The soybean field pictured above needs help. The ugly, tall weeds dotting the field are marestail. If anyone knows what to do with a field like this, it’s Bill Johnson, Purdue University Extension weed control specialist.
When Johnson looks at the field and frowns, it’s not good news.
“It’s a good bet these marestail are resistant to glyphosate and ALS herbicides,” he says. “If these are non-GMO or Roundup Ready 2 soybeans, there are no chemical options for control. The only remedy now is a hand-weeding crew.”
If they are Xtend or LibertyLink soybeans, you have options, Johnson says. You could apply XtendiMax, Engenia or FeXapan if it’s an Xtend variety. If it’s a LibertyLink variety, apply Liberty.
Prevent the mess
The best option is to prevent fields from looking like this, Johnson says. That starts with knowing you have a serious marestail challenge.
“You can get on top of a situation where the pressure is this intense by using burndown herbicides and utilizing herbicides with residual activity,” he says.
The first step is changing your mindset about burndown programs when you know marestail pressure is intense, he says. “Think about a multi-pass burndown program, especially when we have weather like we did in 2018, where it was wet early, then hot and dry in May,” Johnson explains.
Simply adding a pint of 2,4-D to some other product as a burndown probably won’t be robust enough to handle these tough situations, especially if you only make one pass. Count on escapes, he says.
Fields need to be free of marestail at planting. “Plan on a two-pass burndown program from the start,” he says. “It could be a fall application followed by a spring burndown, or two applications in the spring.
“Don’t rely on just glyphosate and 2,4-D. Dicamba and Elevore are slightly better than 2,4-D, although dicamba didn’t work well on marestail larger than 8 inches tall in 2018.”
The second step is applying residual herbicides in the spring to provide control for up to six to eight weeks after planting. Broad-spectrum preemergence products to consider for residual control include a full rate of metribuzin at 5 ounces per acre or more; Valor, Authority or Sharpen at 2 ounces per acre or more; and Zidua.
Once you see a field like the one pictured here, you may need to rethink how you manage marestail, Johnson says. Again, assume that when you see escapes like these, you’re likely battling both ALS- and glyphosate-resistant marestail.
One option is tillage if you’re on level soils and soil erosion isn’t a major concern. If you opt for that, plan thorough tillage close to planting.
Another option would be growing corn until you get marestail under control. You could also stay with soybeans, but choose a different herbicide resistance trait than glyphosate. As noted above, Xtend dicamba-tolerant soybeans and LibertyLink varieties are options.
This article is part of a series on real-life weed control options. Recommendations here are not all-inclusive. For more details, see the 2019 Ohio, Indiana and Illinois Weed Control Guide.