When it comes to weed control, no two seasons are the same. This year, many forecasters are predicting a hot, dry summer. What challenges will this bring?
For one thing, some weeds that thrive better under hot, dry conditions, says weed expert, Robert Blackshaw with Agriculture and Agri-Food Canada in Lethbridge, Alberta.
“Some weeds that have a different photosynthesis mechanism, and C4 plants such as corn, thrive under warm conditions,” he says. “We also have C4 weeds, such as kochia and redroot pigweed, so those would definitely be an issue.”
A bigger concern for farmers is dry conditions during seeding, because that leads to uneven crop emergence and staggered weed emergence, which makes herbicide timing tricky.
“Farmers like to have the entire crop uniform at the correct leaf stage to apply their herbicide and they’d like most of the weeds to be up and present as well, but under drier conditions that often doesn’t occur,” says Blackshaw.
When it comes to staging the herbicide under these patchy conditions farmers probably just have to do it when the majority of their crop is at the correct leaf stage and has some weeds growing. “They pretty much need to spray at that stage and perhaps consider a second application of a herbicide if they have a second flush of weeds,” says Blackshaw. “Some herbicides are safe enough that you can use them on the crop at a little later growth stage, and others are not, so they have to be careful about that.”
In dry conditions, there are often flushes of weeds during the growing season after rainfall. A second application may not be possible depending on the product window or if reduced yield potential of the crop makes it uneconomic.
It’s also not as easy to kill weeds with a post-emergent herbicide application during drought conditions, which seems counterintuitive, says Blackshaw. “We might expect that if the weeds are stunted and under stress due to dry conditions, it might be easy to finish them off with a herbicide but for most of our herbicides it’s the exact opposite of that,” he says. “If the weeds are actively growing and very healthy then that allows the herbicide to exert its full effect and weeds are killed the best under those growing conditions. But if plants are under stress due to heat and drought, then weed control is actually reduced with a herbicide.”
Contact herbicides will be the most effective during drier conditions because they don’t need to move within the plant to exert their full effect. But herbicides that need to translocate though the plant — such as Roundup, or Group 1s like Poast, Select, Horizon or Axial — will not be as effective.
“The majority of herbicides do require movement within the plant often to the growing point of the plant to exert their full effect and that’s greatly reduced under drought conditions,” says Blackshaw. “As well, the herbicide entry into the weed can be reduced under hot dry conditions sometimes because the plants produce thicker cuticles and deposit more wax on the outer surface of their leaves to try and inhibit moisture loss and so that acts as a barrier for uptake of the herbicides into the plant as well.”
Be cautious about using spray additives or surfactants in an attempt to increase absorption of the herbicide into the plant. “Herbicides have gone through a very rigorous registration process and evaluation and the companies have selected the best additive for that herbicide, so whether it’s already included in the herbicide formulation, or they suggest what is the best surfactant to add to a herbicide, farmers should stick with those recommendations,” says Blackshaw.
In some cases, says Blackshaw, especially if conditions are dry in the fall after harvest, it might be worth gambling on getting some control from a soil-applied herbicide applied in the fall or early spring, followed up with a post-emergent herbicide later on. “If it was soil-applied last fall and there’s some snow melt or rainfall this spring those herbicides will be activated in the soil and be effective,” says Blackshaw. “If we have very dry soil conditions those herbicides will be less effective, just as the post-emergent herbicides will.”
Don’t reduce seeding rates
One thing farmers should not do, says Blackshaw, is reduce seeding rates. “If farmers have a reasonably high seeding rate, it’s a little bit of an insurance policy if they do have dry conditions that are going to reduce germination and emergence,” he says. “A little higher seed population will help you get a decent crop up and makes it more competitive crop with weeds.”
Although it doesn’t help much with weed control during the growing season, farmers can consider using a desiccant or swathing at harvest time to dry down weeds. “It doesn’t solve your weed problem but it could reduce the amount of green material that would affect the crop in storage,” says Blackshaw.