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: 1) persistence of the herbicide, 2) herbicide rate, 3) soil characteristics, 4) amount of rainfall during the season following the application, 5) length of interval between herbicide application and planting the rotational crop, 6) sensitivity of the crop to the herbicide, and 7) 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 occasional carryover include atrazine, chlorimuron, clopyralid, 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 (Figure 1). 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.
When diagnosing early-season problems in crop fields, a basic understanding of herbicide mode of action is essential to determine whether herbicides are contributing to the problem. Following are a few photos illustrating symptoms associated with herbicide injury.
HG 2: imazethapyr, cloransulam, chlorimuron. ALS inhibitors are systemic and affect new growth by inhibiting synthesis of amino acids. The distinct symptom of Group 2 herbicides is bottle brush roots on corn, but stunting and chlorosis are also associated with carryover. Symptoms can be subtle and difficult to identify.
HG 4: clopyralid. Growth regulators are systemic and mimic the activity of auxin, a plant hormone. Typical symptoms are epinasty and distorted leaf venation. Soil residues of clopyralid typically do not cause the uniform distortion of leaves associated with drift of 2,4-D or dicamba.
HG 14: fomesafen. Fomesafen is the one PPO inhibitor that occasionally injures corn following use in soybean. While there is little translocation with postemergence applications, residues in the soil move via the xylem to photosynthetically active leaves. Symptoms are very distinctive, veinal chlorosis and necrosis. The mid-vein may break midway in the leaf. Most users have learned to switch to other products after mid-June to reduce risks of fomesafen carryover, reducing the likelihood of corn injury.
HG 27: isoxaflutole, mesotrione. HPPD inhibitors can cause chlorosis and bleaching of foliage via their activity on pigment synthesis. Herbicides move via phoem, resulting in symptoms appearing on new growth.