Travis Legleiter, Weed Science Professional Assistant, Botany & Plant Pathology, Purdue University
A key indicator that a producer may be dealing with a herbicide resistant weed species is a segregating weed population. Although it may sound like a complicated indicator to observe it is actually quite easy. Segregating weed populations refers to the selection resistant plants from sensitive plants in a field with the repeated applications of the same site of action herbicides. It’s as simple as looking for dead plants and perfectly healthy plants side by side in the field following a herbicide application.
Herbicide resistance does not occur instantly across an entire field, but rather an initial resistance event occurs in a small number of plants. When this initial herbicide resistance occurs in a few plants, the population then becomes a segregating population with resistant individuals and susceptible individuals. Then repeated applications of single site of action herbicides will kill the sensitive plants and allow the resistant plants to survive and reproduce to build the resistant population until the entire population or field is resistant. The application of single sites of action is often referred to as selection pressure, as in selection of resistant individuals from the susceptible plants.
When a weed population is segregating it should be easy to see the segregation following a single site of action herbicide. When a producer observes a perfectly healthy weed next to several dead plants this is an indicator of a segregating population. When observing these dead and live plants you are seeing the resistant plants surviving to reproduce and build the population, while the susceptible plants are not allowed to reproduce.
Observing a segregating population should be a positive event, as this is early on in the resistance development in a population and there are only a few individuals that need to be controlled rather than a whole field. When a producer sees a segregating population they should change their herbicide program or use cultural practices to eliminate the resistant individuals that currently exist. The continued use of the same herbicide will only make matters worse as you are simply continuing to select out the resistant individuals in the segregating population.