Herbicides and Variable 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 this spring has been dry weather and how it impacts herbicides. 

There are two primary ways we see herbicides affected by dry weather, activation of preemergence herbicides, and efficacy of postemergence herbicides. We will address each here separately.

Dry weather and preemergence herbicides

In the spring it is not uncommon to rush planting into moist soil in between rain events, hoping to catch the next rain for activation of our preemergence herbicides. However, sometimes Mother Nature does not cooperate with timely rain, and we get extended dry periods which can impact activation of the herbicides. Conditions have been excellent for crop planting, but the extended dry period may mean herbicides are sitting on the soil surface with no activating rainfall within even the next 7-10 days; in this time, weeds germinate with no symptoms and it can look like a failure of control. 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; this is discussed in a prior article Bob Hartzler wrote in 2021. Except for the rare case, this does not help us with weeds that emerge before an activating rain. We should plan an early postemergence application to clean those weeds up and avoid competition with our crop. If we delay the application to after a rain event, we will get activation of our preemergence product and get control with the postemergence application. Don’t let those weeds get too large waiting on rain; waterhemp has already emerged in many fields and plants must be <4” tall to assure consistent control. 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 recently planted and not yet sprayed with herbicide, farmers could wait to spray until rainfall is more certain in the forecast. As noted above, this comes with risk of emerging weeds gaining substantial size or becoming more challenging to control. Farmers will want to evaluate necessary changes to their herbicide program as some preemergence herbicides must be applied within a few days of planting. Additionally, as weeds emerge during this time, the delayed preemergence (or early postemergence) application will need include effective herbicides 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 but 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.

The hot and dry conditions of the past week have allowed for substantial planting progress but potential problems in weed management in these fields. Scout fields before and after herbicide applications to evaluate weed control and possible crop injury during this challenging weather pattern.