High herbicide carryover in dry areas

Areas receiving less than 125 millimetres of rain are at high risk of experiencing greater than expected herbicide carryover.

That risk factor jumps to serious in areas with less than 75 mm of rain.

The risk categories shown on a map based on data from June 16 to Sept. 14 collected by the Saskatchewan environment ministry and Environment Canada show that the southwestern region is most affected, says Saskatchewan weed specialist Clark Brenzil.

He said rainfall following the application of a residual herbicide is the most important factor in the breakdown of herbicides in the soil. Lower-than-normal rainfall can result in residual herbicides remaining in the soil at higher levels than expected. This increases the potential for herbicide injury to sensitive crops the following year or even two years later.

“Some herbicides are more likely to carry over. We have two pages covering that in our Guide to Crop Protection,” said Brenzil, adding that some products already have restrictions on them, based on normal rainfall.

Soil in the yellow zone on the map may be more sensitive to carryover of the active ingredients clopyralid (Lontrel, etc.) and pyrasulfotole (Infinity, etc).

“The normal rainfall areas are the green areas on the map,” he said. “The light green areas are somewhat unknown because of data smoothing. We have some areas in west-central and down toward the No. 1 Highway where we don’t have enough volunteer crop reporters. They’ve always been a major data source compiling these maps.

“It’s always been a struggle to recruit and retain crop reporters. Folks get busy with their own farming operations and that’s the first thing they drop. And it is voluntary. But it gives a producer the chance to have an accurate record of precipitation on their own farm. I think in the future we may see automated weather stations.”

Producers in high risk areas are basically out of luck when it comes to curing herbicide carryover. Brenzil said all tillage will do is dilute the layer of residue and further dry the soil. It may move the product deeper in the soil profile so the roots can better access it, resulting in an even bigger problem.

He said producers hold the cards they’ve been dealt and now they have to live with it. Farmers already applied the product. Soil temperatures typically do not warm enough in the spring to encourage a buildup of microbes to break down the carryover herbicides.

“A heavy snowfall this winter, or heavy rains early in the spring, will not solve the problem. You’d need an awful lot of rain to wash the residual down through the profile. And you’d need beach sand for soil down to 12-feet deep,” Brenzil said.

“Some herbicides strongly bond to the soil. Some have a weak bond. And we have herbicides that become more strongly bound the longer they’re in the soil. Very dry conditions can drive residual herbicides to bind more strongly to the soil than their mobility rating would suggest.”

Brenzil said farmers who want to put a more precise number on how bad there residual herbicide problem really is can gather soil samples and send them to a lab such as A&L in London, Ont. They will do a bioassay assessment to determine the risk of the specific crop that the farmer wants to plant in that specific field.

The best areas to pull samples would be hilltops because they’re drier and have different pH and lower organic matter levels than other soils on the farm. As a result, they present the greatest risk of crop failure.

Brenzil said the processes that farmers want to see happening in their soil are either microbial breakdown or chemical hydrolysis, in which a molecule of water is added to a substance causing both substance and water molecule to split into two parts.

“In the Group 2 family, higher soil pH generally causes sulfonylurea products to hang around longer. It’s just the opposite for the imidazolinones, where acid soils have more carryover. All the details are in the Guide to Crop Protection. You can find a list of residual herbicides. Make sure to read the label for specific re-cropping information.”

Producers in the serious risk, very high risk and high risk areas who applied a residual herbicide in previous years requiring restrictive cropping practices in the current year should contact the manufacturer of the residual herbicide for rotational crops it supports.

Producers in the moderate risk areas who also have low organic matter or soil pH less than 6.5 or greater than 7.5 should also contact the manufacturer of the residual herbicide used previously.

Producers in the normal risk areas should follow label directions to determine what crops they may plant following the application of a residual herbicide. However, they may still have localized areas that are susceptible to extended carryover due to low in season rainfall.

Brenzil said it’s important to have rainfall records for each field to determine localized risk and correctly follow the label.

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