Updated November 6, 2019:
Effectiveness of Residual Herbicides for Pre-emergence Control of Palmer Amaranth and Morning Glory.
There were significant 2-way by year interactions of Group 14, 5, or 15 herbicides for Palmer amaranth control. In 2015, 2-way combinations of Valor, metribuzin, or Zidua controlled Palmer amaranth at least 93% 4 weeks after planting (WAP). In 2016, all treatments except for metribuzin alone controlled Palmer amaranth at least 89% 4 WAP. In 2017, all treatments except for Zidua alone and Valor plus metribuzin controlled Palmer amaranth at least 87% 4 WAP. Following the POST application of Reflex and glyphosate there were significant Group 14 by Group 5 by year interactions following the application of glyphosate plus Reflex. In 2015, all PRE treatments except Zidua alone controlled Palmer amaranth 92% or greater 6 WAP. In 2016, all PRE treatments controlled Palmer amaranth 92% or greater. In 2017, all PRE treatments except Zidua and Valor plus metribuzin controlled Palmer amaranth 87% or greater.
There was a significant three way-interaction for ivyleaf morning glory control 4 WAP. In 2015, Valor plus metribuzin, Valor plus Zidua, metribuzin plus Zidua, and Valor plus metribuzin plus Zidua controlled ivyleaf morning glory 93% or greater 4 WAP (Table 2). In 2016 and 2017 Valor plus metribuzin, Valor plus Zidua, Valor plus metribuzin plus Zidua, Spartan plus metribuzin, and Spartan plus metribuzin plus Zidua controlled Palmer amaranth 84 to 92%. Following the application of glyphosate plus Reflex there were significant 2-way by year interactions for ivyleaf morning glory control. In 2016, ivyleaf morning glory control ranged from 72 to 100% 6 WAP, but there were no significant differences among treatments. In 2017, all treatments except metribuzin alone and the no PRE treatment controlled ivyleaf morning glory greater than 98% 6 WAP.
There were no treatment differences for yields in 2015. In 2015, soybean yields ranged from 20 to 26 bu/A. Low soybean yields in 2015 were the result of lodging which occurred in other studies as well. In, 2016 and 2017, only the Group 14 herbicides had a significant effect on soybean yield. Treatments with Spartan had higher yields than treatments without a Group 14 herbicide; however, all treatments yielded at least 60 bu/A.
Methods of Cereal Rye Management for Weed Control in Soybeans
Palmer amaranth control at 4 WAP was best when Envive was used with either level of rye. These treatments also provided the highest level of control at 8 WAP, averaging 92% control. Palmer amaranth control was also highest when Envive was applied at 10 days early pre-plant compared to 20 days. Morning glory control from rye, Envive and burndown timing was less than 80% at 4 WAP, regardless of combination. However, at 8 WAP (after the Reflex plus glyphosate treatments) control was at least 94% if rye was used (regardless of other factors) or Envive was used (regardless of other factors). Morning glory is susceptible to both Reflex and glyphosate and this explains the difference between the 4 and 8 WAP treatments. Large crabgrass control was at least 86% control if Envive was used with either low or high levels of rye when rated 4 WAP. At the 8 WAP rating, large crabgrass control was excellent for all treatments.
Soybean yield was highest if rye was part of the system (either high or low levels) and residual herbicide was used and/or burndown was applied at 10 EPP. If rye was burndown at 20 EPP and no residual herbicide was used, weed competition reduced yields.
Interaction of Cereal Rye Cover and Application of Soil-Applied Residual Herbicides
The presence and level of a cereal rye cover crop did not negatively affect pre-emergence herbicide activity. Analysis of water-sensitive papers did not show a difference in droplet coverage as influenced by the level of cereal rye (data not presented). Pre-emergence herbicides with rye often provided better weed control than pre-emergence herbicides alone. However, giant foxtail density was high resulting in variable control of other weed species. Common ragweed control ranged from 84 to 100% in rye only plots, 77 to 99% with Dual Magnum treatments, and 82 to 100% with Valor treatments. Rye only plots that were planted green controlled ivyleaf morning glory better than non-fertilized rye. Dual Magnum plus fertilized rye and rye planted green controlled ivyleaf morning glory better than Dual Magnum alone. Valor treatments with and without rye controlled ivyleaf morning glory 85 to 97%. All treatments, regardless of herbicide or rye level, controlled large crabgrass at least 93%. Rye planted green controlled giant foxtail better than other rye-only treatments. Dual Magnum and Valor plots with rye controlled giant foxtail better than Dual Magnum or Valor alone.
Control of Palmer Amaranth in Double-Cropped Soybeans
Weed density was low in all these trials. Liberty provided 100% control of horseweed, as well as glyphosate plus Sharpen. Overall, broadleaf weed control was good to excellent for all treatments. Large crabgrass control as very good to excellent for all treatments that included glyphosate. Fall panicum control with glyphosate varied depending on the tank-mixed partner. Glyphosate with Liberty, Sharpen, or Anthem plus Aim, resulted in reduced control, presumably due to increased fall panicum leaf burn and reduced glyphosate absorption. Liberty, Ultra Blazer and Storm all provided excellent control of both Palmer amaranth and morning glory. Raptor and Synchrony provided good to excellent control of Palmer amaranth but morning glory control was poor. It was surprising to see such high level of Palmer amaranth control with Raptor and Synchrony, given widespread resistance in Palmer amaranth to these herbicides.
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Our results indicate that a Group 14 herbicide should be included in a pre-emergence tank mixture to control Palmer amaranth and morning glory in soybean. Metribuzin plus Zidua provided a similar level of Palmer amaranth control prior to the application of glyphosate plus Reflex indicating that this combination may help to alleviate selection pressure on Group 14 herbicides when used both PRE and POST. However, control of ivyleaf morning glory with these two herbicide groups was variable, indicating that a different herbicide group is needed for effective morning glory control.
Weed control was better if rye was used in combination with residual herbicides. If cereal rye was not used, then it was critical that the residual herbicide was applied within 10 days of planting, because weed control was reduced if the residual herbicide was applied 20 days before planting. The presence of cereal rye suppressed weed growth and resulted in fewer and small weeds at time of POST applications and improved overall control compared to no cereal rye. The first year indicated that rye did not influence residual herbicides from reach the soil.
The combination of glyphosate and Liberty provided excellent burndown of a wide range of weed species; and was the most consistent. Over a three-year period, the glyphosate and Liberty combination was good to excellent for the species evaluated, with one exception. When large fall panicum was present, the combination was not effective; however, a POST application of Select (clethodim) provided excellent control. The coarse-textured soils of southern Delaware make use of Sharpen impractical due to the waiting period between application and planting. Timely postemergence application of Liberty, Ultra Blazer, or Storm consistently provided excellent control of Palmer amaranth and morning glory. There were very few weeds emerging after the POST application, but additional work should focus on need of residual herbicides for fields infested with Palmer amaranth.