It’s no secret that growers deal with a wide variety of agricultural problems every single day. From fickle weather conditions to unwanted pests, the issues that growers are up against make up a long list. Near the top of that list is herbicide chemical resistance. 

What is Herbicide Chemical Resistance?

Herbicide resistance is the acquired ability of an individual plant to survive a chemical application that would kill a normal population of the same species. 

Herbicide chemical resistance does not happen because of poor performance of a herbicide. Resistant weeds can often survive the application of herbicide at rates that are much greater than the recommended rate.

What Causes Crop Chemical Resistance?

Crop chemical resistance can happen for a variety of reasons, but at the core it is simply that we’ve used the same chemistry on the same field for an extended period of time. Resistance is an evolutionary process.

“What happens with herbicide resistance is that we apply our herbicides and kill the weeds that don’t have a mutation within the plant that allows it to survive. So the resistance in the plant happens ‘naturally’ so to speak.”

– Dr. Breanne Tidemann, a research scientist at Agriculture and Agri-Food Canada
How herbicide resistance develops

It’s like any evolutionary process — resistant plants that were not controlled by herbicide applications will survive, and over some period of time that plant evolution results in a plant population that is resistant to that chemistry. It’s not necessarily bad agricultural or farming practices, it’s just the reality we live in; biology adapts to survive.

Not every herbicide application will find a resistant individual. Resistance isn’t necessarily all that frequent and there are a number of factors that play into how common it is and how complex it can be. Essentially we’re dealing with random mutations in plant populations, selection pressure from the herbicide, and a numbers game in terms of how many plants are being treated to find that one that has the resistant characteristic.

The Effects of Crop Chemical Resistance

Herbicide resistance results in weeds that aren’t controlled. This means they continue to compete with the crop for nutrients, light, water, etc., and take away from the yield of the crop. “We can also see impacts in quality of harvested crop if the weed seeds are the same size as the crop seed that is harvested – for example cleavers species have seeds that are close in size to canola, which makes them hard to get out of the crop sample, which can result in the harvested sample being downgraded and losing value,” says Tidemann. “Weeds that aren’t controlled can also serve as a vector for insects or diseases and help maintain their populations. Herbicides are one of the most effective and efficient ways of managing the weeds and each case of herbicide resistance decreases a grower’s options in terms of strategies for managing that weed.”

There are similarities between herbicide resistance and something like antibiotic resistance. Whether you are dealing with weeds or bacteria, resistant populations decrease options available for control. The worst case scenario is that you’ll end up with a field with limited cropping options, because effective herbicide control options are limited due to resistance. If the crop being grown is tolerant to a specific group of chemistry and the weed population is resistant to the same chemistry, you have a problem!

However, any excess weed pressure you can’t control chokes off your ability to get good yields on your fields. Therefore, even with relatively low populations of resistant weeds, further pressure on the system by using the same chemistries on the same populations will promote reproduction of the resistant biotype. That resistant population will increase and spread, resulting in all of the difficulties that come with managing weedy fields.

Overall, limited options in terms of what crops you can grow and an economic loss due to increased weed pressures are the biggest concerns from a resistance perspective.