Each year, herbicide performance is influenced by weather, and spring 2025 has brought concerns related to dry conditions in the Midwest. Dry weather can significantly impact herbicide efficacy in two key ways: by hindering the activation of pre-emergence herbicides and by reducing the effectiveness of postemergence applications. In an Iowa State University Integrated Crop Management blog, extension specialists Wesley Everman and Meaghan Anderson outline these effects and provide management recommendations for farmers facing these challenges.

For pre-emergence herbicides, timely rainfall is critical for activation. While planting conditions may be favorable, the lack of subsequent rain can leave herbicides sitting on the soil surface for extended periods, during which weeds may germinate and emerge. These weeds show no symptoms of herbicide damage, making it look like the product failed to work. However, many herbicides can remain effective on the soil surface for weeks and will activate with a 0.5 to 1 inch of rainfall. Unfortunately, this does not help with weeds that have already emerged. To manage this, early postemergence herbicide applications should be planned to control early weed flushes and reduce crop competition. Farmers should avoid delaying these applications while waiting for rain, especially with fast-growing weeds like water hemp, which must be controlled under 4 inches tall. In fields not yet sprayed, farmers may hold off on herbicide application until rain is in the forecast, but this carries risk if weeds grow too large or become harder to control. Herbicide programs may need adjustment to include products effective on already emerged weeds.

For postemergence herbicides, dry weather presents a different set of challenges. Stressed weeds may reduce photosynthesis, transpiration, and absorption, leading to suboptimal herbicide performance. Low relative humidity is especially problematic because it speeds up the time of the droplet drying on leaves and promotes thickened cuticles, reducing herbicide uptake. High temperatures combined with low humidity can reduce herbicide absorption and boost volatility risk — conditions under which spraying is discouraged, especially when temperatures push past 90°F.

However, some strategies can mitigate these effects. Spraying systemic herbicides in the early morning or late afternoon, when humidity is higher, may improve outcomes. Adding adjuvants can also slow droplet drying and enhance absorption, but may create additional crop injury risks under stress.

In conclusion, while dry weather aids planting, it creates potential weed management issues. Farmers should scout fields regularly before and after applications to get a grip on herbicide performance and crop safety. Adjusting herbicide timing, formulation, and application techniques can help maintain effective weed control under these challenging conditions, the extension specialists noted.