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No easy answers for some tough herbicide questions

By Matt Reese

This Delaware County field had poor weed control after a June application of Roundup and Sharpen.

It was a tough spring for weed control and herbicide experts like Mark Loux, with Ohio State University Extension, continue to get more questions about various issues.

A recent example is a commercially sprayed, long-term no-till Delaware County field (pictured). While some ragweed was killed, nearby plants were slowed, but not stopped, with a treatment of Roundup/Sharpen after 10 days. Monsanto recommended waiting until it greens back up and hitting it again with 44 ounces of Roundup. Was this an herbicide resistance issue or something else?

Here is Loux’s response to the situation and the photo:

One thing to keep in mind that, aside from glyphosate resistance issues, burndown treatments in no-till were applied a month or more later than they typically are, so we were dealing with bigger and older weeds. One consequence of this is certainly that burndown treatments that work well in late April or early May are being stretched to their limits when they are applied in June. So, we can expect to see more variability in control. Sharpen treatments have to be applied with MSO (methylated seed oil), so the failure here could be due to the omission of that, although I think this message has gotten out. I am generally hearing about more problems with burndown performance this year.

The other issue of course is that most of the marestail populations in the state have become resistant to glyphosate, and I believe we have a fair amount of low-level glyphosate resistance in giant ragweed. We do not expect to kill marestail with glyphosate alone.

Glyphosate/Sharpen is an effective marestail burndown, although there can be re-growth of marestail after this treatment infrequently, based on comments from dealers and consultants. I am getting lots of questions about postemergence control of marestail soybeans, and these are situations where either the burndown did not work or no burndown was applied prior to soybean emergence. There is not much good news here. The only options for POST control are glyphosate, Classic, and FirstRate, and some populations are resistant to all of these herbicides. The best hope is to apply a combination of glyphosate plus Classic or FirstRate and hope the combination of suppression from herbicides and the soybean canopy will keep the marestail from being a major problem. It’s a long shot, especially this year when soybeans are still small.

Giant ragweed populations can have a low level of resistance to glyphosate, which will be expressed to a greater extent as they age and increase in size. Effective burndown of ragweed often requires the addition of Sharpen or 2,4-D ester, although ragweed can respond to increases in glyphosate rate. Something to keep in mind is that Sharpen is effective on primarily marestail, and while it helps control a number of other weeds when it is applied with glyphosate, it largely fails on these weeds when applied alone. So it’s possible that the combination of large giant ragweed size, age of the plant, and glyphosate resistance will mean that the Sharpen has to carry more of the load for control even when mixed with glyphosate. This might be where the mixture starts to struggle when applied this late in the season.

There have been problems with both giant ragweed and marestail surviving the burndown. I would agree with applying the Roundup at 44 ounces, which is the maximum POST rate, but I would recommend the addition of FirstRate or Classic to hopefully improve control of both weeds. If giant ragweed is the main concern, the other option is a mixture of Roundup and Flexstar, or the premix product Flexstar GT. Flexstar has no activity on marestail, however. Relying on glyphosate alone in the re-spray is risky.

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