Getting good control of Palmer amaranth and common waterhemp in Kansas has become more of a challenge in recent years. Many populations are now resistant to either glyphosate, triazine, ALS-inhibitor herbicides, HPPD-inhibitor herbicides, PPO-inhibitor herbicides – or a combination of those modes of action. As wheat harvest approaches over the next few weeks in Kansas, producers should plan now for good weed control ahead of double crop soybean.
There are several ways producers can try to manage pigweeds in doublecrop soybeans. However, all of them involve higher costs than in the past when one or two postemergence applications of glyphosate could control pigweeds in doublecrop Roundup Ready soybeans.
It is common for pigweed to emerge from the early spring through late summer. Because of the ecology of the winter wheat crop, a higher percentage of pigweed emergence can occur a bit later in the season than on fields where wheat is not growing. This can put additional pigweed pressure on double crop soybean.
The critical weed-free period in soybean is through the V3 developmental stage to avoid economic yield loss. However, due to the vast seed production capabilities of pigweed, a better strategy is to reduce pigweed emergence and ultimately seed production throughout the development of the double crop soybean.
There are non-chemical options for controlling pigweeds after harvest. Tillage is an effective option for weed control prior to double crop soybean planting. However there are a few drawbacks to this practice. Long-term tillage studies in southeast Kansas tend to show a yield advantage to no-till double crop soybean vs. tillage prior to soybean planting. This is likely due to having some residue during a hot and dry part of the season to help cool the soil and conserve moisture. In addition, tillage may slow double crop soybean planting in an already shortened growing season that may negatively impact yield.
Burning wheat stubble can provide good weed control, although many producers have moved away from this practice for a number of reasons including loss of residue, loss of nutrients, depletion of soil organic matter, and safety.
Many times producers rely on chemical control for pigweed control prior to double crop soybean planting. Some of the possible chemical options for controlling pigweeds include the following practices:
- There are a few residual herbicides that can be applied to the standing wheat crop several weeks before harvest, including Prowl H2O and Zidua (check label for wheat growth stage
restrictions for application). However, these treatments are usually applied 90 days ahead of wheat harvest and begin to break down prior to the big flush of pigweeds. Consequently, they have shown inconsistent results in controlling pigweeds in the standing wheat crop.
- Paraquat is an older herbicide but has made a resurgence in use by producers because of how effective it is on pigweeds. Excellent burndown control of emerged pigweeds after wheat harvest has been observed over the last several years. However, paraquat has no residual activity so it does not provide control of later-emerging pigweeds. Therefore, it is critical that it be applied in combination with preemerge herbicides that offer residual control of pigweeds.
Some caution should be taken when using paraquat. It is a contact herbicide, so using a minimum of 15-20 gallons per acre is needed for best control. Also, it is one of the most toxic herbicides to humans so producers need to take steps to avoid coming in contact with chemical and spray droplets in order that no one be exposed to the product. In addition, it is very toxic to nearly all plants. If any drift occurs it will be very obvious because of leaf spotting. Producers should not use this product in drift-prone conditions or near sensitive plants.
- Another possible option for burndown of pigweeds after wheat harvest, if producers choose to plant RR2Xtend Soybean, includes the new dicamba products Xtendimax, Engenia, and FeXapan. These products deliver 0.5 to 1.0 lb. ae/acre of dicamba and can provide excellent control of broadleaf herbicides including smaller pigweed. Early work at K-State would suggest this dicamba system does fit well in an integrated weed control system for pigweed control. Note that these new dicamba products are not labeled for preemerge applications ahead of non-RR2Xtend soybean.
- Regardless of what burndown option a producer uses after wheat harvest, a preemergence residual herbicide is essential for extended pigweed control. A residual herbicide program should include a combination of herbicide modes of action group 5 (triazine such as metribuzin), group 14 (PPO inhibitor such as Authority, Valor, or Flexstar), and/or group 15 (long chain fatty acid inhibitors such as Zidua, Dual, Outlook, or Warrant). These three modes of action provide excellent control of pigweeds when adequate rainfall is received for activation.
By using a combination of two or more of these herbicide modes of action, producers can help guard against selecting herbicide resistance. Also a producer may receive improved control under adverse environmental conditions if two different residual herbicide modes of action are used instead of relying on one. When selecting residual herbicides, consider rotational restrictions to other crops.
- Many residual premixes are currently available for double crop soybean. Careful consideration must be given to ensure that the premix provides the desired, effective sites of action at the appropriate rate. It is not uncommon to find premixes that offer ineffective sites of action for pigweed (e.g., ALS herbicides) or offer a reduced rate.
- Recent K-State research indicates that preemergence treatments that provide effective burndown and offer multiple residual sites of action are best; however, season-long pigweed control in the double crop soybean should not be expected. A timely postemergence application to control pigweed escapes will likely be necessary. Examples of effective postemergence tools include PPO-inhibiting herbicides such as Cobra, Blazer, Flexstar, glufosinate for LibertyLink soybean, or Xtendimax, Engenia, and FeXapan dicamba products for RR2Xtend Soybean.
It is imperative to implement an herbicide program as part of an integrated strategy. An example of this is utilizing narrow row spacing when planting double crop soybean. The narrow row spacing can help shade the soil to reduce pigweed emergence as well as suppress pigweed growth. It is important to consider the ecology of the winter wheat crop proceeding the planting of double crop soybean. Thin wheat stands or low-yielding areas will likely have heavier pigweed pressure at the time of double crop soybean planting when compared to high yielding areas.
Regardless of which weed management system a producer chooses, the inclusion of herbicides with multiple effective modes of action are needed to reduce the risk of further development of herbicide resistance in pigweeds. A combination of an effective burndown of paraquat or approved dicamba products in the RR2YXtend system for control of glyphosate resistant pigweeds, need to include a residual herbicide for extended weed control throughout the growing season.
Harvey County Extension
800 N Main, Room 11, P.O. Box 583
Newton, KS. 67114 316-284-6930