As people begin to assess soybean stands following the low temperature conditions of May 19th, questions are coming my way regarding the possible interaction of frost with soil-applied PPO herbicides. Is it possible? My answer is yes. Is it widely prevalent? As I receive more reports from around the state my current answer is, not likely.
An interaction of frost with soil-applied PPO herbicides is possible because cold temperatures slow the rate of emergence of the soybean through the herbicide-treated soil and the soybean is limited in its ability to metabolize the herbicide. However, the crook stage of the soybean plant that is expressing injury symptoms appears to be targeted to soybeans planted in early May (May 2 to 4 are frequently mentioned). Soybeans planted in early May were just cracking from the soil at the time of the low temperature conditions and were vulnerable to freeze damage.
Many report affected fields were treated with soil-applied PPO herbicides that include Authority-, Valor-, or Sharpen-based products. This is not definitive proof of an interaction but their frequency of use does make sense because many growers see the need to use these products to address the greater risk of herbicide-resistant weed management and wisely chose to initiate their weed management program with a preemergence herbicide.
Differentiating between frost damage and herbicide injury
How do you differentiate between freeze damage and PPO-induced crop injury? That is a good question. My best answer is subjective but is based on the contact-injury symptomology of PPO herbicides that will result in necrotic lesions on the cotyledons and the “crook” of the plant. These lesions are reddish to purplish to brownish in color. More advanced symptoms result in a girdling of the plant below the cotyledons. Bob Hartzler has some nice photos at: http://www.weeds.iastate.edu/mgmt/2005/soilppo2.shtml.
Because frost injury is due to radiation of heat away from the plant I would expect the necrosis of the “crook” of the plant to be more uniform in pattern and lacking any discolored lesions on the cotyledons.
What are the next steps? Regardless of the cause(s) the affected fields should be left alone until early next week. By next week plants will be either dead or alive. Then accurate stand counts can be taken and next steps determined. The recent Crop News blog “Frost injury to soybean” will provide you with more details: http://blog-crop-news.extension.umn.edu/2015/05/frost-injury-to-soybean.html.
Like any pest management tactic, preemergence herbicides come with some risks to your crop. However, when you weigh these risks against the more prevalent and more yield-reducing risks associated with poor weed control and the long-term consequences of herbicide-resistant weeds I think proper use of preemergence herbicides at their full-labeled rates is still the wise choice.