Herbicides and Hot, Dry Weather

Each year brings fresh concerns related to herbicide performance and the weather. We can’t control the weather, and we are often at the mercy of wind, temperature, and moisture whether there is too much, too little, or ideally, just right. A recurring concern has been dry weather and how it impacts herbicides. There are two primary ways we see herbicides affected by dry weather, activation of residual herbicides, and efficacy of postemergence herbicides. We will address each here separately.

Dry weather and residual herbicides

While rainfall is generally a guarantee for Iowa springs, sometimes we get extended dry periods which can impact activation of residual herbicides applied either at planting or with postemergence applications. When residual herbicides sit on the soil surface with no activating rainfall following application, this provides weeds an opportunity to germinate below the herbicide layer and create the appearance of a control failure. This is especially true if rainfall occurred prior to residual herbicide application. What should we expect, and what can we do? 

Residual herbicides will sit on the soil surface for several weeks, depending on the product, and will be activated by the next 0.5-1” rainfall. Some excellent information on activation needed for Group 15 herbicides is available in a previous article “Preemergence herbicides, dry soils, and rain” on the ICM website. Except for the rare case, this does not help us with weeds that emerge before an activating rain. In situations when residual herbicides are applied at planting with no rainfall after, expect to scout fields and plan an early postemergence application to clean emerged weeds up and avoid competition with our crop. It is important to not let those weeds get too large waiting on rain; waterhemp begins emerging in late April to early May in many fields and plants must be <4” tall to assure consistent control. 

A more likely scenario for Iowa farmers to encounter is dry weather after a postemergence application which includes a residual herbicide. In these situations, weeds will emerge if rainfall occurred prior to application or if sufficient moisture exists beneath the soil surface triggering germination. Applicators may question whether to wait on these postemergence applications for a forecast with rain chances or whether to leave the residual herbicide out entirely. The timing of the postemergence application should be driven by the size of emerged weeds, and many residual herbicides can sit on the soil with little degradation and become active when rainfall comes. If weed escapes occur between the postemergence herbicide application and activation of the residual herbicide, watch weed growth to evaluate the density of escapes. If weeds warrant a second postemergence application, do not allow them to get larger than 4” in size before deciding to apply. Dry weather should slow the growth of seedlings, however some species are capable of significant root growth to find soil moisture even when it is dry.

In the unusual situation where the crop was planted and not yet sprayed with a residual herbicide, farmers could wait to spray until rainfall is more certain in the forecast. As above, this comes with risk of emerging weeds gaining substantial size or becoming more challenging to control with continued dry/hot conditions. Farmers will want to evaluate necessary changes to their herbicide program as some residual herbicides must be applied within a few days of planting. Additionally, as weeds emerge during this time, the early postemergence application will need to control emerged weeds like waterhemp.

Dry weather and postemergence herbicides

Throughout the year we can experience dry weather, which can cause adverse effects on our postemergence herbicide efficacy and weed control. This can occur due to several factors including reduced photosynthesis and growth, reduced transpiration and conservation of moisture in the plant, and reduced absorption of herbicides due to low humidity, rapid drying, or thickening of the leaf cuticular wax. In general, if the weeds are stressed and showing signs of moisture stress in the form of wilting, it is not recommended to apply postemergence herbicides. If the plant looks normal, applications can be made, however be aware that low humidity can impact the level of control observed with some herbicides. 

Relative humidity is the more important moisture component when it comes to herbicide efficacy. It determines how fast a droplet will dry on the surface of the leaf and also determines the transpiration of the plant when associated with soil moisture. To determine relative humidity, we have to consider temperature as well. When temperatures are high and relative humidity is low, the droplet will dry faster, reducing the amount of time a herbicide can enter the plant (uptake ceases when the drop dries out), and ultimately reducing herbicide activity. Plants which have grown in low humidity also tend to develop a thicker cuticle, the waxy surface of the leaf, that helps preserve moisture and also acts as a greater barrier to herbicide entering the plant. In high temperatures with low humidity, volatility risk increases, so avoid spraying under low humidity conditions when the temperatures are greater than 90°F. Generally, we expect systemic herbicides like glyphosate to be more affected than contact herbicides like PPO-inhibitors when relative humidity is low, but research has shown that low humidity impacts glufosinate activity negatively as well. 

To overcome the effects of low humidity, there are a few tactics to consider. First, determine if the weeds are wilting, if yes do not spray. Second, consider the time of day you make your herbicide applications. Systemic herbicides may perform better if applied in the morning or late afternoon. Third, adjuvants may be added to the spray solution to delay drying of the droplet on the leaf and/or increase the rate of absorption into the plant. These can also increase the chance of crop injury under stressed conditions, so keep this in mind. Dr. Hartzler wrote a detailed article on spray adjuvants. Additionally, the Crop Protection Network has published a resource on the specific role of different adjuvants in herbicide applications. 

When selecting herbicides and adjuvants during dry conditions, avoid mixing too many products in one tank. We often tank-mix several products to ensure multiple modes of action for control, residual activity, and to save time. An example of a hot mixture would be glyphosate plus a Group 14 (PPO-inhibitor) plus Group 15 (Dual, Zidua, Harness, Outlook), plus an oil adjuvant (MSO or COC). If anything else were added, including nitrogen fertilizer, insecticide, or fungicide, the combination would increase the oil in the mixture, further enhancing absorption and creating more “leaf burn”. While this may increase weed control, it can also increase crop injury under hot conditions. This is especially important to consider if crops are small, so weigh the pros and cons carefully. 

For optimizing weed control under dry conditions the order of decreasing effectiveness is: Methylated Seed Oil (MSO) followed by Crop Oil Concentrate (COC) and then Non-ionic Surfactant (NIS). Adding a nitrogen fertilizer like AMS or UAN can help improve the efficacy of postemergence herbicide applications as well but may increase crop response.  Do not apply UAN to an emerged crop under stress conditions. Always check the herbicide label for recommendations on adjuvant and nitrogen fertilizer additions, not all products are compatible and severe crop injury or death may occur. 

Summary

There are no miracle products that will solve our challenges. In dry conditions, MSO, COC, and nitrogen fertilizer are all products that can increase weed control with most herbicides. Under normal circumstances NIS will generally be sufficient, however COC or MSO may help penetrate a thicker cuticle and improve control. Consult your Iowa State University Extension field agronomist and agricultural service provider to find the right product for the herbicides you intend to apply. It is important to select adjuvant types recommended on the herbicide label, as some can lead to excessive crop injury if applied incorrectly. Scout fields before and after herbicide applications to evaluate weed control and possible crop injury during challenging weather patterns.