Multiple herbicide-resistant (MHR) Palmer amaranth is an increasing management concern for Kansas grain producers. The main purpose of this research was to 1) characterize the resistance levels to glyphosate (Roundup PowerMax) and mesotrione (Callisto) in an MHR Palmer amaranth population collected from Stafford County, KS, compared to a known herbicide-susceptible (SUS) population; and 2) to evaluate the effectiveness of preemergence (PRE), PRE followed by (fb) early post emergence (EPOST), and PRE fb late POST (LPOST) herbicide programs for controlling this MHR population in Roundup Ready and LibertyLink corn. To achieve these objectives, a whole plant dose-response study was conducted in a greenhouse at the Kansas State University Agricultural Research Center near Hays, KS; and a field study was conducted in Stafford County, KS (from where the MHR population was originally collected). Doseresponse study indicated that the MHR population had 7.2-and 3.5-fold resistance to glyphosate and mesotrione, respectively, on the basis of visual control (LD50 values). Results from field study indicated that PRE application of Clarity + Acuron in combination with Aatrex, Callisto, Dual II Magnum, or Sencor fb a sequential EPOST application of Acuron alone or in combination with Aatrex, Callisto, or Dual II Magnum provided 80 to 95% control of MHR Palmer amaranth population in corn throughout the season. Furthermore, control with PRE applied Clarity + Acuron fb a LPOST application of Acuron + Status or Acuron + Liberty averaged 85% at the final rating. Based on these results, the tested Palmer amaranth population from Stafford County has evolved multiple resistance to glyphosate and mesotrione. Two-pass herbicide programs, including PRE fb EPOST or LPOST investigated in this research can provide adequate control of this population throughout the season in Roundup Ready and LibertyLink corn.
Late-season control of Palmer amaranth in wheat stubble is a challenge for Kansas producers. The objective of this study was to determine the effectiveness of POST herbicide programs (with multiple modes of actions) for late-season control of Palmer amaranth in postharvest wheat stubble. The study was conducted at the Kansas State University Agricultural Research Center in Hays, KS, in 2019. The study site had a natural seedbank of Palmer amaranth that emerged immediately after wheat harvest. All selected herbicide programs were tested 3 weeks after wheat harvest, when Palmer amaranth plants had attained a height of 2 to 2.5 feet with inflorescence initiation. Twenty-four herbicide programs comprising Roundup PowerMax, Clarity, 2,4-D, Aatrex, Gramoxone, Sencor, Valor SX, Spartan, Sharpen, Authority Supreme, Kochiavore, Panther MTZ, and Huskie applied alone or in tank-mixtures were tested at recommended-use rates. All herbicide treatments were arranged in a randomized complete block design with four replications. Visual Palmer amaranth control was assessed at 2, 4, and 8 weeks after treatment (WAT) by using a rating scale of 0-100% (where 0 = no control and 100% = complete plant death). The aboveground Palmer amaranth biomass and seed production were determined by harvesting plants from a 10.7-ft 2 quadrat placed at the center of each plot 8 WAT. All tested herbicide programs, except Kochiavore and a tank-mixture of Huskie + Aatrex provided > 88% control of Palmer amaranth 8 WAT. In contrast, late-season control of Palmer amaranth did not exceed 71% at 8 WAT with Kochiavore or a tankmixture of Huskie plus Aatrex treatments. Consistent with visual control (%), a majority of those tested programs significantly reduced shoot dry weights (>77% reduction) and seed production (>93% reduction) of Palmer amaranth compared to nontreated weedy check. Overall, these results suggest that several POST herbicide programs exist that growers can utilize for effective late-season control of Palmer amaranth in postharvest wheat stubble.
Two kochia accessions (KS-4A and KS-4H) were previously identified from a corn field near Garden City, KS, with multiple resistance to glyphosate (Roundup PowerMax), dicamba (Clarity), and fluroxypyr (Starane Ultra). The objectives of this research were to (1) determine the response of these kochia accessions to preemergence (PRE) and postemergence (POST) applied atrazine (Aatrex) in doseresponse assays, and (2) determine the effectiveness of alternative POST herbicides. Seeds of a known susceptible kochia accession (SUS) collected from research fields in Hays, KS, were used for comparison. Greenhouse experiments were conducted at the Kansas State University Agricultural Research Center near Hays, KS, in a randomized complete block design with 4 to 12 replications. For Aatrex PRE doseresponse assay, germination trays (each 10-× 10-inch) containing field soil were used. Fifty seeds from each accession were separately sown on the soil surface in each tray. PRE applied Aatrex doses, including 0, 1/4X, 1/2X, 1X, 2X, and 4X (1X of Aatrex = 32 oz/a) were tested. Emerged kochia seedlings from each tray were counted 28 days after treatment (DAT). For Aatrex POST dose-response assay, kochia plants from SUS and KS-4H accessions were grown in 4-× 4-inch pots containing commercial potting mixture. The same doses of Aatrex (as for PRE dose-response) were tested on 3-to 4-inch tall kochia plants. In a separate greenhouse study, the SUS and KS-4H accessions were also tested with alternative POST herbicides. Data on percent visual control and shoot biomass were collected at 21 DAT in both Aatrex POST and alternative POST herbicide studies. Results indicated that the effective dose (ED50 values) of PRE applied Aatrex required for 50% reduction in seedling emergence of KS-4A, KS-4H, and SUS was 129, 7, and 1 oz/a, respectively, indicating 129-and 7-fold resistance in KS-4A and KS-4H accessions. Furthermore, the KS-4H accession showed 248-fold resistance to POST applied Aatrex, as compared to SUS accession. Among alternative POST herbicide programs, Gramoxone, Huskie, Talinor, and Sharpen alone or with 2,4-D provided excellent control (96-100%) of SUS and KS-4H accession at 21 DAT. In conclusion, these results indicate that dicamba/ fluroxypyr/glyphosate-resistant kochia from Garden City, KS, are also highly resistant to PRE and POST applied atrazine. However, alternative POST herbicides such as Huskie, Talinor, Gramoxone, Sharpen alone, or with 2,4-D were effective control options for this multiple resistant kochia accessions.
Kochia populations with multiple resistance to glyphosate and dicamba are an increasing concern for growers in the High Plains region, including Kansas. A field study was conducted at the Kansas State University Agricultural Research Center near Hays, KS, to evaluate and develop herbicide options for controlling glyphosate-and dicamba-resistant kochia in Roundup Ready 2 Xtend soybean. The study site was uniformly infested with a glyphosate-and dicamba-resistant kochia population prior to soybean planting. Fifteen herbicide treatments (programs), including PRE alone and PRE followed by (fb) POSTapplied herbicides, were investigated in a randomized complete block design with four replications. All PRE treatments included Roundup PowerMax for control of other weed species. Results indicated that a single PRE application of Spartan alone or with Engenia, Panther PRO, and Zidua plus Valor SX provided season-long control of glyphosate-and dicamba-resistant kochia. A single PRE application of Engenia or Zidua alone only provided 70 to 78% kochia control at 6 weeks after POST (WAPOST) application. However, PRE-applied Zidua alone or with Engenia, Engenia alone, Spartan + Zidua, and Spartan + Sencor followed by a sequential POST application of a tank-mixture containing Engenia and Roundup PowerMax provided ≥ 95% kochia control at 6 WAPOST. Kochia biomass reduction was > 92% with a majority of the treatments; exceptions were Zidua PRE alone (59% reduction), Engenia alone (76% reduction), and Zidua + Engenia (88% reduction) treatments. Soybean grain yield for a majority of the tested treatments did not differ, and ranged from 23 to 25 bu/a. These results suggest that effective PRE herbicide options are available for managing glyphosate-and dicamba-resistant kochia in Roundup Ready 2 Xtend soybean.
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