We are receiving calls about weeds breaking through soil-applied herbicides. With all of the recent rainfall and especially if reduced herbicide rates were used, a post-emergence herbicide may be necessary to clean up some of the escaped weeds, says Dwight Lingenfelter. However, keep in mind there are crop height restrictions on many of the post herbicides.
For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products please refer to Table 2-15 in the Mid-Atlantic Field Crop Weed Management Guide and check the most recent herbicide label for specific use guidelines. Or, refer to the CDMS Applied Intelligence or Greenbook herbicide label for additional use information.
Horseweed/marestail in soybean
Use the highest rate possible of Classic, Synchrony (use highest rate on STS bean only), and FirstRate and full adjuvant systems (see label) may help control glyphosate-resistant marestail, says the Penn State University weed scientist. However, there are populations that are both ALS- and glyphosate-resistant. In these cases, the above products will not provide control.
High rates of Liberty can be effective post-emergence on marestail if LibertyLink soybean varieties were planted. According to Mark VanGessel, University of Delaware, horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and outcompete them.
Additional glyphosate applications at higher rates will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them. It is always best to treat the horseweed plants soon after they start regrowing from the burndown application.
Palmer amaranth and waterhemp in soybean
So far we have not received as many reports about Palmer and waterhemp problems in crops as in the past few years. It is likely due to the cool, wet conditions we have experiences most of the spring. However don’t be complacent, there is still plenty of time for it to emerge and cause problems. So keep watching for these weeds in your fields.
If you are in a situation where a soybean field did not receive a PRE/residual herbicide application and Palmer is now over 4 inches tall, the best management option in this situation is to start over, otherwise adequate control of Palmer or waterhemp will not be obtained.
Consider terminating the entire field and replant with a LibertyLink variety to allow for additional herbicide options. Make sure to start clean and include a residual at planting followed by additional residual herbicides mid-season.
For details on what herbicides to consider please refer to our Herbicide Recommendations for Noxious Pigweeds article.
Also, make sure to scout fields early to determine what kind of pigweed species are in your fields. The sooner you ID them, the easier it is to manage them. Once Palmer or waterhemp reaches 4 inches tall, they are very difficult to control.
Weed Management Considerations in Double-Crop Soybeans
With barley harvest being completed and wheat coming off now, 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. However, even with fewer weeds emerging, if they are present in combination with potentially 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 more difficult to control. In full season soybeans, we have been strongly 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. So, what are the options?
- For glyphosate-susceptible marestail, apply glyphosate at 1.5 lb ae/A (44 fl oz of Roundup PowerMax, 48 fl oz 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.
- Add 1 oz of Sharpen (or equivalent rates of Optill or Verdict) to glyphosate. Keep in mind, Sharpen helps with burndown but does not provide much 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 horseweed stumps.
- Use Liberty at 29 to 43 fl oz in the burndown application. This is fairly good on marestail, but not that 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.).
- Gramoxone is a burndown option as well and has some activity on marestail, but good coverage is essential.
- Plant a LibertyLink soybean variety and apply Liberty (or other labeled glufosinate product) POST as needed.
- The Xtend soybean system can be used, but we typically don’t recommend it at this time of year due to drift and volatility concerns to surrounding susceptible crops.
- Plant a Roundup Ready soybean and apply glyphosate POST if marestail is not a problem.
- 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.