Now is the time to control weeds in double-crop soybeans. With barley and wheat harvest in various stages across Pennsylvania, some double-crop soybeans have been planted. Many more acres of beans will soon be planted across the state.

Extension agronomist Dwight Lingenfelter explains weeds are generally less of a problem in double-crop soybeans than in full-season beans because of the delayed planting date. But even with fewer weeds emerging, when they are present and in combination with potential dry weather, the effects on yield can be noticeable.

A weed-free start is one of the most critical steps for weed management in double-crop soybeans. One of the biggest weed concerns is in managing glyphosate-resistant marestail or horseweed. Most of these populations may also be ALS-resistant. Marestails that are cut off during small grain harvest will recover, branch out, and probably be even more difficult to control than they were before.

In full-season soybeans, 2,4-D LVE in the burndown program can provide control. However, most farmers are not willing to wait the necessary seven 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 pounds ae per acre (40 fluid ounces of Roundup PowerMax3 or 64 fluid ounces of a 3 pounds ae per gallon product). Include 1 pint per acre of 2,4-D LVE in the burndown and wait seven days to plant.

If planting an Enlist E3 variety, Enlist One or Duo could be applied and then soybeans could be planted immediately. But be cautious of drift to sensitive areas. Add 1 ounce 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 methylated seed oil and ammonium sulfate to the spray mixture when using Sharpen-containing products. 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, you must wait 14 days before planting. Valor and Authority-containing products provide residual control of emerging Palmer amaranth and waterhemp seedlings.

Use Liberty or other generic version at 32 to 43 fluid ounces 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 at 32 fluid ounces 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 marestails if they are less than 3 inches tall, and good coverage is essential. Adding metribuzin improves control. Products 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 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 or Enlist E3 soybean variety and apply Liberty (or other labeled glufosinate product) post as needed. Glufosinate provides good control of marestail, Palmer, waterhemp, ragweed, cocklebur and others. 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 or Duo to broaden the weed control spectrum in E3 soybean fields.

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-emergence 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. For residuals to be effective, they need to be incorporated by at least half-inch of rain within seven to 10 days of application and before weed seed germination.

There is a debate about which of the 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 more rainfall (over three-quarters of an 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 (one-third to one-half inch).

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