October is a good month for fall weed control, according to Saskatchewan Agriculture weed control specialist Clark Brenzil.
“September isn’t the best time to control winter annual weeds because many more will germinate before freeze-up,” he said.
“The best timing for an application of inexpensive phenoxy herbicide – 2,4-D and MCPA – is in October, as close to freeze-up as possible or as early in the spring as possible.
“Delaying applications in the spring will allow winter annual weeds to get too big to be controlled with phenoxy applications, and a more costly glyphosate application will need to be used,” said Brenzil.
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Winter annual weeds, such as flixweed, stinkweed and shepherd’s purse, germinate in fall and, after wintering under the snow, produce seeds and die relatively early the next year.
They have a high tolerance for cold and are the first plants to set seed in the spring. Some, such as pygmyflower and yellow whitlow-grass, complete most of their life cycle before field activity begins.
The rapid maturity of winter annuals in spring makes fall applications more efficient. Delaying control until later in the spring enables weeds to use nutrients and water needed for the crop.
It also increases the likelihood that chemical applications will be delayed due to weather, with the possible result that weeds are not controlled at all.
“Even if no winter annuals are apparent, if you know you have populations of these weeds, spots missed during a fall application will be very obvious come spring,” said Brenzil.
Rates of four to six ounces of active ingredient per acre (113 to 170 grams active ingredient per acre) of 2,4-D are sufficient to control winter annual plants.
He said at a liberal price of $6.80 per litre for a 600-ester formulation, a fall application of 2,4-D will cost $1.29 to $1.90 per acre plus application costs. Glyphosate applications in the spring can cost about twice that amount, at $4 for a half-litre-per-acre application plus application costs.
“Research at the University of Saskatchewan has found that pulse crops can tolerate fall applications of 2,4-D up to 12 ounces active ingredient per acre (0.68 litres per acre of 500 g per litre formulation) but spring applications are likely to cause injury even at six ounces,” said Brenzil.
A late-fall application is also the optimal time to apply 2,4-D or glyphosate for the control of narrow-leafed hawk’s-beard and dandelion in minimum tillage fields.
To get reasonable control of these weeds, a minimum rate of 12 ounces active per acre (0.68 litres per acre of 500 g per litre formulation) is needed. If 2,4-D is used in the fall, it may interfere with control of narrow-leafed hawk’s-beard with a glyphosate burn-off applied in the spring.
“While it is true that a glyphosate application will control both grasses and broadleaf weeds in one pass, controlling broadleaf weeds with 2,4-D will sometimes allow a lower rate of glyphosate (0.3 litres per acre) to be used on the annual grass weeds and spring-germinating broadleaves that emerge in the spring. The relative cost of the two methods is about the same,” said Brenzil.
If perennial weeds are present in the field, he added, glyphosate is the product of choice, and the rates will have to increase beyond those needed for annuals alone.
