Production Blog

Cool Temps Hurt Burndown

Pam Smith
By  Pam Smith , Crops Technology Editor
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Early warm up got winter annual weeds excited, but the cool temperatures that followed are making it tough to get on burndown treatments. (DTN photo by Pamela Smith)

DECATUR, Ill. (DTN) -- In the manufacturing world, a burn down chart is a graphical representation of work left to do versus time. The backlog of work is often on the vertical axis and time along the horizontal.

This spring Midwest farmers would say their burn down chart is officially backlogged when it comes to burndown herbicides applications. Roller coaster temperatures have kept the sprayers sidelined and a good number of fields are officially furry after winter annuals got excited with the warm up in February and March.

Kevin Bradley and Mandy Bish, University of Missouri weed specialists, indicate growers are wise to hold off on burndown treatments until temperatures warm up. Air temperature before, during and after application can influence control with burndown.

"As a general rule, when air temperatures fall below 40 F for an extended period of time after a burndown herbicide application has been made, weed control will most likely be reduced," they said in a news release. This is especially the case with any burndown application that includes glyphosate (Roundup, Touchdown, etc.), which is a systemic herbicide and needs time to penetrate the leaf cuticles and move throughout the plant in order to have optimum activity. According to Bradley and Bish, weed control will likely be even poorer if you have made a burndown application and there is an extended period of cool, cloudy conditions following that initial drop below 40 F.

By studying more than a decade of weather station info, the weed scientists found 1/3 to 1/2 of the April days in Missouri can fall to 40 F, depending on location in the state. "There have been several years over this time period when we have experienced widespread burndown failures across the state -- most likely this was directly related to the air temperatures experienced before, during, or after the burndown herbicide applications were made in those years," they noted.

Pennsylvania State University extension weed scientists have also issued a reminder that vegetation should be actively growing and capable of intercepting the herbicide spray (e.g. not covered with crop residue) before attempting burndown treatments. They suggest waiting two or three days if possible after the nights have warmed before making foliar-applied applications.

To make this more complicated, all herbicides and weeds may not respond the same. University of Illinois studies have found lower temperatures at application reduced glyphosate activity on henbit, but were less important to common chickweed control with glyphosate. However, temperature did have a significant impact on common chickweed control with paraquat -- control increased as temperature increased.

Overall, increases in temperature significantly enhanced weed control and reduced weed biomass. With henbit, the Illinois studies showed control of less than 80% until applications were made when temperatures were above 76 F.

Holding out for warmer weather is hard when the planting window starts to close, not to mention all the recommendations to get clean early. Weed growth doesn't wait for weather forecasts -- many of our winter annual weeds are already blooming and larger weeds make control more difficult.

"We realize that this might not always be possible and that this decision must also be balanced by the size of the weeds at the time of the application -- you don't want to wait so long that your weeds have exceeded the optimum size for control, as can easily occur with horseweed and giant ragweed at this time of year," Bradley said.

"So if there is no other alternative other than to spray and you know cool conditions are going to persist after application, you may want to increase the rate of glyphosate or whatever burndown herbicide you are using and consider at least one other tank-mix partner to ensure the best chance of burndown success."

Recommendations for Arkansas can be found here: http://bit/…ly/1YiVzg4

Here's a recent report on burndown recommendations from Pennsylvania State University:…



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4/14/2016 | 9:23 AM CDT
If you are a cover cropper using cereal rye such as myself this spring has been especially challenging. A very warm March followed by this cold April has allowed this crop to become very aggressive . This is a good and a bad thing I suppose. When we get it all killed I am pretty sure we will be pleased with the root systems. At any rate I have 1200 acres of it and I find that asking for advice from the "professionals" is rather useless. If it is warm enough that it is growing any the wind is not blowing too hard ,the ground is not too wet and the sun is going to shine on it for a few hours SPRAY IT. These conditions don't occur that often and if you spend a lot of time worrying about what the weather was or will be you will miss a window. This is just advice from some one who must actually kill the stuff not a scientist who is called upon to write a perfect label recommendation.