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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.
Cereal rye cover crop (cereal rye) and preemergence (PRE) herbicides are becoming common practices for managing herbicide-resistant weeds in soybean production. Adopting these two practices in combination raises concerns regarding herbicide fate in soil, given that the cereal rye biomass can intercept the herbicide spray solution, preventing it from reaching the soil. Delaying cereal rye termination until soybean planting (planting green) optimizes biomass accumulation but might also increase PRE interception. To better understand the dynamics between cereal rye and PRE herbicides, a field experiment was conducted to evaluate two soil management practices (tillage and no-till) and two cereal rye termination practices in the planting-green system (glyphosate [1,260 g ae ha−1] and roller-crimper) on the spray deposition and fate of PRE herbicides and soybean yield. The spray deposition was assessed by placing water-sensitive paper cards on the soil surface before spraying the PRE herbicides (sulfentrazone [153 g ai ha−1] + S-metolachlor [1,379 g ai ha−1]). Herbicide concentration in soil (0 to 7.6 cm) was quantified 25 d after treatment (DAT). The presence of no-till stubble and cereal rye biomass reduced the spray coverage compared to tillage at PRE application, which reflected in a reduction in the concentration of both herbicides in soil 25 DAT. Soybean yield was reduced in all three years when the cereal rye was terminated with a roller-crimper but only reduced in one year when terminated with glyphosate. Our findings indicate that mainly cereal rye biomass reduced the concentration of PRE herbicides in the soil due to the interception of the spray solution during application. Although higher cereal rye biomass accumulation can provide better weed suppression according to the literature, farmers should be aware that the biomass can lower the concentration of PRE herbicides reaching the soil, thus intensifying field scouting to ensure that weed control is not being negatively affected.
Soybean planting has occurred earlier in the Midwestern United States in recent years; however, earlier planting subjects the crop to longer durations of weed interference. This may change the optimum timing of POST glyphosate applications, or increase the need for residual herbicides applied PRE to optimize yield. A field study was conducted in 2012 and 2013 near Arlington, WI to determine the effect of planting date, residual herbicide use, and POST glyphosate timing on weed control and soybean yield. Planting dates were late April, mid-May, and early June. A PRE application of sulfentrazone plus cloransulam was applied to half the plots following each planting date. Glyphosate was applied POST to all plots at the V1, V2, V4, or R1 soybean growth stage. Planting date and glyphosate timing did not affect soybean yield in this study. However, averaged across years, planting dates, and POST glyphosate timings, yield increased from 3,280 to 3,500 kg ha−1 when a PRE herbicide with residual soil activity was used. In POST-only treatments, delaying the planting date to June decreased weed density at POST application timing from 127 to 5 plants m−2 (96%) and from 205 to 42 plants m−2 (80%) in 2012 and 2013, respectively. Where a PRE was used, total weed density at POST application timing was always less within planting date, and also declined from early to late planting date 26 to 3 plants m−2 (89%) and 23 to 6 plants m−2 (74%) in 2012 and 2013, respectively. In conclusion, both PRE herbicide use and delayed soybean planting were effective strategies to reduce the number of in-crop weeds exposed to POST glyphosate and should be considered as strategies to reduce the number of weeds exposed to POST herbicides for resistance management.
Increased soybean seed cost has generated recent interest in reducing
seeding rates to improve economic returns. However, low seeding rates result
in reduced established plant stands with slower canopy development, and
canopy development is an important element of integrated weed management
(IWM). Field studies were conducted in 2012 and 2013 in Wisconsin to
determine the trade-off between reduced seeding rates and PRE residual
herbicide use for POST herbicide exposure. Soybean was planted in mid May in
38-cm-wide rows at five seeding rates ranging from 148,200 to 469,300 seeds
ha−1. A PRE application of metolachlor plus fomesafen was made
to half of the plots. One of two POST herbicide programs were sprayed at the
V4 soybean growth stage to determine whether blending herbicide-resistant
(HR) and non-HR soybean cultivars could be a practical alternative to reduce
soybean seed expenses while maintaining the potential benefit of weed
suppression before the POST herbicide application. An increase in seeding
rate did not reduce the density or size of weeds exposed to the POST
herbicide, and furthermore, end-of-season weed density and biomass were not
influenced. In contrast, the use of a PRE herbicide reduced total weed
density and biomass before POST application by 93 and 95%, respectively, in
both years. In 2012, the season was dry early and harvest stands of 161,100
and 264,100 plants ha−1 produced 95% of the maximum yield for the
PRE and no-PRE treatments, respectively. The difference was not repeated in
2013 with adequate early season rainfall. In conclusion, PRE herbicide use
produced maximum yield with fewer plants per hectare by limiting early
season weed competition and reduced weeds exposed to POST herbicide
application thus contributing to HR management (HRM). In contrast, higher
plant densities generated within the seeding rate range of this study did
little to improve IWM or HRM.
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