By Dwight Lingenfelter
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. Some 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, we have been advocating the use of 2,4-DLVE in the burndown program. However, most growers 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?
- For glyphosate-susceptible marestail, apply glyphosate at 1.5 lb ae/A (44 fl oz of Roundup PowerMax or 64 fl oz of a 3 lb ae/gal product).
- 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. But be cautious of drift to sensitive areas.
- Add 1 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. Adding metribuzin improves residual control.
- 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 tank-mix 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 or other generic versions at 32 to 43 fl oz in the burndown application. This is good on marestail, but not as effective on some 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 tank-mixed with Liberty to improve spectrum of burndown control.
- Gramoxone is a burndown option as well and has some activity on marestail if they are <3" tall, and good coverage is essential.
- We do not recommend using dicamba as a burndown in double-crop soybeans since it can be risky. At that time of year, many sensitive crops and ornamentals are very susceptible to dicamba and weather conditions favor dicamba drift. Furthermore, since the June 30 deadline is upon us, Engenia, Xtendimax, and Tavium cannot be legally applied.
- Plant a LibertyLink, LLGT27, Enlist E3 soybean variety and apply Liberty (or other labeled glufosinate product) post as needed.
- Plant a Roundup Ready soybean and apply glyphosate post if marestail or Palmer amaranth are not problems.
- For growers 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 activated by at least 0.5” of rainfall within 7-10 days of application and before weed seed germination. Medium and high solubility products such as s-metolachlor, acetochlor, metribuzin, Outlook, Pursuit, Reflex, and Spartan/Authority tend to perform somewhat better in the summer since rainfall can sometimes be limited for proper activation.
Source : psu.edu