Iowa: Herbicide Carryover Damage – How Come?

Herbicide injury on young soybeans. Photo: Purdue University

Herbicide injury on young soybeans. Photo: Purdue University

Although there is no evidence of widescale problems with carryover injury to corn or soybean, ISUEO field agronomists have received a few reports.

The potential for carryover injury is determined by several factors, including:

  • Persistence of the herbicide.
  • Herbicide rate.
  • Soil characteristics.
  • Amount of rainfall during the season following the application.
  • Length of interval between herbicide application and planting the rotational crop.
  • Sensitivity of the crop to the herbicide.
  • Early-season crop vigor.

The majority of active ingredients used in Iowa are not persistent enough to pose a carryover risk. Products that have half-lives sufficient to result in occassional carryover include atrazine, chlorimuron, clopryalid, cloransulam, fomesafen, imazethapyr, isoxaflutole, and mesotrione.

Atrazine and chlorimuron probably pose the greatest risk, particularly on soils with high pH, but most farmers have learned where and at what rates these products can be used safely.

Due to delayed planting, a high percentage of herbicide applications were made later than normal in 2019. The combination of delayed applications in 2019 and 2020’s record planting pace creates a reduced time frame for herbicides to degrade in the soil.

In addition, certain areas of the state experienced dry weather last summer. These factors could result in crop growth being suppressed by sublethal herbicide residues in some fields. With the products used today stand loss is rare, and crops usually grow through the damage relatively quickly.

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