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Timing critical for fall herbicide application

Choosing the right application window for pre- and post-harvest weed control can be a guessing game. Follow this guide to fall application timing and cool-weather spraying

As fall progresses, post- harvest herbicide application becomes a bit trickier. Effectiveness will be influenced by factors such as temperature, target weeds, the amount of frost damage and the products being used.

Timing is critical.

“One of the primary challenges with a post-harvest herbicide application is whether you are going to have enough time for re-growth after harvest,” says Clark Brenzil, provincial weed specialist with the Saskatchewan Ministry of Agriculture.

This is especially crucial when dealing with perennial weeds. “You want to make sure you have four to six weeks after harvest of open weather with warm growing conditions for that plant to get enough growth on it so you have a target again for application and before you get a killing frost.”

Perennial weeds

Perennials weeds such as quackgrass and foxtail barley are best controlled pre-harvest in early fall when they are still actively growing. But they can be controlled post harvest as long as there is enough regrowth to make the product effective.

“When herbicide is applied on perennial plants in the fall it goes into the root system along with the food material that is being transferred for storage by the plants for the winter, so it is a much more effective way of controlling weeds than applying it in the spring,” says Nasir Shaikh, Manitoba Agriculture, Food and Rural Initiatives’ provincial weed specialist.

Weeds such as Canada thistle can provide more of a challenge because they generally have poor re-growth after harvest. Dandelions, on the other hand, may grow more actively in cool fall conditions than in hot, dry weather.

The odds of having good plant tissue remaining that is warm and pliable enough for the herbicide to be effective drops dramatically past the end of September. “If you are doing perennial weed control with glyphosate or any Group 2s you want to apply the product by the first week of October, and that’s only if conditions are still reasonably warm at that time,” says Brenzil.

Hitting this small window of opportunity becomes harder the further north you go, where harvests can be late and frosts early. This is why Brenzil recommends that farmers in northern areas focus more on pre-harvest rather than post-harvest applications for perennial weed control.

Consistently dry, warm and sunny days both before and after application are needed, especially with herbicides such as glyphosate or Group 2s, to allow the maximum amount of product to move within the plants to the target. “Leaves are often not the target,” says Brenzil. “The leaves are just the receptor. The target is the buds and growing points that the plant is laying down for winter for growth next spring.”

Under cooler conditions, particularly in the case of glyphosate, active ingredients can get bound up in the leaves, which will give good die off in the top growth but poor long-term weed control. “Biological activity basically stops once you get below 4 or 5 C, so if you have evening temperatures that are dipping down below that and your daytime peaks are less than about 15 C then you probably should not be spraying,” says Brenzil.

Frost tolerance

Quickly changing weather conditions can affect the ability of weeds to acclimatize to colder conditions and tolerate a frost. “If it’s really warm and then all of a sudden you get minus two or minus three nights you can get more damage to the plant than if temperatures make a gradual ongoing procession down to reasonably cold overnight conditions,” says Brenzil. “You can have minus 10 degrees under conditions like that and plants like Canada thistle will be fine if temperatures warm back up again.”

Assessing the amount of leaf damage that has been done by a frost is also important. Frost stresses the plant and causes it to shut down — hampering the ability of the chemical to be taken up by the plant. In the case of glyphosate, Monsanto recommends that no more than 40 per cent of the original leaf tissue should be lost before application in order to get adequate control.

Check plants a couple of days after a frost. Keep an eye out for browning caused by acute frost damage, but also for leaves that are green but feel brittle when handled. “These leaves are not going to be good targets,” says Brenzil. “So you basically can count that as frost damage when you are trying to do your assessment.”

If plants are still green with little damage, wait about three days after the frost to give the plants time to start growing again before applying any chemical. If the plants are brown to black and very damaged, it may be best to wait until new growth emerges.

Winter annuals

Winter annual weeds can be controlled quite effectively in the fall, because the plants are young, small and more susceptible to the herbicide. Again, timing is the key factor in getting the best results.

Winter annuals may continue to germinate and grow after the first fall frost, so an application as late into the fall as possible is usually the most effective. Work done at the Scott Research Farm at Saskatoon in the early 1990’s found that annual weed control applications made during the last two weeks of September were not as effective as applications made in mid- to late October.

“I always advise farmers doing their annual weed control to wait until after Thanksgiving because that seems to be a key time to control winter annuals,” says Brenzil. “If they apply between Thanksgiving and freeze-up, as long as the booms aren’t freezing, they will have good results.”

The exception to that rule may be dandelions. Research from the University of Saskatchewan has shown that a post-harvest application gives better and longer-lasting control than a pre-seed application for dandelions. September applications, before the plants went dormant, were more effective than October applications.

Many of the weeds that can be successfully targeted with a winter annual application are members of the mustard family such as stinkweed, shepherd’s purse and flixweed, but not all winter annual weeds will be controlled well at this time. Narrow-leaved hawk’s beard, for example, is not well controlled by a fall application of 2,4-D because the product cannot move within the plant’s system sufficiently well to provide effective control. This is a weed better targeted in the spring.

Straight phenoxy herbicides, such as 2,4-D, are effective if applied post-harvest, without the need to mix glyphosate, but if you want to tank mix, include something that will give good grass control for winter annual grasses such as downy bromegrass, says Brenzil, and pay attention to the recommended guidelines for temperatures at application.

Post-harvest rates need only be about half the rate used in an in-crop application for annual weeds. Saskatchewan’s Guide to Crop Protection recommends 0.28 litres per acre for 2,4-D, or around four to six ounces of active ingredients per acre.

It’s also important to keep in mind what’s going in the ground next year. “What you are doing with that fall application this year is a reference for next year, so you have to consider what you are growing next year,” says Brenzil. For example, a field treated with 2,4-D in the fall will not be a good choice for oilseeds like flax or canola the following year, because they will be sensitive to herbicide residue. Cereals are the best choice following a fall application. One crop that is slightly more tolerant than oilseeds in this situation is peas, so where a cereal crop isn’t a viable choice, farmers could use peas as an alternative crop for the year following fall herbicide application.

If a fall application just doesn’t work out for whatever reason, winter annuals can also be controlled in the spring with a pre-seed burn down.

Spraying considerations

Spray drift tends to be lower during cooler weather because there is less evaporation, says Tom Wolf, a research scientist at Agriculture and Agri-Food Canada’s Saskatoon Research Centre. “Applicators will have a bit more freedom to use finer sprays if that’s important to them, but there are no droplet sizes that are less affected by the cooler weather in terms of efficacy,” he adds.

Farmers should also be aware that, because fall days are shorter and there is less solar heating, there is a much shorter window during which temperature inversions can be avoided. Temperature inversions are a reverse of normal, daytime temperature profiles, where air temperature cools as it rises and thermal turbulence mixes the air, allowing for rapid dispersal of spray drift. At night, especially when no clouds are present, the temperature profile can become inverted, meaning that temperatures rise with elevation, causing poor mixing of the air and making the spray cloud more concentrated, so it will not readily disperse.

Spraying at night or near dusk or dawn is not recommended when conditions are favourable for overnight temperature inversions. Any spray drift may take a long time to dissipate in the morning, although strong sunshine will help speed up the process.

Doubling up

Is it possible to control both winter annual and perennial weeds with one pass in the fall? Probably not. Winter annuals may be present at the time you are controlling perennials, but they may still germinate after perennial weeds need to be controlled. Similarly, spraying late for winter annuals means spraying past the time that the chemical will be effective on the perennials. So walk your fields and decide which weeds are most important to control. †

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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