How Weeds Become Resistant

1957 – The first herbicide-resistant weed

155 – The number of weeds with herbicide-resistance in the U.S. today

16 – No. of weeds in the U.S. resistant to glyphosate

What caused the rapid evolution of herbicide-resistant weeds?

Herbicide-resistant weeds have evolved in response to repeated use of herbicides with the same site of action (SOA).

How does herbicide resistance evolve?

Herbicide applications that eliminate susceptible weeds before they reproduce create a selective advantage for any rare, resistant individuals in the weed population. Reproduction by these escapees then transmits the resistance trait to their offspring, facilitating their survival when exposed to the same herbicide SOA. Sustained use of either the same herbicide or, in some cases, the same SOA, favors survival and reproduction of these resistant biotypes, leading to a weed population in which resistant plants dominate.

Repeating the same control tactics at a given timing, whether a herbicide application or a nonchemical control method, may also result in the evolution of avoidance mechanisms in a weed population by selecting for biotypes that have not emerged, or are outside the optimal growth stage, when control is implemented.

Did biotechnology cause HR weeds?

Herbicide-resistant weeds did not begin with herbicide-resistant crops; resistant weeds have been evolving in conventional crop cultivars worldwide from selection pressure placed on them from repeated use of herbicides.

A plant does not evolve resistance because herbicides cause a genetic change in the plant that makes it resistant. Rather, a few plants with natural resistance to the herbicide survive an application of the herbicide, and as those plants reproduce and each generation is exposed to the herbicide, the number of resistant plants in the population increases until they dominate the population of susceptible plants.

The wide-scale use of any single herbicidal SOA contributes to the evolution of resistance to that SOA, and the unprecedented scale of glyphosate use in glyphosate-resistant (GR) crops has clearly contributed to the number of GR weeds identified in recent years.