Crop rotation vital to slowing resistance in weeds

One of the banes of a farmer's existence is weeds, unwelcome plant-life that sinks in its roots, sucks up moisture and nutrients, and...

One of the banes of a farmer’s existence is weeds, unwelcome plant-life that sinks in its roots, sucks up moisture and nutrients, and denies the crops energy needed to grow robust and strong.

Since the 1930s, farmers around the globe have been able to control weeds – a term which refers widely to all unwanted plants, not just official “weed” species – but herbicide resistance is increasingly becoming a problem, according to Harry Brook, crop specialist with the Alberta Ag-Info Centre in Stettler.

Some weeds are the accidental offshoots of previous crops, the volunteers left behind from a previous year, or blown in the wind. With so many crops today being hybrids that offer some sort of resistance to herbicides, these volunteers are notoriously difficult to exterminate, Brook said.

“When you have roundup-resistant canola volunteers in a field, glyphosate won’t kill it,” Brook said. “It will be there, competing with the new crop.”

Five years ago, a strong wind from an unexpected quarter sent swaths of canola rolling across fields through central Alberta. Seed pods burst open and spilled canola seeds across fields.

In situations like that, volunteers are almost guaranteed to be a problem, Brook said. And, while hybrids do not “breed true,” there’s still the possibility they will germinate.

When it comes to genuine weeds, resistance is increasing year over year.

The kochia plant is one such example of a plant that has become extremely tolerant of several categories of herbicides. While kochia is not big an issue in central or northern Alberta at this time, the tumbling weed is becoming a nuisance in southern Alberta.

“The plant grows, then separates from the stem,” Brook explained. “The tumbleweed rolls around, spilling seeds everywhere as it goes.”

With Alberta’s windy prairies and rolling hills, tumbleweeds can go quite the distance.

In central Alberta, one of the more common weeds that is becoming a nuisance due to herbicide resistance is wild oats, Brook said.

“You won’t find a whole field full of resistant wild oats,” Brook said. “But there’s increasing amounts, and the number climbs every year.”

Herbicides are broken into different classes depending on the method in which the herbicide kills the weeds. Some outright kill weeds, while others prevent the weed from maturing. Others not only kill the weed itself, but migrates through to the roots to ensure the weed does not re-sprout.

Resistances to herbicides have varying degrees. In some cases, the herbicide kills the weed too quickly, before the herbicide can migrate into the roots. This means that though the weed dies, the roots continue to drain moisture and nutrients from the soil and will sprout anew. In other cases, the effect of the herbicide is lessened, or out-right eliminated by the weed’s resistance.

Other weeds will winter well and sprout in the spring before herbicides have been put down, spreading their wealth of seeds before farmers can even get out in the field.

Farmers can help control herbicide resistance in weeds by rotating crops and herbicide types more frequently, Brook said.

“Many farmers use type one or type two herbicides,” he said. “They’re cheap, but a lot of weeds are becoming resistant to those types.”

Resistance to type one is about 15 per cent and growing annually, while in type two, the resistance is at about 25 per cent, and growing.

By using different herbicides, and not swapping between types one and two every other year, weeds will have a harder time building up the resistance that is increasing the difficulty in eliminating weeds from farmers’ fields.


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