Last week’s cold and wet conditions followed by the weekend’s frost creates the potential for herbicide-induced crop injury from soil- and post- applied herbicides as well as reduced postemergence weed control.
The warmer and dryer conditions projected for this week are encouraging for crop recovery. Therefore it is wise to allow for a few days of warm weather for the crops and weeds to recover before heading out to the field to apply any postemergence herbicide. Your crops need time to recover so they can adequately metabolize the herbicide, thus preventing herbicide-induced crop injury and the weeds will need time to recover before they can take up the herbicide and move the herbicide to active growing sites.
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 postemergence weed control and the long-term consequences of herbicide-resistant weeds, I think the use of preemergence herbicides at their full-labeled rates is still the wise choice.
Last week’s cool, wet weather slowed the rate of crop emergence and extended the amount of time the crop is exposed to herbicides in the soil/water solution. Injury potential will vary with herbicide chemistry since some herbicides (e.g. metolachlor and acetochlor) contain crop safeners and soil texture will influence the amount of herbicide bound to the soil and not available for plant uptake.
The most common questions to-date have come from people assessing soybean stands following the low temperature conditions of May 15th. Current questions are in regard to replanting soybeans in soils already treated with soil-applied PPO herbicides. Soil-applied PPO herbicides would include Authority-, Valor-, or Sharpen-based products (Site of Action number 14).
The herbicide label is the final authority but after a quick review of several PPO-labels, soybeans can be replanted into treated soil. However, in order to retain the weed control effectiveness of your herbicide application I would encourage as little disruption of the soil as possible. Reworking the field will dilute the existing herbicide and likely stimulate new weed growth. If necessary, delayed preemergence herbicides options exist for several of the non-PPO soybean herbicides (e.g. Site of Action number 15).
For soybean stands just emerging or yet to emerge, based on lessons learned last year, it is often the crook stage of the soybean plant and the cotyledons that will be expressing injury symptoms.
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.
Regardless of the cause(s) of the soybean injury, the affected fields should be left alone until early next week. In one weeks’ time plants will be either dead or alive. Then accurate stand counts can be taken and next steps determined. Minimizing soil disturbance in any replanted areas will increase the value of the previously applied herbicide.
Last year’s Crop News blog, "Frost injury to soybean," will provide you with more details regarding replanting: http://blog-crop-news.extension.umn.edu/2015/05/frost-injury-to-soybean.html.
To assess frost injury in corn, see "Frost injury to corn seedlings unlikely to greatly impact yield": http://blog-crop-news.extension.umn.edu/2016/05/frost-injury-to-corn-seedlings-unlikely.html