Two good reasons for pre-harvest herbicide

Pre-harvest application can give you better weed control 
and simplify harvest. But know the residue rules

Farming decisions always seem to come back to economics, and whether to do a pre-harvest treatment for perennial weed control or harvest management is no exception. But there are many other considerations such as crop type, geographic location, weather and even trade risks that contribute to that decision, says Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture.

In northern regions of the Prairies a pre-harvest herbicide application is often the only opportunity farmers have to control perennial weeds because the season is too short to allow enough re-growth for a post-harvest application to be effective. But even in the southern prairies a pre-harvest application is a better economic option, says Brenzil. “For a pre-harvest application of glyphosate the recommendation is 360 grams of active ingredient per acre,” he says. “Following harvest, weed re-growth presents a smaller target, so producers have to use about two-and-a-half times more glyphosate to get the same amount of control.”

Bruce Dalgarno, who farms near Newdale, Manitoba, started using a pre-harvest application of glyphosate on his crops around 1994, partly to try and control the Canada thistle and quack grass in his fields. “We’ve certainly seen a change in our weed spectrum in that we’re getting rid of Canada thistle and the quack grass has all but disappeared,” he says.

1. Effective weed control

There are a couple of reasons why a pre-harvest glyphosate application is effective against perennial weeds, says Brenzil. First, perennial plants at this time of year are moving sugars to the roots, which helps translocate the herbicide to the root system and prevent growth the following year. Secondly, prior to harvest there is more plant material present, which provides a larger target to maximize the uptake of the herbicide and give better control.

The other economic advantage is that it’s generally less expensive to deal with perennial weeds in-crop than the following year. “Canada thistle, as an example, is very competitive, so if a producer has a well-established stand no crop will grow underneath it,” says Brenzil. “If there are a substantial number of thistle stands within the field that’s a significant yield loss.”

Dalgarno says using a pre-harvest glyphosate application is fairly economical for him as he has his own high clearance sprayer. “The herbicide cost is anywhere from $3.50 to $4.50 an acre, plus the cost to run the sprayer,” he says. “I’d say it probably costs us around $6 an acre, but it would be more if you had to hire a custom sprayer, although you’re saving the cost of swathing as well.”

2. Harvest quality

For Dalgarno a pre-harvest herbicide treatment isn’t just about weed control; it’s also about ensuring a good quality crop on his farm, which is located amid rolling hills and potholes, which means maturity is all over the map. “We can’t straight cut our wheat without pre-harvesting it because everything is a different moisture level and maturity, so the herbicide also kills the plants and makes it more even by the time we harvest,” he says. “When we used to swath our wheat, and it had lain out there for a week or two we were lucky if we got No. 2, and most of the time it was a No. 3. Now 90 per cent of the time it’s a No. 1, and if the difference is $2 a bushel, at 60 bushels an acre, we potentially could lose $120 an acre by not pre-harvesting it.”

Dalgarno also knows from his own experience that timing of a pre-harvest glyphosate application is crucial. “We’ve run into a few situations where it’s turned cool for a week or two after application and the chemical doesn’t uptake as well,” he says. “So you have to look at all the factors — the weed situation in that field, the crop type and stage, and the value of the crop to determine if a pre-harvest treatment is worthwhile, whether it’s barley, oats, wheat, or any crop.”

“Another challenge with having dead plants sitting out there in the weather for any amount of time is that they’re going to be susceptible to saprophytic fungi and things like that cause quality loss,” says Brenzil, who doesn’t recommend focusing on glyphosate exclusively for its harvest management benefits for a number of reasons.

“If your goal is to control green weed growth, that is one thing, but to control green crop growth carries another range of problems,” he says. ”At 30 per cent seed moisture, the earliest timing indicated on the product label, the crop is essentially ripe. Research has shown that there is a little harvest management benefit at seed moistures less than 40 per cent. But if you apply a herbicide above that moisture level, you’re terminating that plant before the end of its life cycle, and it also terminates filling, so there could be a risk of more green seed, or shriveled and sunken seeds.”

Watch residues

If an application is made too early in the life cycle of the crop plants, there will be a higher likelihood of residual glyphosate within the grain, he adds. “International buyers are more and more concerned about pesticide residues in grain and they’re doing more routine testing of grain that they’re purchasing,” says Brenzil. “If the residues exceed allowable limits there is the risk of a shipment being rejected and the boat turned back, or the buyer maybe negotiates a deep discount on that product so that they can blend it off to meet the maximum residue limits that are required.”

Increasingly grain marketers are taking sub-samples of grain so that they can trace a lot back to the individual farmer if there is an issue. “It’s kind of like traceability, just not in a formal sense but the marketers are going in that direction in order to protect themselves, so producers have to realize that risk is starting to tumble down to them as well, and be aware of those maximum residue limits,” says Brenzil.

There can even be instances where a product is registered for use in Canada but isn’t registered in the countries the grain is being exported to, “Producers need to check with the people that are buying their grain to make sure that they can use that product on the crop they hope to deliver,” says Brenzil.

Brenzil recommends producers plan ahead and consider perennial weed control in the fall before planting a high value crop in the next growing season. This way the perennial weeds are addressed prior to the growth of the high value crop, maximizing their return on that value. “If producers select their primary and secondary crops each January, February or March based on the current commodity prices, and then try to track down products to manage perennial weeds in-crop, they risk losing yield to the weed before in-crop control occurs, if there is an option for control in that crop at all.”

They also run the risk of having a more volatile crop rotation, and volatile economics, and higher risk. The real challenge for producers these days is to diversify and stick with their rotations so they can stabilize their incomes, while being more sustainable over time, rather than trying to capitalize and maximize their incomes from any one particular year or any one particular crop.”

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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