Even slightly residual herbicides can hang around longer than expected given the right circumstances. Herbicides require water, microbes, time and warm soil in order to break down in the soil.
On the surface, weighing the risk to this year’s crop of herbicide injury seems simple. If you follow labels and keep field records you should be fine, right? Not necessarily, says Clark Brenzil, Saskatchewan’s provincial weed control specialist. The challenge is that re-cropping recommendations often assumes that herbicides used in previous years have had “normal” environmental conditions to degrade through natural processes, such as hydrolysis (breakdown through water) and microbial breakdown. Growers may also have a narrow spectrum of eligible follow crops to select from, and labels don’t always carry re-cropping information on minor crops.
The vast majority of herbicides are degraded by being consumed by microbes as food. “Only a few herbicides, such as group 5 (Sencor and atrazine) or Group 2 (Ally and Express Pro), rely on a chemical-only process called hydrolysis to perform a substantial part of the degradation process,” Brenzil says. Depending on the herbicide, temperature and soil pH, microbial and chemical processes can occur relatively quickly or slowly. Slow breakdown equals a residual herbicide. Residual herbicides can come from almost any herbicide group.
As living organisms microbes that degrade herbicides require water — not too much and not too little — to break down herbicides, Brenzil says. But it’s not that simple. “Soil pH, clay content, crop sensitivity and growing conditions in the spring all interact to determine the effect of herbicide residue on a spring crop,” he says. Several of these factors also affect the rate at which microbes consume the herbicide, and pH has a significant influence on the speed of chemical hydrolysis and may also increase herbicide binding to soil organic matter and clay.
Consider it this way — microbes are like Pacman, eating up all the little dots of herbicide molecules. If there isn’t enough water or the pH is such that the molecules are held tightly to soil particles Pacman can’t get eat them. If there is too much water, Pacman runs out of oxygen and keels over before being able to eat up the herbicide. There are, however, some anaerobic microbes (those that thrive in low-oxygen environments) that break down Group 3 herbicides, such as Edge, very quickly. “It’s why wild oats can get away on you in low spots,” he says.
To put all this in perspective, the majority of herbicide breakdown happens in the top three inches of soil. That’s a lot of biological processes happening in a very small amount of soil, and seeing as it’s a water-dependent process, being near the soil surface doesn’t really help matters.
How do you know if you’re at increased risk to carryover? Brenzil says a rule of thumb is that more than six inches of rainfall in June, July and August is necessary for herbicides to break down normally, although that’s no guarantee. Microbes need a minimum soil temp of 4 C to 5 C to work, meaning fall or early spring moisture won’t help breakdown what’s there until the seed is going in the ground. If the growing season was dry, but there’s abundant winter snow you should still be cautious. “Snow won’t help with herbicide carryover this year because nothing happens (to the herbicide) in the cold of winter. Herbicide breakdown depends strictly on in-season rains,” Brenzil says.
Even in areas with good soil moisture this year there may be some question marks. “With the cold weather prevailing through the three main breakdown months of June, July and August,
Snow won’t help with herbicide carryover this year because nothing happens (to the herbicide) in the cold of winter. Herbicide breakdown depends strictly on in-season rains
soil temperatures may not have been sufficient to allow normal breakdown to take place,” Brenzil says. “The rate of breakdown at 15 C is about half of what it is at 25 C.”
HOW TO TEST FOR CARRYOVER
The tricky part in dealing with herbicide carryover is determining if you have it and to what degree. Even minute amounts of active herbicide in the soil can injure susceptible seedlings, but how do you know what’s in there? “Your first step is reading labels carefully,” Brenzil says. Crop-protection companies are quick to update product labels if there are concerns, and new do with the results. “A chemical test gives you a reading for the lowest detectible level, down to less than one part per billion. It doesn’t tell you if that level is going to cause injury or not because it doesn’t consider binding and availability of the residue to plant roots,” he says. The test can also be costly, generally running over $200 per test.
The Alberta Research Council (ARC) offers a lab pot bioassay test that takes two to three weeks on pulse crops and four to six weeks for other crops and is a more affordable $150 for as many crops as you want to test. The downside? You need to send 10 pounds of sprayed soil and 10 pounds of unsprayed soil (from the same or nearby field) for comparison. “The soil sample has to come from the top three inches,” Brenzil says. Too deep and you’ll get a false negative result, telling you it’s safe to seed when it isn’t, too shallow
— Clark Brenzil, provincial weed control specialist, Saskatchewan Ministry of Agriculture
information could have been added to products you use regularly. Beyond following the re-cropping restrictions, the only sure way to know what’s out there is through a bioassay.
The first type of test is a field bioassay but for this to be useful you will have needed to leave an untreated check strip. Because the bioassay strip needs to go to yield, the results really only indicate what you can seed next year, rendering the test impractical for most.
“A chemical analysis of the soil is possible,” Brenzil says. There are several reputable labs that offer a chemical analysis of herbicide residues in the soil down to minute quantities, which is good information, however the test offers no advice on what to and you’ll get a false positive. Proper sampling is critical. “The big benefit is that the test will tell you with high certainty what you can and cannot seed in that field,” he says.
Another option that may be available in the near future aims to be faster and requires far less soil. Brenzil says that Dr. Jeff Schoenau at the University of Saskatchewan has a rapid bioassay test in the works that is showing promise for detecting Everest residues. The test takes one week and is done in the lab by growing seedlings in the soil in question. No labs have picked up this test to date. “Right now, the test has a slightly higher high false negative rate (showing no injury when there actually would be) than some are comfortable with. Additional herbicides are being worked on as well to expand the test’s utility,” he says. This test will suffer from the same sampling challenges as the lab pot bioassay test.
Without testing, you’ll have to depend on your own judgement for 2010. If crops suffered from lack of moisture in 2009, it’s more likely you’ll have more herbicide carryover than expected, Brenzil says. High soil organic matter content and higher clay levels usually offer some buffering capacity and could reduce the risk of carryover, but he says that if you’re worried, it’s best to talk with the manufacturer of the herbicide you’re concerned about, check with the ARC lab on timelines for testing or simply adjust your crop rotation to something more tolerant to the herbicide rather than take the risk on residues you didn’t expect.
Lyndsey Smith is editor of Grainews. Contact
her at [email protected]