Herbicide carryover called major risk for pulses this year

There’s a high risk of herbicide carryover across southern and central Saskatchewan, and some areas have an extreme risk.

Many crops are vulnerable to herbicide carryover, but pulse crops are particularly sensitive to groups 2 and 4 and potentially group 27 chemistries, which can cause crop damage for up to two years after application in dry conditions, said Clark Brenzil, Saskatchewan provincial weed control specialist.

“Generally, it’s only the year following, but if it’s very, very dry, you may have a situation where there’s next to no breakdown. So on our map on our website, if you’re looking in that severe to extreme category where you’ve got less than three inches of rainfall in the growing season. You may find that there’s very little breakdown of that herbicide,” he said.

To show regional herbicide carryover risk, Brenzil built a map that is available at saskatchewan.ca, which displays rainfall amounts in areas of the province from June 16 to Sept. 14, 2020.

“Last year there was an early rain in, I think it was early June, and then after that the tap turned off and there was really no significant rain. A lot of those areas are showing deficiencies for most of the rest of the season,” Brenzil said.

He said the small showers that did fall do not significantly help reduce the amount of carryover because the top layer of the soil, where the herbicides reside, will quickly dry up once the sun comes up.

Some herbicides are broken down by microbial digestion, and this requires moisture.

Microbes largely target the molecule’s carbon, but their nitrogen, phosphorus fluoride and chloride content is also consumed.

“When it’s dry you don’t really build a microbial population that will break down that herbicide residue, and so it just stays put and doesn’t go anywhere,” Brenzil said.

The microbes that break down herbicide are most commonly bacteria, which need warm temperatures to work.

This is why Brenzil’s map starts in June and ends in the middle of September and why the amount of snowfall an area gets has a marginal effect on the microbial degradation of herbicides.

“At snow melt you’ll get some moisture recharge back into the soil, but it’s still cold. So you won’t grow any microbes at that point. You may get some breakdown of the herbicides that utilize the hydrolysis process, but it’s not until those soils have been warmed up for about a month that you end up with a decent enough microbial population in order to have an efficient breakdown of the herbicide,” Brenzil said.

Brenzil said producers should use their own rainfall records to estimate their specific risk of carryover injury because there can be large distances between rainfall reporting sites he used to create the map.

There are indications that soil type can have an effect on herbicide carryover, but there is limited definitive research on this in Saskatchewan.

Brenzil said the difference soil types have in terms of herbicide carryover injury has more to do with how the molecules cling to the soil than it does with breaking the herbicides down.

“What we find is that herbicide sticks more to things like clay and organic matter. So if you have a heavier soil with higher organic matter, you may see less in the way of a problem with herbicide carryover even under dry conditions than someone that has a sandy soil with low organic matter might see,” he said.

Soils with Ph levels outside the neutral soil pH range of 6.5 to 7.5 may increase the risk of carryover of some herbicides.

Brenzil said pulse growers should also be aware of how the chemistries they use in their pulse crops can affect other crops in their rotation.

For a list of residual herbicides that can carry over, refer to the chart in the front of the Weed Control chapter of the Guide to Crop Protection at bit.ly/3qXv44m.

For more information on the risk of herbicide carryover for the 2021 growing season in Saskatchewan, go to bit.ly/36lp0L9.

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