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Widespread occurrence of herbicide-resistant weeds and more variable weather conditions across the United States has made weed control in many crops more challenging. Preemergence (PRE) herbicides with soil residual activity have resurged as the foundation for early season weed control in many crops. Field experiments were conducted in Janesville and Lancaster, Wisconsin, in 2021 and 2022 (4 site-years) to evaluate the weed control efficacy of solo (single site of action [SOA]) and premix (two or more SOAs) PRE herbicides in conventional tillage corn. Treatments consisted of 18 PRE herbicides plus a nontreated check. At the Janesville-2021 site, S-metolachlor + bicyclopyrone + mesotrione, atrazine + S-metolachlor + bicyclopyrone + mesotrione, and clopyralid + acetochlor + mesotrione provided >72% giant ragweed control. At the Janesville-2022 site, none of the PRE herbicides evaluated provided >70% giant ragweed control due to the high giant ragweed density and the lack of timely rainfall. At the Lancaster-2021 site, atrazine, dicamba, and flumetsulam + clopyralid provided <45% waterhemp control, but the remaining treatments provided >90% control. At the Lancaster-2022 site, the efficacy of some PRE herbicides was reduced due to the high waterhemp density; however, most herbicides provided >75% control. At the Lancaster-2021 and Lancaster-2022 sites, only dicamba and S-metolachlor did not provide >75% common lambsquarters control. Group 15 PRE herbicides provided >75% control of giant foxtail. Across weed species, PRE herbicides with two (78%) and three (81%) SOAs provided greater weed control than PRE herbicides with a single SOA (68%), indicating that at least two SOA herbicides applied PRE result in better early season weed control. The efficacy of the PRE herbicide treatments evaluated herein varied according to the soil seedbank weed community composition and environmental conditions (i.e., rainfall following application), but the premixes were a more reliable option to improve early season weed control in conventional tillage corn.
Limited information exists on the global economic impact of glyphosate-resistant (GR) weeds. The objective of this manuscript was to estimate the potential yield and economic loss from uncontrolled GR weeds in the major field crops grown in Ontario, Canada. The impact of GR weed interference on field crop yield was determined using an extensive database of field trials completed on commercial farms in southwestern Ontario between 2010 and 2021. Crop yield loss was estimated by expert opinion (weed scientists and Ontario government crop specialists) when research data were unavailable. This manuscript assumes that crop producers adjust their weed management programs to control GR weeds, which increases weed management costs but reduces crop yield loss from GR weed interference by 95%. GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed would cause an annual monetary loss of (in millions of Can$) $172, $104, $11, $3, and $0.3, respectively, for a total annual loss of $290 million if Ontario farmers did not adjust their weed management programs to control GR biotypes. The increased herbicide cost to control GR volunteer corn, horseweed, waterhemp, giant ragweed, and common ragweed in the major field crops in Ontario is estimated to be (in millions of Can$) $17, $9, $2, $0.1, and $0.02, respectively, for a total increase in herbicide expenditures of $28 million annually. Reduced GR weed interference with the adjusted weed management programs would reduce farm-gate monetary crop loss by 95% from $290 million to $15 million. This study estimates that GR weeds would reduce the farm-gate value of the major field crops produced in Ontario by Can$290 million annually if Ontario farmers did not adjust their weed management programs, but with increased herbicide costs of Can$28 million and reduced crop yield loss of 95% the actual annual monetary loss in Ontario is estimated to be Can$43 million annually.
Cover crops can be utilized to suppress weeds via direct competition for sunlight, water, and soil nutrients. Research was conducted to determine if cover crops can be used in label-mandated buffer areas in 2,4-D-resistant soybean cropping systems. Delaying termination of cover crops containing cereal rye to at or after soybean planting resulted in a 25 to more than 200 percentage point increase in cover crop biomass compared to a control treatment. Cover crops generally improved horseweed control when 2,4-D was not used. Cover crops reduced grass densities up to 54% at four of six site-years when termination was delayed to after soybean planting. Cover crops did not reduce giant ragweed densities. Cover crops reduced waterhemp densities by up to 45%. Cover crops terminated at or after planting were beneficial within buffer areas for control of grasses and waterhemp, but not giant ragweed. Yield reductions of 14% to 41% occurred when cover crop termination was delayed to after soybean planting at three of six site-years. Terminating the cover crops at planting time provided suppression of grasses and waterhemp within buffer areas and had similar yield to the highest-yielding treatment in five out of six site-years.
Accurate weed emergence models are valuable tools for scheduling planting, cultivation, and herbicide applications. Multiple models predicting giant ragweed emergence have been developed, but none have been validated in diverse crop rotation and tillage systems, which have the potential to influence weed emergence patterns. This study evaluated the performance of published giant ragweed emergence models across various crop rotations and spring tillage dates in southern Minnesota. Across experiments, the most robust model was a mixed-effects Weibull (flexible sigmoidal function) model predicting emergence in relation to hydrothermal time accumulation with a base temperature of 4.4 C, a base soil matric potential of −2.5 MPa, and two random effects determined by overwinter growing degree days (GDD) (10 C) and precipitation accumulated during seedling recruitment. The deviations in emergence between individual plots and the fixed-effects model were distinguished by the positive association between the lower horizontal asymptote (Drop) and maximum daily soil temperature during seedling recruitment. This finding indicates that crops and management practices that increase soil temperature will have a shorter lag phase at the start of giant ragweed emergence compared with practices promoting cool soil temperatures. Thus, crops with early-season crop canopies such as perennial crops and crops planted in early spring and in narrow rows will likely have a slower progression of giant ragweed emergence. This research provides a valuable assessment of published giant ragweed emergence models and illustrates that accurate emergence models can be used to time field operations and improve giant ragweed control across diverse cropping systems.
The pH and hardness of water used as agrochemical carrier can influence herbicide efficacy. The objective of this research was to determine the role of carrier water pH and hardness on saflufenacil efficacy and solubility. Saflufenacil was mixed in eight different carrier waters with one of five pH levels (4.0, 5.2, 6.5, 7.7, 9.0) or one of three hardness levels (0, 310, 620 mg L−1) and applied POST to common lambsquarters and giant ragweed in a field experiment and to field corn in a greenhouse experiment. Solubility testing was also completed on saflufenacil mixed in the five pH levels used in the field and greenhouse experiments. Water hardness did not influence the efficacy of saflufenacil on common lambsquarters, giant ragweed, or field corn. Control of giant ragweed or common lambsquarters in field experiments was reduced by up to 56% when saflufenacil was applied in water with a pH of 4.0 compared with water with a pH of 7.7. When nonsoluble saflufenacil was removed from the spray solution, saflufenacil efficacy on field corn in the greenhouse was reduced by 61% or more when applied in water with a pH of 4.0 than when applied with water with a pH of 5.2 or higher. When nonsoluble saflufenacil was applied with the soluble saflufenacil in the spray solution, at least a 7% reduction in control of field corn was observed when applied in water with pH of 4.0 as compared with saflufenacil applied in water with pH of 5.2 or higher. Solubility of saflufenacil was (1) 10.1 mg L−1 in water with a pH of 4.0, (2) 3,461.4 mg L−1 in water with a pH of 7.7, and (3) > 5,000 mg L−1 at a pH of 9. Some degradation of parent saflufenacil was detected in the pH at 9.0 treatment, with only 90% of added product being recovered after 3 d of storage. This research provides information on how saflufenacil efficacy and solubility is influenced by carrier water pH and potentially explains some differences noticed between field applications of saflufenacil.
The development of crops resistant to 2,4-D, dicamba, and glufosinate may provide new options for the management of glyphosate-resistant (GR) giant ragweed and other herbicide-resistant weeds. A fallow field study was conducted in 2011 and 2012 to determine the control of GR giant ragweed with 2,4-D and dicamba applied alone and in combination with glufosinate or fomesafen. Dicamba and 2,4-D tank-mixed with glufosinate or fomesafen provided the highest level of control at 10 or 20 days after application (DAA). At 30 DAA, all herbicide treatments provided > 88% control of giant ragweed except glyphosate, glufosinate, and 2,4-D alone at 0.56 kg ae ha−1. Glyphosate, glufosinate, and 2,4-D alone at 0.56 kg ae ha−1 also had the highest number of giant ragweed plants (> 5.8 plants m−2) and highest biomass (> 19.2 g m−2). Contrast statements between 2,4-D and dicamba indicated no differences among treatments containing these herbicides. However, contrast analysis indicated that herbicides applied alone resulted in 56, 58, and 61% control while tank-mix combinations of 2,4-D or dicamba with glufosinate or fomesafen resulted in 86, 91, and 93% control, respectively. Herbicides applied alone also had more giant ragweed plants and biomass per m−2 than herbicides applied in tank-mix combinations. Tank-mixing combinations of 2,4-D and dicamba will be important for effective control of GR giant ragweed.
Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m−1 of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m−1 row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m−1 of cotton row for NAWF and 0.8 m−1 or 1.6 m−1 of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m−1 row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m−1 row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss.
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