It’s Saturday, 10:24 am on May 18. It’s raining again.  Like many growers in Georgia, I have had lots and lots of rain to deal with (>13” of rainfall since April 1). This has made it very hard to get things done in the field and it has also led to increased herbicide injury in some peanut fields (Figure 1).

Just because an herbicide is labeled for use in a particular crop, it does not mean that a grower will never see any type of injury.  An appropriate lead in for the general concept of herbicide selectivity.

Herbicide selectivity, simply defined as acceptable crop tolerance plus efficacious weed control, is not really a simple process.  In fact, it’s a very complicated process of both physical (time, position, plant morphology, and environment) and physiological (herbicide absorption, retention, movement, and metabolism) factors. 

Although all of these factors are worthy of more discussion, I would like to focus my attention in this article on the environment and plant metabolism as they relate to some of the 2024 production season issues that many growers have experienced.     

In wetter years such as this, injury potential from preemergence herbicides, such as Valor or Brake, is often increased because more herbicide is readily available in the soil solution for crop uptake/absorption.  Additionally, heavy rains can cause “splash-back” of soil-applied herbicides onto young or tender peanut plants and/or what I like to call plant compaction-related injury (intense rainfall, over a short time period, physically pushes small peanut plants flat on the ground directly into the soil + herbicide solution thus increasing herbicide exposure and injury).

Metabolism or detoxification-based selectivity is one of the most common methods by which crop plants are tolerant to herbicides.  Generally, there are various enzyme systems in plants that can alter or degrade the structure of an herbicide, thereby making it in-active.  When plants are under stress, such as too wet, too dry or too hot, and not actively growing, these metabolic processes are slowed down or reduced, which can often result in increased crop injury after an herbicide application. 

Sound familiar?

Farmers cannot control they weather.  However, they can help reduce the potential for herbicide injury to occur by using labeled herbicides rates, following labeled application times and routinely calibrating their sprayers.  Just so you know, the weed science community is actively working on label-reform to make labels more user friendly and easier to read.   

When mad weed scientists like me are trying to develop a safe and effective herbicide recommendation that will work over many different environments and applicator skills, we try our best to make sure that we have at least a 2X margin for crop safety (selectivity). 

However, sometimes Mother Nature throws us a weather curveball, such as above-average rainfall this year, and we get more crop injury than anticipated or desired. When herbicides are applied according to the label, early season crop injury usually tends to be cosmetic, or hurts your feelings, and transient, which does not last all season long or reduce yield.  

As always, good weed hunting!