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Recent rains help crops and weeds

Herbicide-resistant weeds make controlling weeds even harder

Most Manitoba farmers needed the rain, but wet fields have delayed weed spraying, says Manitoba Department of Agriculture weed specialist Jeanette Gaultier.

Dry soils delay weed emergence, but the recent rains and warmer weather have triggered germination, creating a green carpet of weeds in some fields.

“The crop is going to pop, but so are the weeds,” Gaultier said during the Eastman CropTalk webinar June 2.

Early weed control is best for maximizing crop yield potential and herbicide efficacy, Gaultier said later in an interview.

Tank mixing herbicides with two or more modes of action is also the best way to delay increasing the population of herbicide-resistant weeds (short of not applying a herbicide), she said.

“Companies have made it very easy now for producers,” Gaultier said. “They have prepackaged everything, but sometimes what might seem like a good broad-spectrum tank mix you might find not all the components work on all weeds, so it’s worth it sometimes to make sure you are getting more than one mode of action working on the weed, especially if you do have some problem weeds.”

So far it has been “a fantastic year” for dandelions.

“With its big taproot it didn’t care that it was a little bit dry (earlier) and it has been doing quite well (with the rains),” Gaultier said. “We’re also seeing thistle and sow thistle.

“There are definitely other things coming up out there as well and it can be very specific to fields.”

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photo: Agriculture and Agri-Food Canada

Wet soil weeds

Barnyard grass, curl dock and smart weeds like wet soil too.

“So this means you might want to be scouting for some of those species, especially since some of these like the smart weeds can be difficult to control depending on what the crop is,” she added.

Spring wheat is very competitive against weeds and there are many herbicide options, especially for broadleaf weeds.

Wild oats and green foxtail are the top two worst weeds in Manitoba wheat.

The timing of wild oats emergence relative to wheat stage as well as the number of wild oats, can have a big impact on wheat yields, Gaultier said. For example, if a field has four wild oats per square metre and it’s one leaf stage behind the spring wheat, wheat yield will be cut by one per cent. However, if those four wild oats are one leaf ahead of the wheat that could cut wheat yield four per cent.

“We usually use five per cent yield loss as our threshold (for applying a herbicide),” Gaultier said. “Usually you can easily get your money back when you start spraying at that point.

If the wild oats are ahead of a wheat crop only four plants per square metre is required to justify controlling them.

“If the wild oats are ahead of your wheat crop you only need four plants per square metre to warrant controlling them. If the wild oats are at the same stage as your wheat crop you are looking at eight plants per square metre. And if your wild oats are behind your wheat crop then you are looking at about 16 plants per square metre. The numbers do change quite a bit based on the staging.”

Other considerations

Yield impact is only one consideration for spraying weeds; adding new weed seeds to a field is another. Some farmers may opt to spray weeds at a lower threshold, especially when dealing with herbicide-resistant weeds, she said.

“Having resistant wild oats in a field is the norm and not the exception now in Manitoba,” Gaultier said, based on statistics gathered seven or eight years ago.

A general and herbicide-resistant weed survey is planned for Manitoba this year.

“I expect that these numbers will jump,” she said.

“Wild oats and green foxtail are not only the most common weeds in wheat, but with the resistance problem they are also sometimes the ones that are hardest to deal with. Definitely guys with both Group 1 and Group 2 resistance are looking for other options.”

About 55 per cent of Manitoba wild oat is herbicide Group 1 resistant.

“We all know that there are different levels of cross-resistance (within a herbicide group) between the chemistries depending on what mutation you have,” Gaultier said. “So in some cases you still might be able to use certain Group 1s, or you might just have populations that are completely resistant.”

Eighteen and 11 per cent of Manitoba wild oat is Group 2 and 8 resistant, respectively.

“Even if you haven’t used the chemistry in a long time those populations pretty much tend to stay status quo (herbicide resistant) in your seed bank,” Gaultier said.

Focus is a Group 14 and 15 herbicide new to Manitoba, which controls green foxtail and wild oats in wheat, Gaultier said.

And Valtra is recently registered Group 14 product that suppresses green foxtail in wheat.

Some laboratory research with Group 8-resistant Manitoba wild oats has shown they are also resistant to Group 14 and 15. However, that resistance hasn’t been seen in a field situation, Gaultier said.

“So I am not recommending that guys shy away from this product,” she said. “They should try them to see if they work for them.

“I suggest if you are having trouble that way (with Group 1 and 2 resistance) you might give these a try. If you are not getting the level of control that you like you might want to consider going back to some of these older modes of action.”

These include pre-seed herbicides such as Avadex (Group 8), trifluralin (Group 3) and Fortress (Group 3 and 8).

About the author

Reporter

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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