Source: Dr. Mark Loux, OSU

Current forecast is for fairly warm temperatures through late evening Tuesday evening, followed by a substantial drop in temperatures and chance of snow, followed by cold/cool temperatures through the weekend.  Primary question concerning this scenario seems to be whether it is okay to apply wheat or burndown herbicides prior to this cold snap.  Some things we know about herbicides and cold weather:

– Herbicides applied to an emerged crop just prior to or during cold weather may be more injurious compared with favorable weather conditions.  During cold weather when plants are not actively growing or growing slowly, the rate of translocation and metabolism of herbicide by the plant slows down, which can mean an accumulation of herbicide that is not being metabolized.  This can increase the risk of crop injury since metabolism of herbicide by the crop, or conversion to an inactive form, is what allows that herbicide to be safely used on the crop in the first place.  For some herbicides, there is such a large margin of safety with regard to crop safety that this is all inconsequential.  For others the margin is narrower and issues such as cold weather and sprayer overlaps are more important.  The inclusion of safeners in herbicide formulations reduces the risk of injury, usually be increasing the rate of metabolism, but may not completely solve issues that arise because of adverse weather or too high a dose.  So with regard to this week and risk of injury to wheat, we would recommend avoid applying herbicide once the cold weather starts (from Wednesday on), until warm weather resumes.

– There is less certainty in making a recommendation about whether to treat wheat on Tuesday prior to the cold weather.  We have seen instances in corn where application just prior to cold weather has resulted in greater injury.  Wheat is actively growing now under favorable weather, and should readily translocate and metabolize herbicides.  Much of this process occurs within the first few hours of application.  Temperatures do not really start to plunge until early Wednesday morning per the forecast.  While it’s somewhat of a guess, it seems that application during the first part of Tuesday would be possibly safer to the crop than later in the day.  Past experience has shown us that some wheat herbicides are just generally safer than others, so one option would be to omit the ones that have stricter growth stage guidelines or have more of a history of causing injury.  Having said this, in our research we have really not experienced injury from small grain herbicides applied per label.

– With regard to efficacy of burndown herbicides and cold weather, some of the same principles apply.  Applying herbicide from Wednesday through the weekend, when weeds are not actively growing, is not recommended due to the likely loss of activity.  Susceptible weeds metabolize herbicide slowly anyway, so the issue is a lack of translocation within the plant and the inability of herbicide to do it’s thing at the active site when plant processes are shut down.  This is the type of cold weather we referenced in the recent article about dandelions, when we have observed control of this weed to plummet.  We have also observed extremely slow control of overwintered annual weeds during cold weather.

– We would recommend going ahead with burndown herbicide applications on Tuesday, prior to the cold.  As with wheat, weeds are actively growing under favorable weather so we assume herbicides will work.  It’s still a bit of a guess, but it could be a while before field conditions and weather are suitable for application again.

– This is the type of scenario that makes us want to remind everyone again that a few dollars of herbicide in the fall can help avoid some of the nasty burndown issues that develop when spring conditions are less than optimum.  Just saying.