Double-Crop Soybeans: Weed Management Considerations

With barley and wheat harvest in various stages, double-crop soybean planting will soon follow in some areas of the state. Weeds are generally less of a problem in double-crop soybean because of the delayed planting date. But even with fewer weeds emerging, when they are present and in combination with potential dry weather impacts on yield can be noticeable. A weed-free start is one of the most critical aspects of weed management in double-crop soybean. One of the biggest weed concerns is in managing glyphosate-resistant marestail or horseweed. Most of these populations may also be ALS-resistant. Marestail that are cut off during small grain harvest will recover, branch out, and probably be even more difficult to control. In full season soybeans, the use of 2,4-DLVE in the burndown program can provide control. However, most farmers are not willing to wait the necessary 7 days after 2,4-D application in double-crop and there is potential for drift at this time of year. So, what are the options?

• In most cases, an easy option would be to add 1 fl oz of Sharpen (or equivalent rates of Zidua Pro, Optill or Verdict) to glyphosate to control marestail stumps. Keep in mind, Sharpen at this rate helps with burndown but does not provide much residual control. For effective control, make sure to add MSO and AMS to the spray mixture when using Sharpen-containing products. Adding metribuzin improves residual control.
• Or include 1 pt per acre 2,4-DLVE in the burndown and wait 7 days to plant. If planting an Enlist E3 variety, Enlist One/Duo could be applied and then soybeans planted immediately. However, larger marestail stumps might not be completely killed. Also, be cautious of drift into sensitive areas.
• For glyphosate-susceptible marestail, apply glyphosate at 1.5 lb ae/A (40 fl oz of Roundup PowerMax3 or 64 fl oz of a 3 lb ae/gal product).
• Add FirstRate (or Authority First or Sonic) or a chlorimuron-containing product (Authority XL, Canopy, Envive, Synchrony, Valor XLT, etc.) to improve control if the population is not ALS-resistant. (Valor, Authority, and metribuzin provide residual activity; but not much burndown activity.) Also, root uptake of Valor XLT/Envive may help kill marestail stumps. Keep in mind that if you tankmix other PPO (group 14) products that contain Valor or Authority with Sharpen, there is a 14-day wait period before planting. Valor and Authority-containing products provide residual control of emerging Palmer amaranth and waterhemp seedlings.
• Use Liberty Ultra (19 to 34 fl oz) or other generic glufosinate version at 32 to 43 fl oz in the burndown application. This is good on marestail, but not as effective on other weeds such as large annual grasses and Canada thistle. You can tank-mix with other herbicides to improve performance and provide some residual control (Sharpen, metribuzin, etc.). Glyphosate (32 fl oz) can also be tankmixed with Liberty to improve spectrum of burndown control.
• Gramoxone is a burndown option as well and has some activity on marestail if they are less than 3 inches tall, and good coverage is essential. Adding metribuzin improves control.
• Product like Reviton and Elevore can be used for burndown, but Reviton does not provide effective marestail burndown. If using Elevore, you must wait 14 days after application to plant soybeans, although it provides excellent control of marestail.
• We do not recommend using dicamba as a burndown in double-crop soybeans since it can be risky. At this time of year, many sensitive crops and ornamentals are very susceptible to dicamba and weather conditions favor dicamba drift. However, if you are in a location conducive to its use, Engenia, Stryax, and Tavium can be applied as long as all application guidelines are followed.
• Plant a LibertyLink, Enlist E3, or XtendFlex soybean variety and apply Liberty (or other labeled glufosinate product) post as needed. Glufosinate provides good control of marestail, Palmer amaranth, waterhemp, ragweed, cocklebur, and other weeds. Under hot, humid, and sunny conditions in conjunction with AMS in the tank and good spray coverage, glufosinate can be a very effective herbicide option. It can be tank-mixed with Enlist One/Duo to broaden the weed control spectrum in E3 soybean fields.
• Plant a Roundup Ready or Plenish soybean and apply glyphosate post if marestail or Palmer amaranth/waterhemp are not problems.
• For farmers interested in non-GMO soybeans, a number of post herbicides are still available, but many of the broadleaf products have some potential for soybean injury, and they are not effective for control of marestail.
• Residual herbicides can be a critical component for weed control in double-crop beans, especially if Palmer and waterhemp are problems in your fields. However, for residuals to be effective, they need to be incorporated by at least ½ inch of rainfall within 7-10 days of application and before weed seed germination. However, there is a debate over which Group 15 herbicides provide the necessary residual for effective control of weeds like Palmer and waterhemp. In general, products that contain pyroxasulfone (Zidua, Anthem Maxx, etc.) tend to provide the most effective control of these problem weeds but they require higher amounts of rainfall (>¾ inch) to activate them. Since rainfall can sometimes be limited for proper activation, medium- and high-solubility products (Group 15 and others) such as Dual Magnum, Warrant, metribuzin, Outlook, Pursuit, Reflex, and Spartan/Authority may perform somewhat better in the summer under conditions with less moisture (⅓ to ½ inch).