1.6 kg ai ha−1. Twenty four plots were marked, each measuring 4 × 5 m. Six treatments 1) IR-maize coated, 2) IR-maize coated + brown dolichos, 3) IR-maize coated + black dolichos 4) IR-maize uncoated, 5) IR-maize uncoated + brown dolichos, 6) IR-maize uncoated + black dolichos were laid out in a randomized complete block design replicated four times. IR-maize was planted at a spacing of 90 × 45 cm and 2 seeds per hole. Weeds were sampled from a one meter squared quadrat 21 and 42 days after planting (DAP). The data was subjected to analysis of variance using Genstat version 12.0. Eighteen (18) weed species were prevalent before the experiment. Interaction of dolichos and herbicide coated IR-maize had no significant (P > 0.05) effect on weed species composition and density. Dolichos significantly (P < 0.05) reduced the density of Portulaca quadrifida L. and Paraknoxia parviflora L., and increased Eleusine indica L. Weed species composition decreased by 14% (21 DAP) and 33% (42 DAP) in plots with dolichos compared to no cover. Maize yields were significantly (P < 0.05) higher in plots with dolichos than without. From the findings, this study recommends dolichos integration with coated IR-for weed management and increased maize yields in drylands.

Agronomic Performance and Genotype × Environment Interaction of Herbicide‐Resistant Maize Varieties in Eastern Africa

ABSTRACTStriga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze severely affect maize (Zea mays L.) production in sub‐Saharan Africa. A single Striga plant produces a large number of seeds that form a bank of viable but dormant seed in the soil until they get a chemical signal from suitable maize host roots. Imidazolinone‐resistant (IR) open‐pollinated maize varieties (OPVs) developed for Striga control were tested in diverse environments in four countries of eastern Africa in 2004. The objective of the study was to assess the agronomic performance of IR maize and genotype × environment interactions (GE) for grain yield (GY) and the number of emerged Striga plants across 17 environments under Striga‐infested and Striga‐free conditions. In the combined analysis of variance across Striga‐infested and Striga‐free locations, mean squares for genotypes and GE were significant for most measured traits. The best IR maize variety (STR‐VE‐216) outyielded the Striga‐tolerant and commercial genotypes by 113 and 89%, respectively, under Striga‐infested conditions. IR OPVs supported significantly fewer emerged Striga plants relative to the check varieties. Under Striga‐free conditions, IR OPVs showed GY advantage over commercial varieties. Under Striga‐infested conditions genotypic variance () was larger than genotype × location variance () for GY and number of emerged Striga plants at 12 wk after planting. The genetic correlations among locations under Striga‐infested conditions were high (0.990), suggesting little GE between most environments used. Cluster analysis of genotypes under Striga‐infested locations revealed two major groups that separated the IR OPVs from the check varieties. The outstanding performance of selected IR OPVs indicates that their use for Striga control would reduce the Striga seed bank while benefiting farmers with high GY.

Simulated Genetically Modified Triticale: Adventitious Presence from Volunteers in Four Cropping Systems

ABSTRACTHarvesting volunteer genetically modified (GM) crops is a likely source of adventitious presence (AP) that could preclude coexistence with conventional crops. Spring triticale (×Triticosecale Wittm. ex A. Camus) is proposed as a biofuel and, with genetic modifications, as a platform crop for bioindustrial products. We investigated seed production of simulated volunteer triticale in four western Canadian cropping systems: glyphosate‐resistant (GR) canola (Brassica napus L.); glufosinate‐resistant (GLUR) canola; field pea (Pisum sativum L.); and imidazolinone‐resistant (IR) wheat, in 2006 and 2007 at two locations, and quantified crop yield losses, triticale seed viability, and AP of triticale in harvested crops. Fecundity of volunteers in the absence of herbicides varied between locations, from 3506 to 9280 seeds m–2, and with crop: GLUR canola = GR canola &lt; IR spring wheat &lt; field pea. Herbicides applied preseeding (PRE) reduced triticale densities between 30 and 80%, and survivors produced between 178 and 2052 seeds m–2. Herbicides applied in‐crop (POST) were more effective and reduced triticale densities between 8 and &gt; 99% and fecundity between &lt;1 and 1231 seeds m–2. The combination of PRE and POST applications reduced fecundity below that of PRE or POST alone in 3 of 16 crop years. The AP was &gt;1% of the crop (weight/weight) in 3 of 13 crop years when both PRE and POST herbicides were applied. Volunteer triticale is a probable source of AP in subsequent crops that may reduce acceptability in some GM sensitive markets.

Herbicide Applied to Imidazolinone Resistant-Maize Seed as aStrigaControl Option for Small-Scale African Farmers

Strigais a major constraint to food production in Africa. Most technologies developed for the eradication ofStriga asiaticafrom the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha−1provide near season long control ofStrigaand can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improveStrigacontrol when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing floweringStrigaplants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season longStrigacontrol and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa whereStrigais problematic.

The U.S. Witchweed Eradication Effort Turns 50: A Retrospective and Look-Ahead on Parasitic Weed Management

The U.S. Witchweed Eradication Effort Turns 50: A Retrospective and Look-Ahead on Parasitic Weed Management

Hybridization in a Commercial Production Field between Imidazolinone-Resistant Winter Wheat and Jointed Goatgrass (Aegilops cylindrica) Results in Pollen-Mediated Gene Flow ofImi1

Imidazolinone-resistant (IR) winter wheat allows selective control of jointed goatgrass with the herbicide imazamox. However, the spontaneous hybridization between jointed goatgrass and IR winter wheat threatens the value of the IR technology. The objectives of this study were to determine if F1hybrids collected in a commercial production field under IR winter wheat–fallow rotation in Oregon and their first-backcross progeny (BC1) carried theImi1gene and were resistant to imazamox, and to analyze the parentage of F1and BC1plants. The average seed set of the F1spikes was 3.3%, and the average germination of BC1seed was 52%. All F1and BC1plants tested carriedImi1. Jointed goatgrass plant mortality was 100% when treated with imazamox at 0.053 kg ai ha−1, compared to 0% for IR winter wheat and BC1progeny. All F1plants had jointed goatgrass as the maternal parent; whereas, most BC1plants (85.7%) were produced with IR winter wheat as the paternal backcross parent. Although the backcrossing of F1hybrids with jointed goatgrass is very low, it demonstrates the potential for introgression ofImi1from IR winter wheat into jointed goatgrass under natural field conditions.

Adventitious Presence: Volunteer Flax (Linum usitatissimum) in Herbicide-Resistant Canola (Brassica napus)

Flax is in the process of development as a crop for bio-industrial and nutraceutical products predicated on the use of genetic modification. Before genetically modified (GM) flax is commercially released, effective management practices should be developed to minimize adventitious presence (AP) of GM volunteer flax in subsequent crops. Field research was conducted at four locations during 2007 and 2008 in central Alberta to quantify and mitigate AP of volunteer flax in glufosinate-resistant (GR) and imidazolinone-resistant (IR) canola. A single preplant application of glyphosate at 1,250 g ae ha−1in GR canola reduced volunteer flax density from 54 to 3 plants m−2and seed production from 5,963 to 233 seeds m−2. Similarly, the recommended rate of POST glufosinate (600 g ai ha−1) alone effectively controlled volunteer flax and reduced flax seed viability to &lt; 8% and AP to 0.2%. A combination of preplant (glyphosate) and POST (glufosinate) at recommended rates reduced volunteer flax seed production, yield, and AP to near zero in GR canola. Glyphosate applied preplant was equally effective in IR canola, reducing volunteer flax density from 56 to 2 plants m−2, and seed production from 5,571 to 472 seeds m−2. Imazamox + imazethapyr applied POST at all the rates poorly controlled volunteer flax and, even in combination with preplant glyphosate, cannot be recommended for control of flax volunteers in IR canola.

Participatory evaluation of integrated pest and soil fertility management options using ordered categorical data analysis

Maize is becoming the major food crop around Lake Victoria. Major constraints to its production are Striga, stem borer, and declining soil fertility. Innovative integrated technologies have been developed: the ‘push–pull’ system (intercropping with Desmodium and surrounded by Napier grass), soybean and Crotalaria rotations, and imidazolinone-resistant (IR) maize seed. In 12 demonstration trials in four villages in Siaya and Vihiga districts (Kenya) and two villages in Busia (Uganda) in 2003 and 2004, 504 farmers evaluated all cropping systems and a mono-cropped continuous maize, each cropped with IR or local maize, and supplemented or not with fertilizer, totaling 16 treatments. Farmers evaluated all treatments for yield, resistance to Striga and stem borer, improvement of soil fertility, and provided an overall evaluation score, using an ordered scale of 1 (very poor) to 5 (very good). Data were analyzed using ordinal regression, estimating log odds ratios. The results show significant preferences for all treatments over the control. Push–pull with IR and fertilizer had the highest log odds ratio (2.93), so the odds of farmers preferring this treatment are 18.7 times the odds that farmers prefer the control. The odds ratios for the other push–pull combinations were generally highest (9–15), followed by the rotation systems with Crotalaria (3.5–7.0), and soybeans, especially with IR maize and fertilizer (odds ratio of 5.7). In mono-cropping systems, IR maize was only appreciated in combination with fertilizer, and then only in 2004. Push–pull and Crotalaria were more appreciated in 2004 than in 2003. Farmers in Vihiga had a stronger preference for push–pull, and those in Busia for soybean rotations. Significant differences among farmers were observed, but the effects were small. Women appreciated push–pull more than men, while other technologies were gender-neutral. Older farmers were more likely to prefer push–pull and Crotalaria with fertilizer. Livestock ownership was not found to have an effect on technology preferences. Measured yield, stem borer and Striga infestation all had significant but small effects, although their inclusion did not eliminate the treatment effects, indicating that other factors are still important. OLS of the scores for different criteria on the overall score shows yield to be the most important criterion (coefficient of 0.40), followed by soil fertility enhancement (0.25) and Striga resistance (0.13). Labor saving (0.09) and stem borer resistance (0.03) are relatively minor criteria. This research shows that scoring and analysis with ordinal regression is a convenient way to solicit and analyze farmers’ preferences for new technologies, with wide applicability in farming systems and participatory research. Its application here shows that farmers like the new technologies, especially push–pull, but that there are substantial differences between years, sites and farmers. The use of this method can be very helpful to define and focus further research and formulate specific and targeted recommendations for agricultural extension.

Field-Scale Time of Weed Removal in Canola

Field-scale experiments were conducted at several western Canada locations to determine the importance of early weed removal over variable landscapes. In eight of 10 cases, imidazolinone-resistant (IR) canola yield decreased linearly as herbicide application (15/15 g/ha imazamox/imazethapyr or 15/15 g/ha imazamox/imazethapyr plus 150 g/ha clopyralid) was delayed beyond the one- to two-leaf stage. In two of 10 cases, canola oil content also decreased as herbicide treatment was delayed. Canola yields at all environments (location by year combinations) averaged 2,073, 1,872, or 1,650 kg/ha when treated at the one- to two-, three- to five-, or six- to seven-leaf stage, respectively. Assuming canola prices from a low of $250/t to a high of $650/t, growers could lose $50 to $131/ha, respectively, by delaying herbicide application from the one- to two- to the three- to five-leaf stage, or $106 to $275/ha, respectively, by delaying herbicide application from the one- to two- to the six- to seven-leaf stage.

Response of Heterozygous and Homozygous Imidazolinone‐Resistant Spring Wheat Genotypes to Imazamox

Imidazolinone‐resistant (IR) spring wheat (Triticum aestivum L.) was released commercially in Canada in 2004, providing growers with a new opportunity to selectively control a broad range of weeds in wheat, including volunteer cereals and annual grasses. Imidazolinone (IMI) herbicides inhibit acetolactate synthase in susceptible (S) plants, but IR wheat plants have an altered target site that confers resistance to IMI herbicides. The IR trait has served as a phenotypic marker in a number of gene flow studies, but the response of IR F1 wheat hybrids to various doses of imazamox has not been adequately characterized. Therefore, a greenhouse study was initiated to assess the whole‐plant response of several spring wheat genotypes with different parental backgrounds to increasing doses of imazamox. No differences in biomass accumulation prior to imazamox application were detected among genotypes. Biomass accumulation after foliar treatment with imazamox was similar within all groups of genotypes, with no significant differences detected within F1 or S genotype groups. The resistance factor (GR50 ratio) of the single IR genotype tested in the study was six‐fold greater than the mean of the F1 genotypes, which was itself six‐fold greater than that of the S genotypes examined. Differential mortality was observed between some S genotypes at 3.9 g a.i. ha−1, but no differences between genotypes were observed at any other dose. The results of this study indicate that for the spring wheat genotypes assessed, parental background and market class do not affect spring wheat tolerance to imazamox. Furthermore, results indicate that the IR trait in spring wheat should provide a satisfactory marker for pollen‐mediated gene flow studies.

The potential of a herbicide resistant maize technology for Striga control in Africa

Striga is an obligate parasitic weed that attacks cereal crops in sub-Saharan Africa. In Western Kenya, it has been identified by farmers as their major pest problem in maize. A new technology, consisting of coating seed of imidazolinone resistant (IR) maize varieties with the imidazolinone herbicide, imazapyr, has proven to be very effective in controlling Striga on farmer fields. To bring this technology to the farmer, a sustainable delivery system needs to be developed, preferably with substantial participation of the private sector. To help extension agents and seed companies to develop appropriate strategies, the potential for this technology was analyzed by combining different data sources into a Geographic Information System (GIS). Superimposing secondary data, field surveys, agricultural statistics and farmer surveys made it possible to clearly identify the Striga-prone areas in western Kenya. Results found that Striga affected a maize area of 246,000 ha annually, with a population of 6.4 million people and maize production of 580,000 tons, or 81 kg/person. Population density in this area is high at 359 people/km 2. A survey of 123 farmers revealed that 70% of them have Striga in their fields. A contingent valuation (CV) survey indicated that farmers would, on average, be willing to buy 3.67 kg of the IR-maize seed each at current seed prices, sufficient to sow 44% of their maize area. By extrapolation over the maize area in the zone, total potential demand for IR-maize seed is estimated at 2000–2700 tons annually. Similar calculations, but based on much less precise data and expert opinion rather than farmer surveys or trials, gives an estimate of the potential demand for IR-maize seed in Africa of 153,000 tons.

Jointed Goatgrass (Aegilops Cylindrica) by Imidazolinone-Resistant Wheat Hybridization under Field Conditions

Gene flow between jointed goatgrass and winter wheat is a concern because transfer of herbicide-resistance genes from imidazolinone-resistant (IR) winter wheat cultivars to jointed goatgrass could restrict weed-management options for this serious weed of winter wheat cropping systems. The objectives of this study were (1) to investigate the frequency of interspecific hybridization between IR wheat and jointed goatgrass in eastern Colorado, and (2) to determine the gene action of the IR acetolactate synthase (ALS) allele in IR wheat by jointed goatgrass and in IR wheat by imidazolinone-susceptible (IS) wheat backgrounds. Jointed goatgrass was sampled side-by-side with IR wheat and at distances up to 53 m away in both experimental plots and at commercial field study sites in 2003, 2004, and 2005. A greenhouse-screening method was used to identify IR hybrids in collected jointed goatgrass seed. The average percentage of hybridization across sites and years when IR wheat and jointed goatgrass were grown side-by-side was 0.1%, and the maximum was 1.6%. The greatest distance over which hybridization was documented was 16 m. The IR ALS allele contributed 25% of untreated ALS activity in jointed goatgrass by IR wheat F1 plants, as measured by an in vitro ALS assay. The hybridization rate between wheat and jointed goatgrass and the expression of the IR wheat ALS allele in hybrid plants will both influence trait introgression into jointed goatgrass.