Herbicide Carryover Across the Prairies

Thursday, August 18, 2022

What to expect and what you can do with the added challenges of herbicide residue during 2022's seeding season.

By Kristen Lutz

With the drought, extreme heat and residual impacts of the pandemic, what else will producers encounter? Unfortunately, the challenges for growers may not be over just yet. The adverse weather conditions a large region of Western Canada experienced are a recipe for herbicide carryover, adding another challenge to this year's growing season.

Adverse weather effects

"Most of the herbicides we use break down within weeks after application, but there are some that persist in the soil for months or even years," says Colleen Redlick, technical marketing specialist for herbicides/fungicides at BASF. "When we have environmental conditions that lead to greater than expected persistence of these herbicides, and then you plant a sensitive species on that field, we can see crops become injured and yield reduced. And this is what really defines herbicide carryover or herbicide persistence," she explains.

The environmental conditions from the 2021 growing season will be a big factor for herbicide carryover. All experts agree that moisture in the soil is the driving force behind herbicide breakdown. "Herbicide carryover is connected to herbicide degradation and herbicides are expected to degrade at a certain rate based on environmental conditions," says Jeremy Boychyn, agronomy research extension specialist with Alberta Wheat & Barley Commission.

"The most common form of herbicide degradation is microbial degradation and microbial degradation depends on a number of environmental factors. This year we saw a significant decrease in rainfall and when this happens, we are going to have reduced activity. The microbes are not degrading herbicides, causing them to remain in the soil and cause issues in the following year," he continues.

"Microbial activity is at its highest when the soil is warm and moist and herbicide breakdown happens relatively quickly. There is less potential for herbicide carryover and for the herbicide residue to affect the crops in the following year," says Sarah Anderson, agronomy manager at Saskatchewan Pulse Growers.

"Some herbicide degradation is done through chemical hydrolysis. Chemical hydrolysis is also largely regulated by soil moisture and some water molecules will break the herbicide into smaller and less active pieces," she continues.

Experts agree that the widespread drought across Western Canada will be a big factor in the increased presence of herbicide carryover. "The breakdown of herbicides is greatly affected by moisture and the moisture-holding capabilities of the soil. Ultimately, if you don't have moisture, whether this is fog, snow, or rain, you won't see very much herbicide breakdown," says Ted Moir, western territory manager at Prograin.

"A lot of precipitation during the growing season, between June 1 and Aug. 31, was less than 125 mm of accumulated rainfall. This level is typically used as a guideline for herbicide carryover," says Anderson.

Boychyn continues to explain, "rainfall below 150 mm can put you at high risk for herbicide carryover in the following year. Anything around 160 mm is definitely a grey area; it's risky. And above that is typically not as much of a risk."

Some areas experienced substantial rainfall after Sept. 1, during harvest season. Since precipitation is directly correlated with herbicide degradation, there is the possibility that this late- season rain could reduce the effects of herbicide breakdown in future years. However, when rain occurs outside this season, there's no evidence to prove its effects, based on a variety of factors.

"Absolutely (the rains) will have some impact and we know those soils were still quite warm.

"Could they mitigate some of the impacts? For sure!

"But do we feel comfortable enough to say that if you got X amount of rain after Sept. 1, your risk goes down? No, we don't.

"Most of the third parties and manufacturers want to stick to the Sept. 1 rule because that's when we know the microbial degradation will be taking place," explains Redlick.

"What is also important to remember, is that those soils which saw that late rain last fall were quite dry at the time. The rain that did occur was quickly moving through the surface and down to deeper levels. So, the surface layer, where herbicide degradation should really be occurring, dried out relatively quickly. The microbial activity may not have been enough to sustain significant herbicide breakdown," Anderson remarks.

Heavy snowfall over these winter months may also be an option to get additional moisture in the soil. However, "temperature is also very important," says Moir. "If we have cool soil temperature, we will not have the microbial activity which is needed for breakdown. It starts becoming the chicken and the egg problem between moisture and temperature."

All experts agree. Snow may result in well-wetted soils come spring, but this, unfortunately, is not the answer to resolve herbicide carryover.

Potential risks

Crop injury is evident in this year's growing season with the dry conditions from 2021. There is a variety of characteristics that come with injury caused by herbicide carryover, and each crop will be affected differently.

"Different crops react differently with different herbicides," says Redlick.

"It will look different depending on the active ingredient," says Anderson. They all have different mechanisms that work on the plant. "Generally speaking, it adds stress to the plant, and so plants affected by herbicide carryover are at a disadvantage to plants that are able to tolerate herbicides residues."

"It could come in the form of stunted growth or development, root pruning could occur, which depletes the ability of the plant to absorb nutrients from the soil. There is also necrosis that could happen to the leaf tissue and decreases photosynthetic area on the whole plant. These combined effects could lead to a reduction in yield," she continues.

Crop damage may vary between crop and herbicide chemistry, but they all seem to result in reduced yield.

Different herbicides can impact the plant at different stages of growth, and although they may emerge as normal, growers will likely see herbicide residue effects during their growth cycle.

Most commonly experts focused on imidazoline (IMI) based products when speaking of herbicide residue for this upcoming spring. "For all of Western Canada, when discussing a certain chemistry having more impact, those are going to be the IMI chemistries," says Boychyn. "The reason why it is the most talked about is because of its risk patterns and the chemistries involved. It has a few different active herbicides, it's used in a wide range of areas and can impact a wide range of crops. So, in terms of the extent of risk, this herbicide group typically poses the biggest risk. With IMIs, we see a risk when we reach low precipitation much like what we saw this past summer," he explains.

"IMI based products are not germinations inhibitors, so you won't see anything really early and the crop will emerge as normal. Then, when we get some rainfall, those crops start growing and do not rely on seed reserves anymore and they take up those herbicides that are in the soil. That's when we first see the symptomology," says Redlick.

Assess & make educated decisions

Unfortunately, producers have to deal with the hand they are dealt, and this growing season may not go exactly as planned. "It would be great if we could pull something off the shelves and use it to soak up the herbicide residue, but those tools are not available to us. What we need is time," says Anderson.

"Do an honest assessment based on products applied in 2021 and even 2020 growing season. Talk to experts, look at online resources and look at product labels and plan the next steps based on what you know about the product. Make educated decisions and don't make best guesses," advises Moir.

"Western Canada has been good about thinking about herbicide rotation and crop rotations that work better with herbicide residues. The carryover issues that have been expressed on us in the last couple of years have made us think a little more about our applications, and it really is the right product, right place and right time," he continues.

There are several resources growers can use to adjust their rotations accordingly. Figure 1 (provided from BASF) outlines the sensitivity level of various crops and which are more likely to be affected by herbicide residue. Experts recommend using resources such as this coupled with information on geographical precipitation to make decisions moving forward.

"Saskatchewan Ministry of Agriculture produces an herbicide risk map and a lot of the province is considered high risk for herbicide carryover. If you look at these maps or consult weather station data a lot of the precipitation was below 125 mm which is our guideline for herbicide carryover," Anderson explains.

"If possible, get a field-by-field report of how much rain fell during the growing period," says Boychyn. We were very dry, but some areas did see pockets of rainfall which could resolve some issues. Make sure you reach out to your chemical support and extension groups and talk about risk and your options."

One positive that will come out of this year's higher herbicide residue is increased awareness of herbicide stewardship. Redlick spoke specifically about Group 2 herbicides, which include IMI based products. "Limiting Group 2 application and putting it just where the product is needed to have a successful crop, particularly when dealing with more persistent active ingredients like IMI herbicides. We need to have these conversations as well as bringing it into focus that herbicide persistence is a reality under drought conditions and management is important."

"It's good to have these conversations from both agronomic and economic standpoints. Some growers have current practices that could be improved and we hope that growers have conversations to bring in more variety into their crop rotations. We may see more drought and we need to start looking at rotations that both manage herbicide persistence and maintain profitability," she says.

Final thoughts

Herbicide residue will be an added challenge for producers during this seeding season. Conversations around rotations and herbicide application between producers and chemical companies alike are the first steps into finding solutions for another successful year.

"Ultimately, there is nothing that could prevent this. Product stewardship and knowing the potential effects is the best way to manage this challenge.

"This is a difficult balancing act, especially in products that require microbial breakdown.

"If we could guess the weather, we wouldn't be in this situation," Moir concludes. BF