Features Agronomy Weeds
Post-harvest weed control saves moisture and nutrients

Making a post-harvest burndown a planned part of a regular fall routine will help control some of the worst moisture and nutrient-robbing weeds before they can rob crop yields.

Spring photo of untreated field (left) compared to fall post-harvest application of Express SG plus glyphosate (right). Photo courtesy of DuPont.

And some of the toughest weeds, like dandelion, are best controlled before spring. “Spraying post harvest is something that a lot of producers have moved away from with direct seeding,” says Ken Sapsford, research assistant at the University of Saskatchewan’s department of plant sciences. He is also a member of the team that produced the research on the benefits of spraying early even if it means seeding later. “What we’re seeing, depending on weed spectrum and particularly if they have a dandelion problem, is that post-harvest is the best time to attack those weeds.”

He says there are problems with dandelion showing up because farmers have opted away from fall applications. “We get a lot of questions about dandelion,” says Sapsford. “The simple answer is to get out there with a fall treatment. If you have some residual in there it will help control winter annuals as well.”

Sapsford says the other issue is that in a wet fall, winter annuals like flixweed, narrow-leaved hawk’s beard and shepherd’s purse can really take off.  “All of those weeds are easier to control in fall than in spring,” he says. “Those weeds can get fairly large if you leave them until spring and you’re forced to seed later,” he says. “Sure you’ll be able to kill them with the treatments you put on but they can suck out a lot of surface moisture and that’s the seedbed for
your crop.”

In some situations, pre-harvest applications provide effective perennial weed control but they do not always hit the target. “Some low-growing or heavily fertilized crops can fall over and shadow plants underneath,” says Clark Brenzil, provincial specialist in weed control with Saskatchewan Agriculture. “Depending on the thickness of the canopy, you may not get good coverage of weeds such as dandelion with pre-harvest applications.”

The light intensity at ground level increases once the canopy gets removed by harvesting and allows the germination of plants like Canada thistle and dandelion, says Brenzil. “As you progress into the fall, winter annual and biennial plants begin to establish themselves.”

Wait for new growth
It is important to leave time for the weeds to re-grow before spraying after harvest. It may seem counterintuitive but there has to be a leaf-surface target for the spray to hit. Brenzil says that growers should wait four to six weeks after harvest for new growth before spraying creeping perennial weeds like Canada thistle. Because the amount of leaf surface area will be less after harvest, the rate being applied needs to be higher to get the same amount of herbicide into the plant.
 
“Another thing to consider is day length,” says Brenzil. “The number of daylight hours can have a big impact on movement of the systemic herbicide to the roots of perennial plants. Sunlight hours vary widely when you compare northern areas like Meadow Lake or the Peace River district to communities along the US border.”

“What happens in southern regions is perennial weeds shift their growth earlier from the reproductive to the rosette stage when they start moving sugars into the roots,” says Brenzil. “This movement of sugars carries systemic herbicides with it and improves their effectiveness. In southern regions, a pre-harvest application made in the middle to end of July in an early maturing crop may not be as effective as a post-harvest treatment for that same crop in early September.”
 
Areas further north have longer daylight hours in July and August, meaning they make the transition to the rosette stage later. Northern areas typically also have later-maturing crops. This means that a pre-harvest application to control perennial weeds is the only practical option in most years, as harvest is later and first frost is earlier.

Of course, the first hard frost ends any opportunity to spray no matter how much light there is. “If you have a frost that doesn’t physically damage the plants and you get warm weather after the frost, you can still control those weeds fairly readily,” says Rick Holm of the University of Saskatchewan. “The crucial question is, has frost damaged the weeds? If not, ask yourself if there will be enough warm weather to get the herbicide to work adequately.”

Good time to target dandelion
Sapsford says that late September to early October is the best time to spray for dandelions. For winter annuals, wait until after October 15 to spray. He says some of the choices for this timing, such as a tank-mix of glyphosate and a formulation like Express Pro, have extended control, which will control some early-germinating weeds the following spring, as well. “Products with residual control can still control later-germinating winter annuals, even if they aren’t out the day you sprayed,” says Holm.
 
A tough weed like narrow-leaved hawk’s beard (NLHB) is best controlled in October, as is flixweed. It is an inefficient moisture user so it removes a lot of moisture from the soil early in the spring.

Even with a post-harvest burndown growers are still likely to require a pre-seed burnoff. “If you put the post-harvest treatment on fairly late and catch most of the winter annuals that were going to emerge and are seeding the field really early the next spring, there might not be a strong case for putting the pre-seed treatment on,” says Holm. “But as soon as the soil starts to warm up in the spring you have spring-emerging annuals as well that need to be taken care of with a pre-seed burnoff.”

Holm says that where perennial weeds are a major concern, he would time his fall application to maximize control of the perennial weed species and worry about controlling winter annuals in the spring.
 
To control some of the tougher weeds, a tank-mix of Express SG or Express Pro with glyphosate may be a good idea. “Growers use products from the Express family in a post-harvest situation to improve control but also so they can hit weeds with actives from two different groups to help manage weed resistance,” says Jon Gough, product manager for cereals and oilseeds for DuPont.

A product like Express SG provides control of key broadleaf weeds, including ones that glyphosate tends to miss, providing a cleaner field to seed into. Plus, there is the flexibility to seed any cereal crop or selected pulse crops, including peas.

New Express Pro can be sprayed prior to spring wheat, durum or winter wheat, or spring barley, but canola and peas cannot be seeded until 10 months after application.

While controlling annual and perennial weeds after harvest is a good idea, the controlling factor is whether Mother Nature will let growers do it. But in years when conditions allow, fall post-harvest control is a good practice that can provide better weed control.