Recent weather extremes have created favorable conditions for soil crusting and herbicide carryover, two separate phenomena that can cause poor early stand development in agronomic crops. The agronomic crops team has received several questions about how soil crusting and herbicide carryover might be impacting early crop performance, so this article is intended to review these issues in the context of recent conditions in Ohio.
Soil Crusting
Soil crusting is a hard, compacted layer of surface soil that forms under wet, warm conditions like what we’ve seen in many parts of Ohio these last few weeks. Soil crusts form when heavy rainfall detaches soil particles that are cemented together when the soil dries. Soil crusting inhibits seedling emergence, especially for broadleaf crops like soybean, reduces water infiltration, and increases runoff and erosion. Fine-textured soils (high silt and clay content) with low organic matter and frequent tillage have the highest risk of crusting.
Dr. Laura Lindsey and Dr. Osler Ortez address the agronomic implications of soil crusting and replanting decisions in this CORN Newsletter article from 2022.
Herbicide Carryover
Herbicide carryover refers to crop injury that occurs when an herbicide applied to a previous crop remains in the soil at a high enough concentration to damage the current crop. Each herbicide active ingredient has a different period that it will remain active in the soil, ranging from not at all to several months or years under the right conditions. The duration of carryover is determined by a combination of the active ingredient’s chemical structure and environmental conditions that affect how quickly the herbicide is degraded by chemical or microbial processes. Each herbicide label will indicate plant-back intervals for sensitive crops that represent how long it typically takes for various application rates of that herbicide to break down to safe concentrations under “normal” environmental conditions. Note that there are several factors that can extend these intervals, including the rate applied, soil characteristics (high pH, finer texture, low organic matter), and weather. Generally, degradation is sped up under warm, wet conditions and slows when it is either cold or dry. Last summer’s drought during the warmest months of the year and cool temperatures this spring likely slowed microbial degradation of herbicides, so the risk of herbicide carryover is probably higher now than in recent years. Herbicides with longer carryover periods to be aware of include atrazine (Aatrex), metribuzin (Sencor), clopyralid (Stinger), chlorimuron (Classic), fomesafen (Reflex), mesotrione (Callisto), imazethapyr (Pursuit), clomazone (Command), and pre-mixes containing these active ingredients. Consult each herbicide label to determine plant-back intervals for these products, including any additional precautions based on soil and weather conditions, and refer to last year’s spray logs to determine whether enough time has passed to safely plant a sensitive crop. If you are concerned about carryover in a particular field, you can conduct a bioassay with the soil and crop you are concerned about using the methods outlined in this article by Dr. Eric Jones (South Dakota State University Extension).
How Soil Crusting and Herbicide Carryover Might Interact
There are a few ways that soil crusting might interact with herbicides to increase carryover risk or otherwise interfere with herbicide efficacy:
- Pushing through a soil crust is stressful for seedlings and also delays emergence, potentially causing increased herbicide uptake that may exceed a crop’s tolerance. The combined stress of both pushing through a soil crust and herbicide carryover may also add up to increased seedling injury and reduced stand count, even if herbicides have had time to break down and residual concentrations are relatively low.
- Soil erosion that occurs alongside crusting can carry herbicides attached to soil particles and cause them to accumulate at higher concentrations in low spots in a field, increasing the risk of carryover damage. This damage would be patchy and may be indistinguishable from issues caused by waterlogging alone.
- Soil crusting may cause residual herbicides applied this year to remain on top of the soil surface, where they have limited contact with germinating weed seeds and are prone to being washed off during the next rainfall.
It may be hard to determine what exactly causes seedling injury and poor stands, as there are several other factors, including diseases, cold/wet soils, and planting issues that might also interact with and add to problems caused by crusting or carryover. If you do decide that replanting is necessary, light tillage before replanting would break up a soil crust and disperse any remaining herbicide carryover if weather allows, and it makes sense for your cropping system.
As a last note, we’ve also received a few questions about what the best residual herbicides are for the current wet conditions. Dr. Amit Jhala provides a great overview of this topic in this article from UN-L Extension.