The more you spend, the less they work

If you want to keep using herbicides, give them an occasional rest and try a winter cereal or a heavier seeding rate

Neil Harker says that when you no longer have the big hammers in the tool box, it’s time to use the little ones.

For the Agriculture and Agri-Food Canada (AAFC) weed scientist, wild oat herbicides are the big hammers, and the little ones are integrated weed management practices.

Harker says that although western Canadian producers spend more herbicide dollars on wild oats than any other weed, those herbicides are becoming less and less effective. Due to so much selection pressure, resistance to Group 1 and Group 2 herbicides is on a steep rise.

“I see resistance happening where producers are using these groups over and over again. There are farms in southern Alberta where we have resistance to Group 1 and Group 2 herbicides, as well as some Group 8 herbicides,” says Harker.

Wild oat is a difficult weed to manage at the best of times. The summer annual spreads by seed, but has variable germination and lies dormant for three to four years, or in special circumstances up to 10 years.

In less competitive crops like flax or field peas, wild oat can gain a swift foothold, but even in more competitive crops like barley or canola, producers can see a yield loss of 20 to 30 per cent, says Harker.

Try winter wheat

Harker recently published results from a four-year direct-seeding study looking at the impacts of crop life cycle, crop species, crop seeding rate, crop usage and herbicide combination on wild oat management and canola yield.

The study, which ran from 2010 to 2014 on eight sites across Canada, found that integrated management systems that combine a variety of cultural practices against weeds and limit herbicide use prolong the life of the herbicides.

“We wanted to see if some of these other methods could be as good as using 100 per cent wild oat herbicide in canola-wheat-canola-wheat rotations,” says Harker. “We found that in terms of wild oat emergence, biomass, canola yield and wild oat seed numbers in the seed bank, some of those treatments were just as effective as herbicides.”

For example, putting winter wheat in the rotation so that it gets ahead of wild oat in the spring means that for “a whole year you don’t need a wild oat herbicide,” says Harker.

The researchers also found that doubling seeding rates for crops like wheat, rye, triticale or barley can be extremely effective against wild oat — especially when used in combination with a winter cereal.

Harker also cites a research study that suggests Group 1 and Group 2 herbicides are good for at least 10 years of applications, meaning that after 10 years producers will start to see some resistance developing.

Herbicides like glyphosate can go for about 20 years before resistance starts developing, but selection pressure increases with the number of acres receiving the herbicide.

Hugh Beckie, an AAFC research scientist and specialist in herbicide resistance, says most western Canadian growers currently have wild oat resistance to Group 1 and Group 2 herbicides.

There is still no documented wild oat resistance to glyphosate, but Beckie believes Canada is at risk for resistance developing in a variety of weeds including green foxtail, cleavers and wild oat.

For several years, Beckie has done risk assessments for herbicide-resistant weeds in tandem with weed surveys in Prairie provinces, comparing management practices in producers’ fields with and without resistance.

“Those with good rotations had better resistance. Other practices like weed sanitation, mowing uncontrolled weed patches and cleaning equipment meant a lower probability of resistance,” says Beckie.

Modes of action

Currently, mixing modes of action is the most common advice producers hear when it comes to managing weed resistance.

Mixing modes of action means slowing resistance, says Harker, but eventually this practice will select for multiple resistance. In other words, it’s a short-term solution that could seriously downgrade land quality.

Beckie says use of a burndown product like glyphosate, or residual products like Avadex or trifluralin (both Group 8) can help reduce the weed population pre-plant.

But the only lasting solution for wild oat control lies in the use of integrated weed management practices. “We’ve talked about integrated weed management for decades and the message is the same, but it’s the only solution to managing weeds. It’s repeated often but it can’t be emphasized often enough,” he says.

“Producers realize that it’s important, but because of time pressures, they tend to rely on herbicides — the big hammer. We just keep telling them, use best management practices as consistently and frequently as possible.”

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