Herbicide Resistance – Coming To A Field Near You

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Herbicide resistance in weeds is a growing problem affecting many commonly used products used in Alberta and around the world. Harry Brook, crop specialist at that the Alberta Ag-Info Centre, explains herbicide resistance and the upcoming webinar that addresses the issue.

“We frequently hear about resistance – whether it is microbial, fungal or weed control – and it is the same problem,” says Brook. “When you have a great tool, like an antibiotic or herbicide, and it does a bang-up job of controlling a problem, it becomes your go-to answer to that particular bug or weed. However, the success of that control product carries the seeds of its own destruction. Overuse leads to resistance and a search for another magic bullet. There is not an inexhaustible supply of magic bullets.”

Brook explains further, “With herbicides, using the same herbicide group sharpens the selection pressure on the weeds affected by that herbicide. Every plant population has small numbers of naturally occurring resistant plants, or ones that do not respond as well, to a given herbicide. Repeated use of that herbicide or group will give the resistant plants free rein to grow and proliferate. Over a short period of time this can result in fields dominated by weeds not responsive to your herbicide and a massive seed bank, giving you trouble for years in the future.”

Brook says that there are several telltale signs that indicate a potential problem with herbicide resistance:

    • Unexplained weed patches in the field even when the majority of that weed species was killed.
    • Problem patches in no particular pattern and obviously not a sprayer miss.
    • Other weeds controlled by the herbicide are killed but weeds next to them appear untouched.
    • This herbicide or another from the same group used this year was noted with a problem last year.
    • Field history indicates extensive use of one particular herbicide group.

Each herbicide is classified by the way the herbicide kills the target weeds, called the mode of action. A limited number of modes of action are available, despite the yearly proliferation of generic herbicides.

“There has been no new mode of action discovered in almost 30 years,” he adds. “If you hear of any new herbicide it is – at best – a new compound found in an existing group, creating a false impression. If you have a weed that is resistant to one herbicide in a particular herbicide group, it is resistant to all the herbicides in that group, even the new ones.”

Record keeping is essential to determine if there is a resistance issue. It notes the repeated use of particular herbicides or groups. It is also useful in planning alternate weed control strategies to combat those resistant weeds.

Herbicides are just one way to control weeds. “Delayed planting prior to seeding can help cut down weed populations, “explains Brook. “In areas of better moisture, heavier seeding of the crop can add crop/weed competition as an effective way to keep weeds from hitting your yields. If you have cattle, chaff collection can be an effective way to remove weed seeds. A diverse and varied crop rotation that incorporates perennial forages can also be very effective in reducing weed seed production and carryover. And ultimately, localized or patch cultivation can sometimes be the most effective way to control particular weeds.”

He notes that another effective method to slow the spread of herbicide resistance is to use more than one mode of action on the particular weed. “We are seeing this as producers have gone back to using Avadex or Treflan as a preplant application to the soil then applying a Group 1 or 2 post emergence herbicide once the crop is up. This extends the useful life of a herbicide but not forever. Other tools, besides herbicides, need to be used to keep herbicides as a valuable weed control option.”

“Foxtail barley is an increasingly problematic weed in much of the province,” says Brook. “It does not respond well to herbicides but is very easy to control with light cultivation. Wild oats are a concern as there are populations of wild oats in the province resistant to three herbicide groups. Grassy herbicide control only comes from about 5 herbicide groups. In crop, you are mostly limited to Group 1 or Group 2 herbicides.”

Brook adds that herbicide resistance is not a problem that is going away. “Record keeping, planning and including all possible weed control tools is essential to herbicides effective into the future. Don’t abuse and overuse, it will get you in the end.”

For more information about this issue, register for the Herbicide Resistance: Coming to a Field Near You webinar, taking place Wednesday, December 12, 2018 at 10 a.m.

Source: Alberta Agriculture and Forestry