Imidazolinone-resistant Wheat Research Articles

Jointed goatgrass (Aegilops cylindrica): A Review

AbstractIn 1994, the National Jointed Goatgrass Research Program was initiated with funding from a special USDA grant. The 15-yr program provided $4.1 million to support jointed goatgrass (Aegilops cylindricaHost.) research and technology transfer projects in 10 western states. These projects resulted in approximately 80 refereed manuscripts, including journal articles and extension publications. The research covered various topics related to the biology and ecology of jointed goatgrass as well as its management and control in wheat (Triticum aestivumL.) production systems. This review summarizes the research on jointed goatgrass published after Donald and Ogg’s 1991 review, most of which was conducted as part of the USDA-funded National Jointed Goatgrass Research Program. Specific topics that were studied and reviewed here includeA. cylindricagenetics, especially as it relates to gene flow and hybridization rates with wheat and fertility of the resulting hybrids; vernalization requirements; seed dormancy, longevity, and germination requirements; competitiveness with wheat; and herbicide resistance acquired through evolution or gene flow from wheat. With respect to management, a wide variety of practices were evaluated, including various tillage types and frequencies; crop rotations, especially diversified wheat production systems that include spring-seeded annual crops; competitive wheat cultivars, seeding dates, seeding density, and row spacing; fertility management, including nitrogen application timing and placement; and field burning. Finally, many studies evaluated the use of herbicides, especially the introduction of imazamox in imidazolinone-resistant wheat cultivars, as well as comparison of adjuvant systems and application timings. In addition to the many management practices that were studied individually, several integrated management systems were evaluated that combined crop rotations, tillage, and herbicide programs. Between 1993 and 2013, weed scientists in 14 western states estimated that jointed goatgrass infestations decreased by 45% to 55% and attributed the reduction to the implementation of more diverse crop rotations, improved cultural practices, and use of imazamox-resistant wheat technology. This is evidence that the practical implications of the National Jointed Goatgrass Research Program have been successfully implemented by growers throughout the western United States.

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.

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.

Adventitious Presence of Herbicide Resistant Wheat in Certified and Farm‐Saved Seed Lots

ABSTRACTVarietal purity in wheat (Triticum aestivum L.) seed production is necessary for agronomic uniformity and to enable potential market segregation. We conducted a survey of certified and farm‐saved seed samples using a nontransgenic imidazolinone‐resistant (IR) wheat cultivar in 2004 and 2005 in eastern Colorado. The objective was to compare varietal purity based on type of seed producer and IR wheat history. Ninety‐two samples of non‐IR varieties were taken from certified and farm‐saved seed growers, who either produced or had never produced IR wheat. Adventitious IR seeds were detected using a seed‐soaking technique in samples from each producer type and each IR production history. Levels of IR seed ranged from 0 to 11.28%. One certified sample and three farm‐saved samples exceeded the 0.1% threshold for off‐types in certified wheat seed. Using a two‐factor analysis, farm‐saved production class and positive IR history increased the estimated proportion of adventitious seed. Based on grower interviews, higher levels of adventitious seed presence were associated with volunteer plants from previous crops of the resistant cultivar and mechanical mixture during harvesting. Production practices for certified seed address these factors and may need to be strengthened if more stringent purity criteria are adopted. This information is important for risk assessment and policy development for potential commercial release of transgenic wheat varieties.

Influence of Imazamox Rate and Tank-Mix Combinations on Winter Annual Broadleaf Weed Control and Yield in Imidazolinone-Resistant Wheat

Abstract Four field trials were conducted during the 2003-2004 and 2004-2005 wheat-growing seasons to evaluate the effect of imazamox rate and tank-mix combinations on winter annual broadleaf weed control and yield in imidazolinone-resistant (IR) wheat. Results from these trials indicate that timely fall applications of imazamox will provide good control of henbit and common chickweed in IR wheat, which are some of the most common winter annual broadleaf weeds encountered in many wheat production systems in the United States. No benefit in weed control was observed by increasing the imazamox rate from 35 to 44 or 53 g/ha or by the addition of a thifensulfuron plus tribenuron or 2, 4-D tank-mix combination. Results from these trials also indicate that the addition of a 20% urea ammonium nitrate (UAN) fertilizer solution to imazamox may cause some visual injury to IR wheat and that the addition of 2, 4-D or a 20% UAN fertilizer solution can reduce IR wheat yields compared to applications of imazamox alone.

Imidazolinone-Resistant Wheat Acetolactate Synthase In Vivo Response to Imazamox

Several experiments were conducted to evaluate the utility of an in vivo acetolactate synthase (ALS) assay for comparing sensitivity to imazamox among imidazolinone-resistant wheat cultivars/lines. Ten single-gene imidazolinone-resistant winter wheat cultivars/lines, one two-gene and four single-gene imidazolinone-resistant spring wheat cultivars/lines, and three pairs of heterozygous and homozygous imidazolinone-resistant winter wheat lines were evaluated in the assay experiments. Additionally, a dose-response assay was conducted to evaluate the tolerance of several imidazolinone-resistant wheat cultivars to imazamox on a whole plant level. The I50value (i.e., the imazamox dose that inhibited ALS activity by 50%) of the winter wheat cultivar ‘Above’ was 54 to 84% higher than the I50values of 99-420, 99-433, and CV-9804. However, based on the results of this study, it is unclear whether genetic background or market class (hard red winter vs. soft white winter) influences the level of ALS inhibition by imazamox. Teal 15A, the two-gene imidazolinone-resistant spring wheat cultivar, had an I50value that was two to three times greater than the I50value of the single-gene imidazolinone-resistant spring wheat cultivars/lines. The heterozygous imidazolinone-resistant wheat lines had I50values that were 69 to 81% less than the I50values of the homozygous lines. In the whole plant dose response, theR50values (i.e., the imazamox dose that reduced biomass by 50%) of the susceptible cultivars Brundage 96 and Conan were 15 to 17 times less than the homozygous single-gene imidazolinone-resistant winter and spring cultivars/lines, whoseR50values were about 1.7 times less than theR50value of the two-gene imidazolinone-resistant spring wheat line, Teal 15A. The results of the in vivo ALS imazamox assays and the whole plant imazamox dose-response assay were similar, indicating that the in vivo assay can be used to accurately and quickly compare resistance between imidazolinone-resistant wheat cultivars/lines.

Interspecific Hybridization: Potential for Movement of Herbicide Resistance from Wheat to Jointed Goatgrass (Aegilops cylindrica)

The transfer of herbicide resistance genes from crops to related species is one of the greatest risks of growing herbicide-resistant crops. The recent introductions of imidazolinone-resistant wheat in the Great Plains and Pacific Northwest regions of the United States and research on transgenic glyphosate-resistant wheat have raised concerns about the transfer of herbicide resistance from wheat to jointed goatgrass via introgressive hybridization. Field experiments were conducted from 2000 to 2003 at three locations in Washington and Idaho to determine the frequency and distance that imidazolinone-resistant wheat can pollinate jointed goatgrass and produce resistant F1hybrids. Each experiment was designed as a Nelder wheel with 16 equally spaced rays extending away from a central pollen source of ‘Fidel-FS4’ imidazolinone-resistant wheat. Each ray was 46 m long and contained three rows of jointed goatgrass. Spikelets were collected at maturity at 1.8-m intervals along each ray and subjected to an imazamox screening test. The majority of all jointed goatgrass seeds tested were not resistant to imazamox; however, 5 and 15 resistant hybrids were found at the Pullman, WA, and Lewiston, ID, locations, respectively. The resistant plants were identified at a maximum distance of 40.2 m from the pollen source. The overall frequency of imazamox-resistant hybrids was similar to the predicted frequency of naturally occurring acetolactate synthase resistance in weeds; however, traits with a lower frequency of spontaneous mutations may have a relatively greater risk for gene escape via introgressive hybridization.

Herbicide-Resistant Grass Weed Development in Imidazolinone-Resistant Wheat: Weed Biology and Herbicide Rotation

A general life cycle model was modified to demonstrate how agronomic practices and weed biology factors affect the rate of appearance of herbicide-resistant downy brome, jointed goatgrass, and wild oat in Pacific Northwest wheat cropping systems. The model suggests herbicide rotation strategies for cropping systems that include imidazolinone-resistant wheat as a weed management tool. Simulation of continuous annual imidazolinone-resistant winter wheat and imazamox herbicide use resulted in the resistant soil seed banks of downy brome, jointed goatgrass, and wild oat surpassing their susceptible soil seed banks in 5, 7, and 10 yr, respectively. Reducing the initial seed bank density of downy brome before beginning a rotation that includes imidazolinone-resistant winter wheat reduces the likelihood of selecting for herbicide-resistant biotypes. The best simulated management option for reducing the total jointed goatgrass soil seed bank in low-precipitation areas is an imidazolinone-resistant winter wheat–fallow rotation. Rotations that include winter and spring crops and rotations that include non–group 2 herbicides minimize herbicide resistance selection pressure and reduce the wild oat soil seed bank.

Control of Volunteer Herbicide-Resistant Wheat and Canola

Volunteer crops resistant to glyphosate and other herbicides pose a potential problem for Pacific Northwest (PNW) growers that rely on glyphosate for control of volunteer crops and weeds during fallow and before planting. Herbicides for control of volunteer herbicide-resistant wheat and canola in PNW conservation tillage systems were evaluated during 2000 and 2001 near Ralston, WA, and Moscow, ID. Paraquat + diuron controlled glyphosate- and imidazolinone-resistant wheat ≥90%, and glyphosate controlled imidazolinone-resistant wheat 88 to 96% 14 d after treatment (DAT). Glyphosate- and imidazolinone-resistant wheat were controlled only 58 to 85% with quizalofop-P and clethodim 14 DAT. By 21 DAT, imidazolinone-resistant wheat control with clethodim and quizalofop-P was ≥93%, but the longer time period required for control to reach an acceptable level could increase disease and insect problems associated with volunteer wheat. Volunteer glyphosate-resistant canola was controlled 92 and 97% 14 DAT and 76 and 98% 21 DAT with paraquat and paraquat + diuron, respectively. Treatments that contained glyphosate controlled imidazolinone- and glufosinate-resistant canola >84% 14 DAT. By 21 DAT, control of imidazolinone- and glufosinate-resistant canola was 94 to 98% with paraquat + diuron and all glyphosate treatments, except glyphosate–isopropylamine salt (IPA) + glufosinate (88 to 93%) and glyphosate-IPA + paraquat (67 to 85%). In these studies, paraquat + diuron was the best alternative to glyphosate for controlling volunteer herbicide-resistant wheat and canola.

Italian Ryegrass (Lolium multiflorum) Control in Imidazolinone-Resistant Wheat

Imazamox ammonium salt at 53 g ae/ha applied postemergence in the fall to imidazolinone-resistant wheat controlled Italian ryegrass 98 to 100% 10 wk after treatment (WAT). Control 22 WAT was 88 to 98% at two locations and 55% at a third location. Imazamox was more effective applied in fall to three- to four-leaf Italian ryegrass than when applied in spring to one- to three-tiller Italian ryegrass. Split applications, with 27 g/ha applied in fall and spring, were no more effective than 53 g/ha applied in fall. Pendimethalin preemergence in combination with fall-applied imazamox increased control 22 WAT 10 to 33 percentage points at two of three locations. Imazethapyr ammonium salt plus imazapyr isopropylamine salt applied at 47 plus 16 g ae/ha and imazamox at 44 or 53 g/ha were similarly effective, whereas imazethapyr at 70 g/ha was ineffective. Control by imazapic ammonium salt at 70 g ae/ha was equal to or greater than control by imazamox at 44 or 53 g/ha. Imazamox, imazethapyr plus imazapyr, and imazapic controlled diclofop-resistant and -susceptible Italian ryegrass. Thifensulfuron plus tribenuron mixed with imazamox increased Italian ryegrass control in field and greenhouse experiments, whereas dicamba reduced control compared with imazamox applied alone. Control by imazamox plus 2,4-D was similar to or greater than control by imazamox alone.

Resistance to an imidazolinone herbicide is conferred by a gene on chromosome 6DL in the wheat line cv. 9804

An induced mutation of the common wheat (2n = 6x = 42, AABBDD genomes) cultivar ‘Fidel’ has been shown to provide resistance to the imidazolinone class of herbicides. This class of herbicide gives broad-spectrum weed control including the weedy relative of wheat, jointed goatgrass (2n = 4x = 28, CCDD genomes). Because wheat and jointed goatgrass share a common genome, genes present on the D genome may transfer between the two species as a result of natural hybridization and selective pressures. Our objectives were to determine which genome of common wheat contained the herbicide resistance gene in the mutated Fidel and to genetically map its position. We investigated the chromosomal location of this gene using both durum (2n = 4x = 28, AABB genomes) and common wheat (6x) backgrounds. From crosses of durum wheat genotypes as the recurrent parent with mutated Fidel (cv. 9804, resistant), only BC1plants containing chromosome 6D (inherited from cv. 9804) were resistant to applications of labeled rates of imazamox, an imidazolinone herbicide. No other D-genome chromosome was absolutely associated with herbicide resistance. To confirm this chromosomal location and genetically map the position of this gene, two populations of F3families from the cross of cv. 9804 to the common wheat cultivars ‘Cashup’ and ‘Madsen’ were screened for reaction to imazamox, followed by genetic mapping with microsatellite markers. Two linked microsatellite markers were associated with the resistance trait, and one of them,Xgdm127, was located to chromosome 6D using aneuploid stocks, confirming the location of this gene on 6D. These results indicate that this resistance gene is in the genome that common wheat shares with jointed goatgrass. Therefore, imidazolinone-resistant wheat will need to be carefully managed to minimize the occurrence and spread of resistant jointed goatgrass, whether such plants arise because of hybridization with resistant common wheat or by independent mutation, a frequent occurrence with this herbicide class.

Effect of Imazamox Soil Persistence on Dryland Rotational Crops1

Imazamox is an imidazolinone herbicide being developed for weed control in imidazolinone-resistant wheat (IMI-wheat) cultivars and various legume crops. In a series of studies conducted under a range of dryland cropping environments in the Pacific Northwest United States, imazamox applied to IMI-wheat or pea injured barley and canola grown 1 yr after imazamox treatment in low-rainfall, low–soil pH locations of Oregon. Injury was not observed in higher rainfall locations near Pullman, WA. Non–herbicide-resistant wheat planted 1 yr after IMI-wheat treated with imazamox was not injured. Of particular concern for imazamox carryover are low-rainfall areas with low-pH soils. Reduced soil moisture appears to limit imazamox degradation. Imazamox sorption is reduced in low-pH soils, which increases its bioavailability, thereby increasing the potential for injury to rotational crops such as barley, canola, and spring wheat.Nomenclature: Imazamox; barley, Hordeum vulgare L.; canola, Brassica napa L.; pea, Pisum sativum L.; wheat, Triticum aestivum L.Additional index words: Barley, canola, carryover, Clearfield™, herbicide-resistant wheat, pea.Abbreviations: IMI-wheat, imidazolinone-resistant wheat; PNW, Pacific Northwest.