By Mark Loux, Extension Weed Scientist

It’s possible to find just about everything in this year’s weed control situation — cover crops that didn’t die, marestail that didn’t die, early burndown plus residual treatments that worked but are now breaking because soybeans haven’t been planted, pre-emergence herbicides that did not or may not receive enough rain, and of course, more cressleaf groundsel than in an average year. Some comments on a few of these:

1. Where burndown has yet to be applied for no-till soybeans, and the field has big marestail, we would suggest pulling out all the stops. Wait another week to plant soybeans, and apply a mix of glyphosate, 2,4-D, Sharpen, and metribuzin. Be sure to include MSO and higher spray volumes (20 gpa) are likely to be beneficial. Where it’s not possible to wait a week, omit the 2,4-D and ideally switch to Liberty Link soybeans so there is the option of applying glufosinate post-emergence to try to control marestail escapes. It’s also possible to switch from glyphosate to glufosinate in the burndown mixture. This will definitely help with marestail, but there is the risk of less control on larger grasses and a few other weeds. Where glufosinate is used in both the burndown and post-emergence treatments in LibertyLink soybeans, be sure to know the total amount that can be applied per season among all treatments. 

2. Where burndown plus residual treatments were applied early, and are now becoming reinfested with weeds, the best strategy is to apply another burndown before soybeans are planted or emerged if possible. This is a mandatory step in fields with emerged marestail. Consider including a reduced rate of residual herbicide since much of the activity of residual applied a month or more ago has dissipated. Where the initial burndown consisted of glyphosate plus 2,4-D, something like glyphosate plus Sharpen could be applied now. A mixture of metribuzin plus either Gramoxone or glufosinate could also work. There are various options depending upon what was used in the initial burndown and how long it will be until soybeans are planted.

3. We have also received questions about the possibility of early post-emergence application of residual herbicides in soybeans, where it was not possible to apply residuals before soybeans emerged. Most residual herbicides cannot be applied once soybeans have emerged, due to risk of severe injury. Scepter always had a good fit in this situation, and apparently is likely to be sold again at some point by AMVAC. Pursuit has substantial activity on a range of grass and broadleaf weeds, as long as they are not resistant to ALS inhibitors (group 2). FirstRate has activity on broadleaf weeds that are not ALS-resistant, and products containing fomesafen (Flexstar) have activity on certain broadleaf weeds. 

Some products that contain dimethanamid (Outlook), metolachlor, pyroxasulfone (Zidua), or acetochlor (Warrant) can be applied early post-emergence for residual control of grasses, nightshade and Amaranthus species (redroot pigweed, waterhemp, Palmer amaranth).  Several newer premix products combine two of the individual herbicides just mentioned, and these are described and rated in the “Weed Control Guide for Ohio, Indiana, and Illinois.”  These include Prefix and generic equivalents, Anthem, Torment and Warrant Ultra among others. 

Most anything applied early post-emergence for residual would still need to be mixed with glyphosate or glufosinate for control of emerged weeds. And a final caution is that many of these products have little or no residual activity on marestail and giant ragweed — check our ratings during the selection process to make sure the product addresses the weeds of concern.