Spring weather conditions have provided little opportunity for field work, including herbicide applications for weed control in winter wheat. While temperatures have fluctuated and many fields have been too wet to spray, wheat and winter and early summer annual weed growth has not slowed down. This has made many of us anxious about not having our wheat fields sprayed.
However, while weed control in winter wheat is important, it is also important to note that applying herbicides under the wrong conditions can lead to increased chances of crop injury and reduced weed control. This is where we are at for the next few days across most of the state with colder than normal temperatures and chances for frost. To make sure we are getting the most out of our herbicide applications, there are a few things to keep in mind prior to spraying these fields.
Several herbicides labeled for weed control in winter wheat have specific label instructions that state applications should be made when weeds are actively growing. Herbicides should not be applied when the crop is under stress from very cold temperatures, when there are wide fluctuations in day/night temperatures, when a frost has occurred or when temperatures are below freezing prior to, at or immediately following herbicide applications.
In 2021, we conducted an experiment where we applied several herbicides when the evening temperature was near freezing (32 degrees Fahrenheit) and the daily temperature was approaching 50 F. Several of these herbicides, especially the ALS-inhibiting (Group 2) herbicides, caused significant injury to wheat (Figure 1). With one of these herbicides the injury could be observed throughout most of the growing season and resulted significant yield loss. This injury was not observed at locations when these herbicides were applied when temperatures were warmer.
To protect wheat from herbicide injury and to maximize weed control, a good rule of thumb is to only apply herbicides to winter wheat when the daily temperature is 50 F or higher. Following this rule of thumb helps avoid possible injury and can improve weed control.
Once temperatures start to warm up, it will also be important check the stage of growth of wheat in the fields that you are planning on spraying. All herbicides have a maximum wheat growth stage for application listed on the label. Late herbicide applications can lead to excessive crop damage that can cause kernel abortion and blank wheat heads that can ultimately reduce yield.
Some of the more restrictive herbicides that are used in winter wheat are the plant growth regulator herbicides. Most plant growth regulator herbicides need to be applied prior to winter wheat jointing (Feekes stage 6). Other herbicides that need to be applied prior to Feekes stage 6 are some of the key grass herbicides: Osprey, Osprey Xtra and PowerFlex.
If wheat is at or beyond Feekes stage 6, it will be important to look for alternative herbicide options that can be applied to wheat that is further along. Figure 1 in Chapter 3 of the Weed Control Guide for Field Crops (E-434) from Michigan State University Extension provides a good diagram of the minimum and maximum wheat growth stages of herbicides used for weed control in winter wheat.
For more information on weed control in winter wheat, consult Chapter 3 of the MSU Weed Control Guide for Field Crops (E-434).