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Weeds Shift To Meet New Farming Practices

The widespread adoption of zero or minimum tillage has led to subtle changes in weed populations in Manitoba, according to the provincial weed specialist, Nasir Shaihk.

In the 1970s, when tillage was widespread, the top three weeds were volunteer barley, dog mustard and field horsetail. An examination of weed survey results stretching back over the past three decades shows new weeds are rising to the top.

A survey from 1994 of the 15 most abundant weeds under zero tillage found that green foxtail was in seventh place, followed by dandelion and barnyard grass at 13th and 14th spot, respectively.

But in an identical survey from 2002, green foxtail had risen to the top slot, and barnyard grass had moved to fourth place. Dandelion moved to the ninth spot.

“Volunteer canola, which was the No. 1 weed in 1994, had moved down to the 10th spot,” said Shaihk. “So, you can see that there has been some shifts in the weed flora.”

Wild oats, wild buckwheat, Canada thistle, and lamb’s quarters, have stayed more or less the same over that period of time.

One of the biggest drivers behind the shift to conservation tillage has been the plunge in the price of glyphosate, which has fallen from $25 per litre in the early 1980s down to about $3/litre for generic products, said Shaihk.

Also, because it is much cheaper, it is often the first weed control tool that farmers reach for, he added.

“It is extensively used now under all conditions for pre-seeding, in-crop, pre-harvest, and for spraying winter annuals,” said Shaihk, in a presentation at the recent ManDak Zero-Tillage Association annual workshop.

There are two factors that most affect weed seed survival, the length of time it can survive in the soil, and the rate that dormancy breaks, which is largely dependent on weather.

Kochia typically lasts only two years in the soil, green foxtail up to 15 years, and wild oats, up to nine years. Red root pigweed can last 40 years, and curled dock, 80 years.

In Manitoba, 15 herbicide-resistant weeds have been identified, and some of them are resistant to more than one mode of action.

“Unfortunately, green foxtail and wild oats, which are the major weeds in Manitoba, are already showing multiple resistance to different herbicide groups, particularly (Groups) 1, 2 and 8,” said Shaihk.

“Fortunately, we do not have glyphosate-resistant weeds so far.”

But he noted glyphosateresistant giant ragweed has shown up in Ontario on a field where the farmer had been repeatedly using the chemical for seven to eight years.

Haisheng Xie, who has conducted herbicide-resistance assays on seed submitted by farmers at Minto-based Ag-Quest Laboratories since 1996, said that the problem seems to be getting worse every year.

Of the over 5,000 samples sent in mainly from the Canadian Prairies, but also North and South Dakota, Minnesota and even Florida, wild oats and green foxtail were the most common weeds tested.

There has also been increase in demand for testing Group 2 resistance in broadleaf weeds such as cleavers, wild mustard, stinkweed, shepherd’s purse and kochia.

Group 1-resistant wild oats ranged from over 80 per cent for the FOP-based herbicide subgroup, 68 per cent for DIM, and 34 per cent for DEN.

For green foxtail, Group 1 resistance has in recent years risen from 78 per cent to 83 per cent, and Group 3 from 78 to 89 per cent.

Recently, there has been increasing demand for multiple testing within one weed sample, he said, and some wild oats samples have been found to be resistant to as many as five different herbicides.

No weed samples received by Ag-Quest have been resistant to flamprop, glufosinate, MCPA/ bromoxynil and triallate, and barnyard grass, Russian thistle, wild buckwheat and yellow foxtail have not shown any resistance to any herbicide, said Xie. daniel. [email protected]

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