Weeds can still take in chemical if they have green material, but growers must wait until they are actively growing
There is a good chance it’s too late for a fall herbicide pass on fields lying under knee-deep snow, but in areas where snow is gone or retreating, there may still be time for an effective fall treatment.
Tammy Jones, provincial weed control specialist at Manitoba Agriculture, said as long as weeds have green material that wasn’t annihilated by freezing temperatures, they can take in herbicide, but growers need to wait until they are actively growing.
“You want it to get up to around 10 C where you are getting that plant growing, where photosynthesis is happening and sugars are moving through the plant,” Jones said.
“You spray the plant during the warmest part of the day so that the chemical is translocated to the growing point and is effective on that plant.”
She said if the plant is not growing, it will not take the chemical into the roots no matter how green it is, and if it is growing very slowly, there’s more time for the herbicide to break down.
During the 2016, 2018 and 2019 harvests unfavourable weather in many growing regions had producers focused on getting the crop in, and there was little time for post-harvest herbicide applications.
This may be starting to show on some fields because fall is the best time to deal with perennials and winter annuals.
“So narrow-leaved hawk’s-beard, dandelion, Canada thistle, even white cockle. With a perennial it’s nice this time of year because it will be translocating sugars into the roots, and that’s where you need to get to kill some of that root system,” Jones said.
“With winter annuals, if you wait until the spring, sometimes they are outside the growing stage that you would want for applications.”
If freezing weather has already smoked the weeds, there may still be time to apply soil residuals on fields dry enough to handle the traffic.
Jones said fall application of a soil residual can be beneficial for a number of reasons.
The first reason is the time it saves growers in the spring.
“You can just bypass that weed control operation, and the perennial or winter annual weeds won’t get a head start on the crop,” Jones said.
“If you didn’t have ideal weed control the year before, then having that residual herbicide to help keep it weed free for a little bit longer also helps.”
Another benefit of using soil applied residual herbicides is it gives growers an opportunity to use different herbicides to control problem weeds.
“They tend to be different groups than what you normally rely on for in-crop herbicide, so it’s rotating through some of that different chemistry to help mix it up and have multiple modes of effective action on a target weed,” Jones said.
She said growers should aim for blanket coverage when using Group 14 and 15 herbicides.
“The analogy is an afghan versus a blanket,” Jones said.
“You can stick your toe through an afghan. So you want to make sure you have a blanket coverage instead of an afghan coverage, so you’re making sure there are no holes for those weeds to come through.”
Products such as Edge, a Group 3 product, and some Group 8 products, should be incorporated into the soil.
Clark Brenzil, Saskatchewan’s provincial weed control specialist, said it’s a good time to apply soil residuals.
“The soils are cool, you’ve got low day length, so what that does is the cool soil reduces the volatility losses,” Brenzil said.
“Group 3s, so Edge and Trifluralin, are susceptible to photodegradation to a certain degree, and by putting it on later in the fall your daylight hours are lower and as a result you lose less because of both those two processes.”
He said a reason some of these products are better applied in the fall is because stubble can lay down over the winter and become a barrier between the granular and the soil.
Harrows work these herbicides into the soil.
“That harrow pass, what it does is it just bounces all of that material and it allows the granular to get in contact with the soil so they integrate with the soil,” Brenzil said.
“When they go through that freeze-thaw cycle over the winter time, those granulars break down and they get a little more integrated. And then you go through with another harrow in the spring time just to kind of stir that material around a little bit.”
Brenzil cautioned that once a soil-applied residual is applied, grower’s crop options can be limited the following year.
“From an agronomic perspective this may not be a bad thing, but from an economic perspective, it might be a bit of a challenge,” he said.