This article was excerpted from the CORN newsletter #19 and written by Alyssa Essman, Assistant Professor, Extension Field Crop Weed Specialist, Dept. of Horticulture and Crop Science.
We’re entering a period of hot, dry weather and the long-term forecast for this season suggests that these conditions are likely to persist during the summer and through harvest. With variable planting dates across the state, there is also a range in current crop growth stages and overall status, which means there are also different weed management activities taking place over the next month. There are some considerations to keep in mind as we aim to manage weeds when temperatures are high, and rainfall is scarce.
- Residual Herbicides – The incorporation or activation of residual herbicides can be insufficient in areas that receive less than ~ 0.5 to 1 inch of rainfall. Without some sort of incorporation or activating rainfall, the product does not enter the soil solution and is not available for uptake by germinating weeds. A layered residual strategy for the control of waterhemp is still recommended, especially where crops were planted early and preemergence herbicides are starting to decrease in concentration and lose efficacy. Rain will still be needed to incorporate these later-applied residual herbicides into the soil profile for maximum efficacy.
- POST Applications – Plants respond to drought stress in part by increasing the thickness of the cuticle, or the waxy outer layer of the leaf, to preserve water. This can result in decreased herbicide absorption, and translocation and metabolism within the plant are also affected. Systemic herbicides are especially at risk of reduced efficacy in these conditions. The use of the right adjuvant product and rate can help to increase absorption by improving leaf coverage and product uptake. Follow the label and herbicide/adjuvant manufacturer recommendations to determine the best adjuvant type and rate in these conditions. Some labels also include specific information related to applications in dry weather. Be aware that an increase in activity from the use of adjuvants can also lead to increased crop injury, and that applications early or late in the day may reduce the risk of injury. Contact herbicides are also more active when applied during hot weather, and crop injury from applications of these products may increase as well. Where residual herbicides were less effective due to lack of rainfall, a two-pass POST program should be considered to manage the potentially higher weed densities.
- Weed Emergence – Dry conditions can delay the emergence of weeds to some degree, especially those in the upper portion of the soil, and those that emerge from smaller seeds. Larger seeded species and those at lower depths will be less affected, as they have more energy reserves and access to more water for imbibition and growth, respectively. In dry weather patterns, weed emergence flushes may occur periodically following rainfall events.
- To Wait or Not to Wait – One question that often comes in when rainfall is limited is whether it’s better to apply POST herbicides soon, when weeds are smaller, or to wait until after a rain event and the potential following flush of weed emergence. Weeds are most effectively controlled when they are smaller and are actively growing. Drought and heat stress can impede control efforts and herbicide efficacy, and large weeds are more difficult to control. This is especially true if the stress-inducing conditions persist and weeds “harden off”. For this reason, it is a good strategy to wait for the extremely high temperatures to pass before making an application, for greater crop safety and herbicide efficacy, and to ensure weeds are actively growing. However, it’s not necessarily better to wait for a rain before making a POST pass. As mentioned, plants that persist through hot and dry weather often produce thicker cuticles, and it can be difficult for herbicides to get into the plant and be effective. For this reason, it might be best to spray when weeds are as small as possible unless there is some definite rain in the forecast.