Weeds Don't Resist. They Adapt.

Voltaire said that “History never repeats itself; man always does.”

George Santayana said "Those who cannot remember the past are condemned to repeat it."

In all facets of life, we often fail to learn from the past.

Even in weed management, we fail to learn from the past.

In his book “Same as Ever,” Morgan Housel highlights that some things never change: "people falling for greed and fear; people persuaded by risk, jealousy, and tribal affiliations; overconfidence and shortsightedness; people seeking the secret to a happy life and trying to find certainty when none exists."

I’d add one more to his list: People succumbing to the seductiveness of Silver Bullets — because we love a great narrative and an easy button.

Many of us fall for the seductiveness of Silver Bullets — whether it is the next “get rich quick” scheme, or weight loss formula (though, GLP-1’s may have me here).

We also fall for them in weed management.

Failure to learn from the past and following our affinity for silver bullets leads to many problems in weed management.

Two weeks ago, I highlighted the sneaky ways weeds could adapt to overcome alternative weed control methods, like laser or mechanical weeding.

This week, I read another comment from a VC surrounding weeds being unable to adapt to mechanical weeding mechanisms (emphasis mine):

The bolded statement is objectively false. Weeds can, and have, adapted to mechanical control methods through phenological approaches (eg: changes in time to seed set), morphological mechanisms (eg: prostrate grow habit) and spectrum shifts on a field basis to rhizome-dominant species, as a few examples.

This is not the first, nor the last time someone will say something to that effect about new methods of controlling weeds.

But we always underestimate how clever Mother Nature can be.

In fact, at one point, the makers of glyphosate made similar claims — and now glyphosate has documented resistance from ~60 weed species worldwide (and growing), with about 20 of those in North America.

Glyphosate was first commercialized as Roundup in 1974.

It worked by inhibiting an enzyme (EPSPS) in the shikimate pathway that plants need to make essential amino acids, but that animals lack entirely, which gave it a sound toxicology profile plus had a broad-spectrum, systemic, non-selective control. It also bound tightly to soil and broke down microbially, so residual carryover was minimal and crop rotation optionality stayed wide open. It was considered a once-in-a-lifetime molecule.

On top, the conventional wisdom was that resistance would never become a serious issue rested on a few specific assumptions: EPSPS is highly conserved with little room for mutation that wouldn't carry a fitness penalty, and no natural analog existed in the environment to pre-condition weed populations.

More than two decades passed without confirmed field resistance.

In 1997, four Monsanto scientists (Laura Bradshaw, Stephen Padgette, Steven Kimball, Barbara Wells) published “Perspectives on Glyphosate Resistance.” The following statements were made in the paper:

They even cited the difficulty in making Roundup Ready tolerance as a rationale for why resistance was unlikely to develop:

Essentially, the conclusion was that because of glyphosate’s unique mode of action, along with the fitness penalty incurred to adapt to it and the difficulty Monsanto itself faced in engineering resistance into crops were all presented as reasons resistance would not evolve in the field.

Obviously, the adaptability of weeds was severely underestimated.

Even accounting for any bias from the Monsanto affiliation — university scientists were asked with similar conclusions, plus cited in the paper is an acknowledgement of conversations with incredibly well respected weed scientist Dr. Stephen Powles¹.

The line of thinking surrounding glyphosate sounds very similar to the logic being applied by alternative weed management companies. But we should learn from the past, not repeat it. Individuals involved with novel weed control mechanism companies have the ability to learn from glyphosate.

Depending on how you count, there are roughly 28 distinct herbicide mechanisms of action today.

HRAC recognizes 24 numbered groups with characterized target sites, plus Group Z, a catch-all holding ~four mechanisms that are distinct from each other and from the numbered groups but with target sites still not entirely known.

However, considering some of these are niche, that leaves us with somewhere around 25 commercially relevant groups.

Then, if we factor non-herbicide mechanisms there are around another 15, leaving us with around 40 unique approaches for tackling weeds

Of those 25 herbicide modes of action ~23 have known resistance, and the ones that don’t are either newly commercialized (eg: Group 28), or have not been used as intensely since being introduced (eg: Group 30), so it’s worth saying… around 90% have resistance…so far, with few on the horizon.

Of the 15 non-synthetic weed management approaches, 10 of them, or about 66% have had documented adaptations by weeds.

For example, with a method like tillage, wild oats have adapted dormancy shifts (when they germinate). With another method, increasing crop competition (higher seeding rates), weeds have adapted shade tolerance. Even in the instance of hand weeding, Vavilovian Mimicry has arisen.

The ones that haven’t had documented weed adaptations either aren’t commercial (eg: microwave) or have been commercialized well under a decade (eg: laser weeding) and likely don’t have significant acre reach at this time meaning less selection pressure.

The stats, and evolutionary biology, illustrate that thinking a specific method will not develop resistance is either ignorant, arrogant, or both.

We all have biases and often strict points of view. In the instance of non-herbicidal weed management companies, their bias often is to eliminate synthetic mechanisms altogether, sometimes through an externalities lens (eg: environmental impact) and sometimes through an altruistic “manage resistance.” In the instance of the latter, they are actually just encouraging future resistance to their own method.

Like I talked about the other week, I acknowledge companies have missions and need a villain, however, it is still important to call out.

Part of the issue with non-herbicidal methods is that framing weed adaptations as “resistance” tricks us into thinking “well, a weed can’t metabolize a laser,” or “a weed can’t resist being cut.”

However, the proper framing isn’t “weed resistance” it is “methods of adaptation.” If we ask, how could a weed make it harder for this method to be controlled, we are more likely to acknowledge all the ways a weed can overcome different methods.

The focus should always be on a plethora of approaches. Consider what Greg Stewart, CEO of Geco Weed Management strategies recently shared in his Linkedin post:

That is what IPM should be about and how new methods should be integrated into farming systems.

Voltaire was right. History doesn't repeat itself, but man does. And in weed management, many companies are at risk of repeating mistakes we should have learned from glyphosate (and 22 other herbicide groups).

¹ In 2015, I was at a Herbicide Resistance Conference in Paris that was filled with weed scientists from around the world. During the event, I was on a full elevator in the hotel. At one point a singular individual got off on a floor while there were still several of us headed to our floors. As soon as the door shut, someone said “Did you see that was Stephen Powles?” to which the another person responded “Yes! I almost asked him for his autograph!” I didn’t tie together who he was at the time, but he has been one of the most influential individuals surrounding herbicide resistance and weed management for the last several decades. Dr. Powles has been published more than 300 times in peer reviewed journals and his work has been cited nearly 30,000(!) times, making him a titan in agricultural science.

Direct kill (energy):

Biological:

Suppression rather than kill: