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Farmers warned about mid-season spraying

CARBERRY, Man. — In the middle of summer, most people want their yard to look neat, tidy and orderly. Many growers feel the same way. They can’t tolerate weeds poking up through the crop in July, tainting the look of what should be a clean field of soybeans or canola.

Spraying glyphosate or another weed killer on a herbicide-tolerant crop will clean up the irritants, but growers should avoid using an in-crop herbicide in July, says a University of Manitoba weed scientist.

“The big extension message, these days, from my perspective is to try and limit the in-crop applications to one per year, no matter what the product is,” said Rob Gulden, who spoke at Crops-a-Palooza, a field day held in late July in Carberry.

That one application should occur early in the growing season, likely in June, when soybeans and other crops are struggling to compete with weeds.

“What’s really important… is early season weed control. Because you lose yield early on when the crop can’t then compensate,” he said, adding that late-season weed control has less influence on yield.

“There’s a tendency to want that weed-free field … to make it look good, but that doesn’t mean it’s going to be the highest yielding field.”

Gulden is pushing the “one in-crop application” message because glyphosate-resistant weeds are now commonplace in Western Canada.

Five years ago, for instance, scientists first found glyphosate-resistant kochia in Manitoba. Now, a much larger percentage of kochia plants are likely resistant.

“Their first survey (in Alberta), they had five percent fields (with) glyphosate-resistant kochia in it,” Gulden said. “Five years later, in their second survey, they’re up to 50 percent of their fields…. The genetics move through the population fast.”

Adoption of row crops, like corn and soybeans, is accelerating the development of herbicide-tolerant weeds because those crops struggle to compete with weeds early in the season. Hence, soybean and corn growers rely heavily on herbicides to keep weeds in check.

Research in Ontario, however, shows there is a critical period for weed control in soybeans — from the first trifoliate (V1) until the third trifoliate (V3). If weeds are managed at that time, growers can maximize their yield potential.

After that time, the plants are larger, the crop canopy closes and soybeans are more competitive with weeds.

Gulden said there are ways to help the canopy close more rapidly.

“If we narrow our row spacing in soybeans and we stick to seed counts of 180,000 per acre … we can shorten that critical (weed control) period to only needing one in-crop application,” he said.

A higher seeding rate costs more money. But in the long-run, growers should have fewer problems with resistant weeds.

“The (short-term) economics say more herbicide and less seed,” Gulden said. “But the consequence of that is we’re really pushing for herbicide resistance.”

A higher seeding rate, tighter row spacing and one in-crop herbicide treatment are reasonable recommendations, said Tammy Jones, a weed specialist with Manitoba Agriculture.

But, in some cases, those strategies may not be realistic.

“In an ideal world, you’re probably going to need a pre-seed herbicide … and then an in-crop (application),” Jones said. “He’s right. That concept is great. (But) the challenge with that is flushing weeds (like kochia and waterhemp)…. That flushing ability, of that type of a weed, is going to be a real challenge to keep under control.”

Waterhemp arrived in Manitoba from the United States a couple years ago.

“Waterhemp is … capable of producing 500,000 seeds per plant that tend to germinate throughout the summer. (And) late flushes can keep coming after the control measure window has closed,” the Manitoba Weed Supervisors Association (MWSA) said in a statement.

Therefore, a second in-crop treatment may be needed to keep waterhemp and the spread of seeds in check. But at the same time, hitting waterhemp repeatedly with herbicides is a recipe for disaster.

A portion of the waterhemp population in America is now resistant to multiple classes of herbicides.

Last year, scientists found waterhemp plants with resistance to six different modes of action in a field in northern Missouri.

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