Mare’s Tail: An Evolved Success

Application of these facts to changes in the population of a weed called Mare’s Tail illustrates the evolution story that Alex Martin predicted his Nebraska farmer and agronomist audience would certainly experience.

Single Mare’s Tail plants can sexually reproduce to generate hundreds of seeds.  This creates populations that consist of thousands of individuals, and genetic variation tends to abound in most populations.  Much of this genetic variation has an impact on plant traits that is difficult to differentiate from variation caused by normal variability in the growing conditions the weeds encounter within a crop field.

As soybean farmers began to use Roundup in the population to control Mare’s Tail in their fields, a vast majority of plants suffered a rapid demise from the application.  However, some plants in the population escaped a lethal dosage of the herbicide from the less than perfect nature of spray applications.  Some plants survived because they were in fence rows, ditches, or neighboring fields. Still other plants germinated after the glyphosate application, then grew fast enough to catch up to the crop and complete their life cycle. These escapees, combined with the natural seed dispersal mechanism of Mare’s Tail, meant that even the most skilled farmers had a return of Mare’s Tail in their fields the next year. Nothing worked better to eliminate competition between the Mare’s Tail weed and the soybean crop than glyphosate so the farmer continued its use.

Some farmers began to notice the first evidence that Mare’s Tail weed populations possessed genetic variation that had an impact on glyphosate selection.  Small patches of the weed would consist of individuals with a variable response to their recent exposure to a Roundup treatment.  Most were dead or soon to be dead. But sometimes, a neighbor of these glyphosate casualties would have clearly stayed alive in spite of herbicide damage.  If they could grow back from axillary buds, these survivors could flower, produce seeds and pass their gene versions for survival on to some of the seeds.  The next year, a patch may have several survivors, a few years later, entire patches survived.  The population was changing.

Farmers who grew Roundup Ready crops every year, and applied glyphosate to control weeds would be applying more selection pressure on the Mare’s Tail population than farmers who rotated corn and soybean and reserved glyphosate for their soybean production year.  The higher the selection pressure, the more rapidly the Mare’s tail population would be expected to evolve.