Managing Chemical Resistance with Crop Rotation

Historically, as an industry, growers have tried to use crop rotation, chemical rotation, and combine multiple modes of action and different groups of chemistry together to manage resistance, and extend the useful life of the chemicals to make them more effective for a longer period of time. A lot of that has been extremely successful, and many growers are able to avoid it. The biggest problem you can run into is when you have narrow crop rotation and very narrow chemical use — that’s when you’ll run into the most accelerated problems with resistance.

Unfortunately there are some weeds out there that have no chemical controls. That’s a different scenario altogether. You can include mechanical tools, but In this extreme scenario, your options become very limited.

But in the less extreme cases —  (i.e. if the resistance is just starting to show up, or it’s not the entire field at risk) — then you have more tools at your advantage to try to keep things under control.

Crop rotation is the practice of planting different crops, one after the other, on the same plot of land in order to improve soil health, optimize nutrients in the soil, and combat pest and weed pressure. A simple crop rotation might involve two or three different crops, whereas a more complex rotation might involve more than a dozen different crops. 

Since different crops have different needs and are vulnerable to different pests, if a grower plants the exact same crop in the same place every year, they continually host the same sets of insects and disease.  Pests and other diseases know what they will be getting and they set up a permanent home there. Decreased diversity also selects for specific species of weeds.  Qualities of crop types, including competitiveness, planting and harvest timing, herbicide options, days to maturity, along with many other factors will naturally select for specific weed species. When this happens, increasing levels of chemical fertilizers and pesticides become necessary to keep yields high while keeping bugs and disease to a minimum.

Crop rotation works by increasing biodiversity, and interrupting the lifecycle of pests, weeds, and other diseases. 

Mechanical tools, like Redekop’s seed control unit, provide another way to fight off resistance. The seed control unit is a flexible and cost-effective solution that has kill rates of greater than 98 percent.

The unit works on any seed that can be physically harvested by killing the seed, and in turn reducing the weed bank and weed pressure in that field. The unit can also be easily turned on and off for growers who have low weed pressure or are dealing with a non-harvestable weed. Remember that the current crop becomes a weed next year! Some our the worst ‘weeds’ are glyph tolerant crops / rotations that are harder to control (eg. barley on wheat).

“We’re not going to cure a hundred percent of the resistant weed issues. But if we can do our job to reduce the weed pressure, we think that means the competitiveness of the crop is going to be higher, herbicides are exposed to a reduced weed population, resulting in an increased effectiveness of the chemistries that we’re using,” says Trevor Thiessen, president of Redekop Manufacturing. “It’s no different than someone trying to weed their lawn. The more weeds you have, the more challenging it is to get a good look at the lawn. So if you can pick out 80 percent of the weeds, the lawn can compete pretty well.”

The ultimate goal when it comes to managing crop chemical resistance is to combine and utilize multiple strategies. By combining crop rotation, multiple different chemistries, and mechanical tools like the seed control unit, you will have the best shot at reducing weed pressure and managing resistance.  

“There are resistant weeds popping up all over the place in western Canada and the US and Europe and Australia. The key is staying ahead of it by using all these tools in our toolbox and being creative enough to not get in a spot where a resistant weed has taken over so completely that the fields are now unmanageable,” says Thiessen. “Now with harvest control tools, we’ve added another piece of the puzzle to help the grower increase his options.”

Total prevention is difficult —  if you are using herbicides, you are applying a selection pressure for resistance. Using the strategies above for management can help delay the evolution of resistance, and if enough other strategies are used it could be a long-term delay.

That means using multiple strategies. ”The reality is that we’re always going to have weeds in our fields. They aren’t going to go away,” says Thiessen. “I think the things that we’re doing — crop rotation, using chemistries, and mixing it up and making sure we’re not cutting rates, and then the third piece is adding new tools like mechanical tools, like harvest control tools. Those are all things to help us with the prevention process as well.” 

However, the best way to prevent crop chemical resistance is simply to not ignore the problem. “If you have resistant weeds popping up, no matter how you want to deal with them, you have to deal with them,” says Thiessen. “In different parts of the world the resistant weeds are starting to grow. Very rapidly. The number of incidents that we saw five or 10 years ago is a fraction in some parts, where now 70 or 80 percent of the field will show some sign of it. So that’s lesson number one — don’t pretend like it’s not going to happen, because it’s probably going to happen. We just need to be aware of it and be proactive.”

Palmer amaranth has the ability to very quickly develop resistance to herbicides. According to Chemical & Engineering News, “Weed scientists and agriculture chemical company experts describe the many ways that biology and growth habits make this king of weeds uniquely difficult to kill or contain. It will likely require brand-new technology—technology that does not yet exist—to regain the upper hand.” 

When Palmer amaranth survives herbicide application, it becomes a major roadblock to growers’ yields. More than other weeds, it outcompetes crops for nutrients, water, and even sun. When uncontrolled, the weed can cause crop yields to drop 50 percent or more. This is likely why the Weed Science Society of America voted it the United State’s most troublesome weed in 2017. 

Annual ryegrass is a serious and costly weed, found commonly in places like southern Australia, as well as North America. Annual ryegrass is highly adaptable and competitive, and can compete with crops very early in the growing stages. It also acts as a host for certain types of bacteria. 

According to the Government of Western Austraila, “many populations of annual ryegrass have developed resistance to both selective and non­-selective herbicides. Repeated use of herbicides from the same mode-of-action group (particularly the high-risk Groups A and B) have lead to herbicide-resistant individuals.”

Kochia has been identified as one of the most dangerous weeds in the West. This drought-loving weed can be difficult to manage, mostly because of its ability to spread and quickly establish itself as a major weed. 

According to The Western Producer, “Kochia, like humans, is a serial out-crosser, meaning that each of a mature plant’s 25,000 seeds are individuals with unique characteristics, accommodating multiple mutations and future natural selection to a variety of perils.” 

Kochia’s ability to resist Group 2 herbicides means it has become hard to control in pulse crops. “Just 21 plants per sq. metre reduce wheat yields by one-third. The plant can be devastating in flax and pulse crops, choking out broadleaf crops for sun and moisture.”

One challenge that Dr. Breanne Tidemann, a research scientist at Agriculture and Agri-Food Canada, foresees is more resistance being selected. “As we continue to rely on herbicides for our weed management we will continue to see more cases of resistance evolve,” she says. “Introduction of new weeds with already established resistance – there are weeds in the US that, particularly with climate change, may make their way north, many of which have multiple herbicide resistance (resistant to multiple herbicide groups) and will be very difficult to manage. Evolution of more multiple resistance in already resistant weeds in Canada.” 

Do environmental changes have an impact? “To some degree, yes,” says Thiessen. “I think the environment and the climate from year to year will change and have some impact on how these crops react. But I don’t think it really changes the trajectory long term of the resistance issue. I think it just changes the fact that a resistant crop in any given year might be more or less dominant.”

A weed’s secret weapon is seed longevity. According to Successful Farming, most annual weedy grass seeds die after two-to-three years, but some broadleaf weed seeds can remain viable for decades.

Why the difference? Weed seed coats play an important role. “Weed seeds that have a hard seed coat are more likely to survive the trials and tribulations that occur when they fall into the soil,” according to Successful Farming. A number of other factors can determine how likely a weed seed is to survive. These include disease, environmental factors, insects, soil, and light. 

A bigger issue, according to Thiessen, are social and political impacts. “As a global trend, we’re not excited about chemical use. So we could see growers in different parts of the world losing options to manage weed resistance because of political or social pressure. By reducing and eliminating chemical use, we could find other problems in our cropping, which will ultimately affect yield and affect the growers ability to grow crops and produce food.”

The rate of innovation in the chemistry market has slowed. So as we look to the future, we can’t rely on some miracle chemical coming onto the market to take care of this issue. It’s not going to get better on its own. Growers need to actively manage it by being proactive and using all the tools at their disposal.