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Heavy Rain May Trigger Injury from Herbicide Carryover

By Clark Brenzil, PAg, Provincial Specialist, Weed Control, Regina

Many areas have received significant rain over the past couple of weeks. While this is good for crop growth, heavy rains can also trigger the symptoms of another concern lurking in some soils to appear.

Because many areas of Saskatchewan received minimal in-season rainfall; the soil residues of certain herbicides may still be lurking in some fields at higher levels than would be expected under more normal rainfall conditions. The cold conditions of spring melt do not allow these residues to degrade. Saturated soils will also reduce the breakdown of some herbicides.

Barley seedlings impacted by Group 2 herbicide carryover
Barley seedlings impacted by Group 2 herbicide carryover (L) compared to unaffected (R)

The recent rains are the most substantial rains that many areas along the western half of the province have received this growing season. The western region is also the area that was most at risk for extremely elevated herbicide carryover risks.

Significant amounts of water entering the previously dry system will liberate the herbicide residues that may still be bound to the solid soil elements in some fields. This allows sensitive plants to take up those residues and begin to exhibit symptoms of injury from that herbicide several days to two weeks after the rainfall event.

Scouting fields over the next couple of weeks will likely reveal whether herbicides carrying over from last year have had an impact or not.

Symptoms vary depending on the herbicide Group or mode of action of the herbicides used in previous years. Product pages in the herbicide chapter of the 2022 Guide to Crop Protection (GCP) will include group numbers on the right side of the page title. The “Recropping Restrictions” chart on page 89 of the GCP will give an indication of potential candidates to persist in soils.

Symptoms of herbicide carryover may include:

Stunting of new growth – Groups 2, 3, 4, 15, 26

  • Epinasty (cupping and twisting of stems) – Groups 2 (minor), 4, 15, 26
  • Miniaturization of new growth – Group 2
  • Pruning or swelling of roots – Groups 3, 4, 15
  • White tissues – Groups 13, 14, 27
  • Necrosis – Groups 5, 14

Once these symptoms are found in a field, no management actions can be taken to reduce the impacts on the crop. There may be claims by some fertilizer providers that the application of micronutrients will lessen the negative impacts. While the symptoms in crops do resemble deficiencies, it is not nutrients the crops are deficient in, but they are deficient in the amino acids that the herbicides are preventing the plants from producing. Only time and the gradual breakdown of the herbicide in the soil will reduce the impact on the crop. In some case, the crop progress may just be delayed by three to four weeks, but in other cases the damage is so profound that the crop will not recover.

The Ministry of Agriculture has information on where high-risk areas are in the province.

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