Herbicide resistance predates herbicides by over 80 years

The Weed Science Society of America (WSSA) recently posted a press release headlined “WSSA Scientists Say Herbicide Resistance Predates Genetically Engineered Crops by 40 Years.” From the WSSA release:

You may think weeds resistant to herbicides are a new phenomenon linked to the overuse of glyphosate in genetically engineered crops, but according to the Weed Science Society of America (WSSA) nothing could be further from the truth. This year marks only the 20thanniversary of glyphosate-resistant crops, while next year will mark the 60th anniversary of the first reports of herbicide-resistant weeds.

The first known report of herbicide-resistance came in 1957 when a spreading dayflower (Commelina diffusa) growing in a Hawaiian sugarcane field was found to be resistant to a synthetic auxin herbicide. One biotype of spreading dayflower was able to withstand five times the normal treatment dosage. That same year wild carrot (Daucus carota) growing on roadsides in Ontario, Canada was found to be resistant to some of the same synthetic auxin herbicides.

And that’s true. But the history of herbicide resistance actually goes back even further than that. There is evidence that individual weeds resistant to herbicides existed long before we even discovered synthetic herbicides. French researchers screened herbarium specimens for a weed called blackgrass (Alopecurus myosuroides). They then tested those blackgrass specimens to see if any of them carried genetic mutations known to confer resistance to a certain type of herbicide (ACCase inhibitors). They found a mutant collected in 1888 near Bordeaux, France and kept at the Montpellier herbarium that had the same kind of mutation that causes herbicide resistance in blackgrass today. The study was published in PLOS ONE in 2013.

A. myosuroides herbarium specimen. (From Delye et al. 2013)

A. myosuroides herbarium specimen. (From Delye et al. 2013)

Herbicides that inhibit the ACCase enzyme were not commercialized until the 1970s, which means that the herbicide resistant plant in the French study was collected over 80 years before the herbicide had ever been sprayed.

Although it is a really cool study, it isn’t actually surprising based on what we currently know about herbicide resistance. Weed scientists have long suspected that spraying herbicides doesn’t cause resistance mutations to occur in the plant; it simply removes all of the susceptible plants so that any plants that are resistant can grow and survive. Over time, continued use of the herbicide selects for the resistant plants to become more common, and to eventually dominate the field. Herbicide use selects for the resistant plants that occur naturally within a weed population. The PLOS ONE study shows pretty convincingly that the resistant plants have been out there for a very long time. And if we continually spray the same herbicides, we’ll eventually find them all!

Comments

  1. Very interesting. So really, weeds are not actively adapting to our management at all – the adapted individuals are already there. This is not an example of mutation in response to environment as stated in some places.

    1. A few questions. Why would a plant have such a mutation in the first place, and why would the mutation be maintained over time if it did not help the plant? Is this the case for all cases of herbicide resistance, or are some due to mutations after the herbicide is being used?

      1. Many of the point mutations that confer resistance to various herbicides don’t have much of a fitness cost to the plant. In the absence of selection pressure, they are neutral. So they persist in the population at low levels. That is presumably the case with this ACCase resistance mutation. There are some mutations that have a substantial fitness cost (like mutations on the D1 protein that confer atrazine resistance). Those tend to be more rare initially. There’s still no evidence that spraying the herbicide induces the mutation, though. Our best guess is that it is simply there at a low level until it is selected for by repeated herbicide use.

  2. Hello,

    Thank you for this very important reminder about the reality of the presence of herbicide-resistant weeds populations.

    Bien cordialement.

  3. Isn’t this natural selection 101? Organisms don’t mutate “in response” to selection pressure, selection pressure simply reduces the frequency of less fit individuals? While there is certainly an answer to “how” mutations occur, isn’t it true that there is often no answer to the question of “why” a plant would have a particular mutation?

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