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Herbicide-resistant weeds invading MonDak





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A field that didn’t get a spring burn down set the stage for an important discussion about a rapidly advancing invasion. Herbicide-resistant weeds in the MonDak.

“Weeds are an issue,” said Tyler Tjelde, irrigation agronomist and manager of the Nesson Valley Research Farm. He was speaking during the irrigation farm’s annual field day in a chickpea field where just a couple of resistant hawksbeard had appeared last fall.

The interlopers were hand-pulled, Tjelde said, in hopes of keeping herbicide-resistant hawksbeard away for another year. But the sunny, yellow flowers that resemble dandelions nonetheless dotted a chickpea field now crowded with invaders like horseweed or mare’s tail and kochia.

Enough weed seeds, Tjelde suggested, to outlast a career.

Some of the weeds, like Roundup-resistant kochia, might be managed with tillage. That has the Nesson Valley research farm’s manager questioning whether some tillage needs to be worked back into land management practices, even though he likes what no-till has done for the farm.

It’s a question for another day, one that Tjelde said he will be exploring further this year.

The chickpea field served as a backdrop for discussion about herbicide-resistant weeds with extension specialist Dr. Joe Ikely and weed scientist Dr. Brian Jenks.

Ikely has been researching glyphosate resistant Palmer amaranth and waterhemp for the last four years, and horseweed for a decade or more.

He had some thoughts on management strategies going forward.

“Weeds don’t happen in a vacuum,” he said. “I’m standing in a no-till field. The weeds are influenced by that no-till environment.”

Resistant horseweed is celebrating a 20-year anniversary in some states, and has by now become a global problem. Control strategies for it have to be considered before anything is planted, Ikely said.

“It’s a winter annual weed,” he said. “It generally germinates in the fall, overwinters, and then bolts in the spring.”

However, it can also emerge in the spring, which makes management much trickier. In fact, in some states, resistant horseweed is shifting toward spring emergence.

Ikely does recommend getting a good burndown in the fall.

“Dicamba and 2,4 D don’t work as well in the cold periods during spring,” he explained.

He also recommends Valor and Spartan, which both have some residual effects that can help keep horseweed from germinating in the spring. They are for fall application.

Then, in spring, he recommends another burndown, this time using a mix of Gramoxone and Sharpen, assuming these are labeled for the crop that will be planted.

The rate is an important consideration.

“Five gallons per acre is not cutting it,” he said. “It needs to be 15 gallons per acre.”

While Liberty is a useful burndown in the South, he doesn’t recommend it in North Dakota. Too many cold days.

So far there is no biocontrol for horseweed, but herbicide companies are working on new products for the horseweed arsenal. Among them, a product called Elevore by Corteva.

The product is compatible for tank-mixing with common burndown and residual partners to control glyphosate-resistant species, including marestail up to 8 inches tall.

“It’s really only good on one or two weeds,” Ikely added. “And horseweed is why it was brought to the market.”

Before, companies only wanted to develop and market herbicides that work on more than one weed, but the widespread nature of glyphosate resistant horseweed made the single-target herbicide economically feasible.

Jenks, based out of Minot, has meanwhile been looking at residual effects of herbicides, and fine-tuning management recommendations.

The weed killer 2, 4-D needs at least a month to degrade before planting certain crops, he said, and Dicamba shouldn’t be used in the fall if the field will be planted to lentils.

Jenks has used Basogran on tiny horseweed plants for as much as 80 percent suppression. For that to work, however, the weed cannot be greater than 4 inches tall.

Narrowleaf hawksbeard

Narrowleaf hawksbeard resembles a dandelion, but rather than a single stem with one flower, it has branched stems with multiple flowers — each of which will produce a legion of new seeds.

Hawksbeard, like horsetail, has a wide window of emergence. It can appear in August, the end of October, and even in spring.

Fall herbicide applications offer better control later – unless you run into the wrong kind of weather.

“Like last fall, we did not have good weather for fall herbicides,” Jenks said.

He recommends a fall burndown using Roundup tank-mixed with 2,4-D, or Panaflex, or Express.

Valor provides residual control, to prevent new emergence of hawksbeard, prickly lettuce and horseweed.

Dicamba can be used in the fall, particularly if the field is returning to wheat, Jenks added. But four months of frozen ground isn’t enough degradation time for many crops in the region.

Some care is warranted with Dicamba.

A spring burndown for control of narrowleaf hawksbeard is also recommended.

And resistant kochia

Resistant kochia has spread throughout the region, and is on both sides of the MonDak now. Kochia tends to drop all its seeds in one spot, forming a mat with hundreds of seeds in one tiny area.

Producers must use full rates to be effective against that mat, which can shelter plants beneath it, Jenks said. He recommends at least 10 to 15 gallons per acre, to get the right coverage.

Some producers are having success by going over the kochia mats twice. Jenks suggested spraying the mats with Valor in the fall, to reduce emergence.

“If there are 100 plants instead of thousands, that makes the burndown better,” he said.

Growers must use more than one mode of action to control kochia. It is continuing to develop resistances, but changing modes of action could help delay this.

“Spartan is good on kochia,” he added. “but you have to be careful with your soil characteristics. Know your soil characteristics so you apply the right rate. And some soils, it shouldn’t be applied at all.”

Help for chickpeas?

Chickpeas, Jenks added, looking at the weedy field around him that missed its spring burndown, is getting a new product soon. It’s called Tough.

“It’s not registered yet,” he said. “It will be at least next year for that for post-emergent broadleaf control in chickpeas.”

This article originally ran on willistonherald.com.

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