Low Temperature Delays Metabolism of Quizalofop in Resistant Winter Wheat and Three Annual Grass Weed Species

Quizalofop-resistant wheat is the core component of the recently commercialized CoAXium™ Wheat Production System. As with other herbicides, quizalofop provides better weed control at early growth stages and under optimum temperature. However, in regions with winter wheat production, quizalofop application may be affected by unpredictable, rapid temperature decreases. Temperature shifts can cause crop injury or impact weed control efficacy. In the following study, we examine the effect of reduced temperature on quizalofop content and metabolism in CoAXium™ winter wheat and three winter weed species: downy brome (Bromus tectorum L.), feral rye (Secale cereale L.), and jointed goatgrass (Aegilops cylindrica Host). Temperature conditions include either 19 or 4.5°C daytime temperatures with tissue sampling over 5 timepoints (1–16 or 18 days after treatment, DAT). Analysis features liquid chromatography coupled to tandem mass spectrometry detection of the active form of quizalofop, quizalofop acid. Quizalofop content trends reveal delayed metabolism under cooler conditions for wheat and weeds. Quizalofop content peaks within 1–2 DAT in the warmer temperatures for all species and decreases thereafter. In contrast, content peaks between 8 and 9 DAT at cooler temperatures except for downy brome. Minimal decreases in content over time generally follow cooler temperature peaks. Further, the absence of differences in maximum quizalofop content in all species suggests absorption and/or de-esterification of quizalofop proherbicide to the active form is not reduced at cooler temperatures. Final dry shoot tissue biomass does not necessarily correspond to differences in metabolism, as biomass of wheat treated with a field rate of quizalofop does not differ between temperatures. Weeds were treated with sublethal doses of quizalofop in order to monitor herbicide metabolism without causing plant death. Under this condition, weed biomass only differs for jointed goatgrass, which has a greater biomass in the cooler temperature.

Distribution and characterization of Aegilops cylindrica host from Iran

Jointed goatgrass (Aegilops cylindrica Host; 2n = 4x = 28, CcCcDcDc) is a tetraploid remote relative of bread wheat (Triticum aestivum L; 2n=6x=42, AABBDD) with two genomes and 28 chromosomes. The diversity center of this species is in the Fertile Crescent and in central Asia and could also be found in many places in Iran. In this experiment, 359 accessions provided by National Plant Gene Bank of Iran (NPGBI) were used. Based on the geographical distribution, the highest distribution of A. cylindrica is found in North, West and North-West regions of Iran. The data on the distribution of A. cylindrica showed that its distribution centers in Iran are more than those reported in the previous studies. Chromosome counting showed that all A. cylindrica accessions are tetraploid (2n=4x=28). Results of factor analysis for nine morphological chromosome traits showed that karyotypic variation within accessions are related to the length of chromosomes and there is difference between the accessions for their total chromosome length, but the karyotype of different accessions are almost the same for the symmetry. Low coefficient of variation in morphological traits as well as symmetric karyotypes of A. cylindrica species observed in this study could lead us to predict that A. cylindrica could be a recently evolved species among the remote relatives of bread wheat.

Seed retention of winter annual grass weeds at winter wheat harvest maturity shows potential for harvest weed seed control

Downy brome, feral rye, and jointed goatgrass are problematic winter annual grasses in central Great Plains winter wheat production. Integrated control strategies are needed to manage winter annual grasses and reduce selection pressure exerted on these weed populations by the limited herbicide options currently available. Harvest weed-seed control (HWSC) methods aim to remove or destroy weed seeds, thereby reducing seed-bank enrichment at crop harvest. An added advantage is the potential to reduce herbicide-resistant weed seeds that are more likely to be present at harvest, thereby providing a nonchemical resistance-management strategy. Our objective was to assess the potential for HWSC of winter annual grass weeds in winter wheat by measuring seed retention at harvest and destruction percentage in an impact mill. During 2015 and 2016, 40 wheat fields in eastern Colorado were sampled. Seed retention was quantified and compared per weed species by counting seed retained above the harvested fraction of the wheat upper canopy (15 cm and above), seed retained below 15 cm, and shattered seed on the soil surface at wheat harvest. A stand-mounted impact mill device was used to determine the percent seed destruction of grass weed species in processed wheat chaff. Averaged across both years, seed retention (±SE) was 75% ± 2.9%, 90% ± 1.7%, and 76% ± 4.3% for downy brome, feral rye, and jointed goatgrass, respectively. Seed retention was most variable for downy brome, because 59% of the samples had at least 75% seed retention, whereas the proportions for feral rye and jointed goatgrass samples with at least 75% seed retention were 93% and 70%, respectively. Weed seed destruction percentages were at least 98% for all three species. These results suggest HWSC could be implemented as an integrated strategy for winter annual grass management in central Great Plains winter wheat cropping systems.

Distribution and characterization of Aegilops cylindrica species from Iran

Jointed goatgrass (Aegilops cylindrica Host; 2n = 4x = 28, CcCcDcDc) is a tetraploid remote relative of bread wheat (Triticum aestivum L; 2n=6x=42, AABBDD) with 2 genomes and 28 chromosomes. The diversity center of this species is in the Fertile Crescent and in central Asia and could also be found in many places in Iran. In this experiment, 359 accessions provided by National Plant Gene Bank of Iran (NPGBI) were used. Based on the geographical distribution, the highest distribution of Ae. cylindrica are from North, West and North West regions of Iran. The distribution data of Ae. cylindrica showed that the distribution centers in Iran are more than those reported in previous studies in Iran. Chromosome counting showed that all Ae. cylindrica accessions are tetraploid (2n=4x=28). Results of factor analysis for 9 morphological chromosome traits showed that karyotypic variation within accessions are related to the length of chromosomes and there is difference between accessions for their total chromosome length, but the karyotype of different accessions were almost the same for the symmetry. Low coefficient of variation in morphological traits as well as symmetric karyotypes of Ae. cylindrica species observed in this study could lead us to more confidently say that Ae. cylindrica could be a recently evolved species among remote relatives of bread wheat.

Jointed goatgrass (Aegilops cylindrica): A Review

In 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.

Resistance Allele Movement between Imazamox-Resistant Wheat and Jointed Goatgrass (Aegilops cylindrica) in Eastern Oregon Wheat Fields

Imazamox-resistant wheat varieties carry theImi1allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F1) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of theImi1allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of theImi1allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for theImi1alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that theImi1allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.