Dry conditions spike herbicide carryover risks

It’s not just herbicides with a persistent nature that can cause injury in the following years if conditions are dry enough. In very dry growing seasons, herbicide actives don’t break down in the soil and will remain there until a rainfall when these still-potent molecules are released back in to the root zone.

As Jason Voogt, of Field 2 Field Agronomy based at Carman, Man., explains in the interview below, certain herbicide products are more prone to carryover and certain crops are more susceptible to injury, but all farmers should be taking a field-specific look to gauge the risk to this year’s crop.

The problem stems from 2017 rainfall — or lack thereof in parts of the prairies. Depending on the herbicide, you made need more than 140 mm from application date to the end of August (winter snow cover does not count!) to fully deactivate the herbicide.

Herbicide carryover risk map for Saskatchewan (source: Saskatchewan’s Ministry of Agriculture; click here to view original)

Voogt says that farmers should check back on their herbicide records and also check rainfall amounts field-by-field. In Manitoba, some fields may have received enough to lower carryover concerns, but others, even just a few miles away, may not have. Those in Saskatchewan’s south and south west areas most certainly did not.

As a rule of thumb, soybeans, dry beans, sunflowers, peas and other pulses are at the highest risk to carryover issues, but corn is not immune. Those who applied Treflan ahead of dry beans in 2017 and were planning to go into corn may have an issue, Voogt says.

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What can concerned farmers do if they suspect they may have an issue with herbicide carryover? Voogt says that there is still time to do a soil test, thought it is slightly more difficult with frozen soil.

“Make sure you sample from a representative area and from more sensitive areas — lighter, coarser soils and low soil organic matter areas will be at the greatest risk,” he says. His company works with AgVise for sampling, which has a lab in South Dakota that tests for herbicide carryover. The test takes two to three weeks and cost around $200 to $300.

Time is running short, Voogt warns, as farmers who suspect or confirm an issue may need to scramble to find alternative seed supplies.

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