Fall is a good time for farmers to get ahead of next year’s weeds by controlling winter annuals and perennials, such as Canada thistle, cleavers and narrow-leaved hawk’s beard. In fact, applying herbicides in the fall may result in increased efficacy of crop control products.
Why it matters: Tackling weeds in the fall offers numerous advantages for the following growing season.
“Fall temperatures trigger the translocation of sugars to the roots in plants in order to survive the winter. Fall-applied systemic herbicides take advantage of this movement and move in the same direction as the nutrients, resulting in the herbicide being translocated into the roots, (which leads to) much more effective control as compared to the spring,” says Nolan Kowalchuk, a technical sales manager from FMC in Alberta.
According to Ian Epp, Canola Council of Canada agronomy specialist for northwest Saskatchewan, winter annual weeds, which germinate in the fall, are easier to kill when they are small. If they are not controlled in the fall, they are the first plants to emerge the following spring, and compete for resources with crops. Additionally, higher doses of chemicals are required to control these weeds in the spring when the plants are larger.
Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture, says fall is the best time to get many soil active herbicides down, so they have ample opportunity to get well integrated into the soil, improving their efficacy and consistency.
“This is particularly important for Group 3 herbicides,” he says. “Group 3 herbicides are volatile, so the cool conditions in the fall help to prevent loss from volatilization; whereas, air and soils are warming in the spring, accelerating vapour losses. Group 3 herbicides are also broken down by sunlight, so the reduced number of daylight hours in the late fall (<12 hours) minimizes breakdown compared with long days in the spring (15-plus hours on May 1 and increasing).”
Other agronomic practices, such as tillage, increased crop seeding rates, decreased row spacing, crop rotation, and growing competitive and cover crops, can be effective in tackling weeds in the field.
Fall herbicide application tips
Fall control of winter annual and perennial weeds offers farmers many advantages, such as time management efficiencies, increased efficacy of crop control products, weeds are easier to kill in the fall rather than the following spring, and fall offers soil active herbicides enough time to get well integrated into the soil. Key fall application tips include the following:
- Start scouting for weeds after harvest in the fall.
- Understand the biology of the weeds.
- Use pre-emergent herbicides with multiple modes of action.
- Control weeds when they are small.
- If you want to till the soil, wait a minimum of 72 hours after herbicide application to perennial weeds.
- Herbicide is unlikely to act on weeds damaged by frost.
- Spray during late morning or afternoon hours, when temperatures are warmer and heavy dew is off the plant.
- Apply herbicides when temperatures are above 10 C.
- Bright, sunny conditions are ideal for facilitating herbicides to move to the roots.
Always follow label directions.
Herbicide resistance: A major challenge
Weed resistance to herbicides is an increasing concern among western Canadian growers. According to Charles Geddes, a research scientist at Agriculture and Agri-Food Canada’s Lethbridge Research Centre, there are 37 unique herbicide-resistant weed biotypes known to exist in Western Canada.
Due to the increase in incidence of herbicide-resistant weeds, Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture, cautions farmers to use integrated weed management techniques to reduce resistant-weed populations in their fields.
“Roughly 60 per cent of wild oats in Western Canada are resistant to Group 1 herbicides and between 25 and 30 per cent are resistant to Group 2 herbicides. Greater than 50 per cent of kochia populations are resistant to glyphosate, and a portion of those are also resistant to dicamba,” Brenzil says.
A Prairie-wide survey of herbicide-resistant weeds was conducted between 2014 and 2017. The results of the survey showed that 59, 57 and 68 per cent of annually-cropped fields are infested with at least one herbicide-resistant weed biotype in Alberta, Saskatchewan, and Manitoba, respectively, Geddes adds.
“The greatest abundance is that of herbicide-resistant wild oat. Group 1-resistant wild oat was found in 58, 59 and 78 per cent of fields where wild oat was sampled in Alberta, Saskatchewan and Manitoba, respectively. Group 2-resistant wild oat was found in 40, 32 and 43 per cent of fields where wild oat was sampled in Alberta, Saskatchewan and Manitoba, respectively. Group 1- and 2-resistant wild oat (multiple resistance) was found in 29, 25 and 42 per cent of fields where wild oat was sampled in Alberta, Saskatchewan and Manitoba, respectively.”
The survey also shows 23.7 million acres of annual cropland were infested with herbicide-resistant weeds in the Canadian Prairies. Herbicide-resistant weeds cost farmers in that region an estimated $528 million annually.