Jason K. Norsworthy

Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

Confirmation and Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) in Arkansas

Failure of glyphosate to control Palmer amaranth was first reported in Arkansas in Mississippi County in June, 2005. The objectives of this research were to (a) confirm glyphosate-resistant Palmer amaranth in Arkansas, and (b) determine the effectiveness of 15 postemergence- (POST) applied herbicides comprising eight modes of action in controlling the glyphosate-resistant biotype compared to glyphosate-susceptible accessions. The LD50 values were similar among three susceptible Palmer amaranth accessions, ranging from 24.4 to 35.5 g ae/ha glyphosate. The resistant biotype had an LD50 of 2,820 g/ha glyphosate, which was 79- to 115-fold greater than that of the susceptible biotypes and 3.4 times a normal glyphosate-use rate of 840 g/ha. The glyphosate-resistant biotype was effectively controlled with most of the evaluated herbicides, but the use of acetolactate synthase-inhibiting herbicides such as pyrithiobac, trifloxysulfuron, and imazethapyr is not a viable option for control of this Palmer amaranth population. Nomenclature: Glyphosate, Amaranthus palmeri S. Wats. AMAPA

Review: Confirmation of Resistance to Herbicides and Evaluation of Resistance Levels

Abstract As cases of resistance to herbicides escalate worldwide, there is increasing demand from growers to test for weed resistance and learn how to manage it. Scientists have developed resistance-testing protocols for numerous herbicides and weed species. Growers need immediate answers and scientists are faced with the daunting task of testing an increasingly large number of samples across a variety of species and herbicides. Quick tests have been, and continue to be, developed to address this need, although classical tests are still the norm. Newer methods involve molecular techniques. Whereas the classical whole-plant assay tests for resistance regardless of the mechanism, many quick tests are limited by specificity to an herbicide, mode of action, or mechanism of resistance. Advancing knowledge in weed biology and genomics allows for refinements in sampling and testing protocols. Thus, approaches in resistance testing continue to diversify, which can confound the less experienced. We aim to help weed science practitioners resolve questions pertaining to the testing of herbicide resistance, starting with field surveys and sampling methods, herbicide screening methods, data analysis, and, finally, interpretation. More specifically, this article discusses approaches for sampling plants for resistance confirmation assays, provides brief overviews on the biological and statistical basis for designing and analyzing dose–response tests, and discusses alternative procedures for rapid resistance confirmation, including molecular-based assays. Resistance confirmation procedures often need to be slightly modified to suit a specific situation; thus, the general requirements as well as pros and cons of quick assays and DNA-based assays are contrasted. Ultimately, weed resistance testing research, as well as resistance management decisions arising from research, needs to be practical, feasible, and grounded in science-based methods.

Red Rice (Oryza sativa) Status after 5 Years of Imidazolinone-Resistant Rice Technology in Arkansas

Certified Crop Advisors of Arkansas and members of the Arkansas Crop Consultants Association were surveyed in fall 2006 through direct mail to assess the current situation of the red rice problem and early impact of imidazolinone-resistant (IMR) rice technology on red rice infestation. The information generated represented 40% (226,800 ha) of rice production areas in Arkansas. Barnyardgrass and red rice were the most problematic weeds, with 62% of fields infested with red rice. The estimated economic loss due to red rice averaged

Differences in Weed Tolerance to Glyphosate Involve Different Mechanisms1

274/ha. Red rice infestation was prevented mostly by crop rotation (96%) and use of certified seed (86%). Of the red rice–infested fields, 38% had light infestation and 26% had severe red rice problems before adopting IMR rice. Thirty-seven percent of infested fields had been planted with IMR rice once and 43% at least twice. Approximately 85% of the consultants reported > 90% red rice control when using IMR rice. The majority (92%) of IMR rice growers rotate to other crops, mostly soybean. Unsuitable field condition was the main reason for growing only rice. After 3 seasons, the consultants perceived that red rice infestation level declined by 77% on average. The herbicide-resistance gene had escaped to red rice in some fields, and 90% of growers are exerting effort to mitigate outcrossing. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, red rice, Oryza sativa L. ORYSA, rice, Oryza sativa L, soybean, Glycine max (L.) Merr

Consultant Perspectives on Weed Management Needs in Arkansas Rice

The cause of differential susceptibility of barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida to glyphosate was examined by measuring the absorption of 14C-glyphosate, quantifying the amount of epicuticular wax, and observing the wettability of leaf surfaces. In greenhouse experiments, the biomass of barnyardgrass and prickly sida was reduced by 95% by Roundup Ultra®. Hemp sesbania and pitted morningglory showed more tolerance, with 66 and 51% average biomass reduction, respectively. Absorption of 14C-glyphosate in a controlled environment did not follow the trend in species susceptibility with barnyardgrass, 30%; prickly sida, 18%; hemp sesbania, 52%; and pitted morningglory, 6%; absorption. The high tolerance of pitted morningglory to glyphosate can be attributed mostly to limited absorption, but the tolerance of hemp sesbania is due to other mechanisms. The addition of nonionic surfactant (NIS) to a low rate of Roundup Ultra® reduced absorption of 14C-glyphosate by barnyardgrass and hemp sesbania, but had no effect on the herbicidal activity. Glyphosate absorption in the four weed species was not correlated with quantity of chloroform-extracted wax or leaf wettability. Pitted morningglory and prickly sida, which contained the least leaf wax, also had smaller contact angles or higher leaf wettability than the species with more waxy leaves. The adjuvant in Roundup Ultra® reduced contact angles of the four species compared to contact angles obtained using deionized water alone. The addition of 0.25% v/v NIS alone to water reduced contact angles more than did the adjuvant in Roundup Ultra® solution. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A. W. Hill # SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP. Additional index words: Droplet contact angle, epicuticular wax, 14C-glyphosate, glyphosate absorption, glyphosate uptake. Abbreviations: CMC, critical micelle concentration; DAT, days after treatment; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; HAT, hours after treatment; NIS, nonionic surfactant.

Soybean canopy and tillage effects on emergence of Palmer amaranth (Amaranthus palmeri) from a natural seed bank.

Certified Crop Advisors of Arkansas and members of the Arkansas Crop Consultants Association were surveyed in Fall 2006 through direct mail to assess current weed management practices and needs in rice from both a research and educational perspective. Consultants reported scouting 228.2 of the possible 567 thousand hectares (40%) of rice grown in Arkansas. Pre-emergence herbicides most often recommended were clomazone (93%) and quinclorac (40%). Propanil (55%) and quinclorac (47%) were the two most commonly recommended postemergence herbicides. Thirty-two percent of the consultants often recommend three or more herbicide applications per field. An average of 37% of the fields were believed to have “serious” or “very serious” weed infestations, and fields were scouted for weeds on average 11 times per growing season. Ninety-two percent of the consultants had “moderate” to “high” concerns with herbicide-resistant weeds. The perceived average additional expense associated with managing a resistant weed in rice was

Herbicidal activity of eight isothiocyanates on Texas panicum (Panicum texanum), large crabgrass (Digitaria sanguinalis), and sicklepod (Senna obtusifolia)

65.60/ha. Propanil-resistant and quinclorac-resistant barnyardgrass were believed to be infesting 24 and 7% of the scouted rice hectares, respectively. Barnyardgrass was the most problematic weed of rice followed by red rice. Northern jointvetch and smartweeds were the two most problematic broadleaf weeds. The number one research need was improved broadleaf weed control. Respondents indicated that research and educational efforts should continue to focus on herbicide performance and development of economical weed control programs. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, northern jointvetch, Aeschynomene virginica (L.) B.S.P. AESVI, red rice, Oryza sativa L. ORYSA, smartweeds, Polygonum spp, rice, Oryza sativa L

Modeling Glyphosate Resistance Management Strategies for Palmer Amaranth (Amaranthus palmeri) in Cotton

Abstract Field experiments were conducted in 2004, 2005, and 2006, at Pendleton, SC, to determine the effects of soybean canopy and tillage on Palmer amaranth emergence from sites with a uniform population of Palmer amaranth. In 2006, the effect of soybean canopy was evaluated only in no-tillage plots. Palmer amaranth emerged from May 10 through October 23, May 13 through September 2, and April 28 through August 25 in 2004, 2005, and 2006, respectively. Two to three consistent emergence periods occurred from early May through mid-July. Shallow (10-cm depth) spring tillage had minimal influence on the cumulative emergence of Palmer amaranth. Increase in light interception following soybean canopy formation was evident by early July, resulting in reduced Palmer amaranth emergence, especially in no-tillage conditions. In no-tillage plots, from 32 or 33 d after soybean emergence through senescence, Palmer amaranth emergence was reduced by 73 to 76% in plots with soybean compared with plots without soybean. Emergence of Palmer amaranth was favored by high-thermal soil amplitudes (10 to 16 C) in the absence of soybean. Of the total emergence during a season, > 90% occurred before soybean canopy closure. The seedling recruitment pattern of Palmer amaranth from this research suggests that, although Palmer amaranth exhibits an extended emergence period, cohorts during the peak emergence periods from early May to mid-July need greater attention in weed management. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; soybean, Glycine max (L.) Merr.

Confirmation and Control of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) in Tennessee

Abstract A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, 2-phenylethyl) isothiocyanates on Texas panicum, large crabgrass, and sicklepod. All isothiocyanates were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g−1 of soil and incorporated. Weed emergence was generally stimulated at the lower isothiocyanate concentrations, but all isothiocyanates provided 37% or more suppression of each species at the highest concentration. Propyl and allyl isothiocyanate were most effective in suppressing Texas panicum, with 50% effective dose (ED50) values of 345 and 409 nmol g−1 of soil. All aliphatic isothiocyanates reduced Texas panicum density by at least 98%. Allyl and 3-methylthiopropyl isothiocyanate were the most effective aliphatics on large crabgrass, with density reductions of 98 and 100%, respectively. All aromatic isothiocyanates reduced large crabgrass density by 86 to 96%. Sicklepod was generally the most tolerant of the three species evaluated, with ED50 values for ethyl, propyl, and butyl isothiocyanate being greater than the evaluated concentrations. Maximum reduction in sicklepod density was 72, 68, 65, and 62%, which was achieved with allyl, benzyl, 3-methylthiopropyl, and phenyl isothiocyanate, respectively. This research shows that soil-applied and incorporated isothiocyanates are effective in suppressing some important weeds of the southeastern United States, but effectiveness of each isothiocyanate varies among species. Application techniques that minimize loss of volatile isothiocyanates may further improve their potential as an effective means of controlling these and other troublesome weeds. Nomenclature: Large crabgrass, Digitaria sanguinalis L. Scop. DIGSA; sicklepod, Senna obtusifolia (L.) Irwin and Barneby CASOB; Texas panicum, Panicum texanum Buckl. PANTE.