Mid-season Weed Control Issues 2018

 

 

As we proceed through the growing season, there are a few issues surrounding weed control and herbicide use. Below are some items to consider.

 

 

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 postemergence herbicide may be necessary to clean up some of the escaped weeds. 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.

 

 

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. However, there are populations in PA that are both ALS- and glyphosate-resistant. In these cases, the above products will not provide control. High rates of Liberty can be effective postemergence on marestail if LibertyLink soybean varieties were planted. According to Mark VanGessel, Univ. 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.

 

 

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.

 

 

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?