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by Kevin Bradley and Mandy Bish
Because of the dramatic swings in air temperature that we have experienced already this spring, we have received a few questions about the effect of air temperatures on our spring burndown applications. Specifically, most people are asking, ‘When is it too cold to apply a burndown herbicide?’, and ‘What are the conditions that lead to poor weed control following a burndown herbicide application?’
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. 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. 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.
As a result of our network of MU weather stations throughout the state, we were able to summarize the average hourly air temperatures during the month of April over the past 13 years for the central (Boone County), northeast (Knox), northwest (Gentry County), southeast (Cape Girardeau County), and southwest (Barton County) areas of Missouri. As illustrated in the line graph below, temperatures are usually at their lowest in April between the hours of 3:00 to 6:00 am, will generally increase from 7:00 am through 2:00 or 3:00 pm, and will start to fall soon after that. Although the graph only shows the average hourly temperatures for this time period, we knew that there were years when air temperatures fell well below 40 F in April and were interested to know how often. This information is summarized for the 13-year time period for each region in Missouri in the bar graph below. As you can see from this graph, historically there can be as many as 1/3 to ½ of the days in April where temperatures fall to at least 40 F, depending on your 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.
To make this even more complicated, all herbicides are not going to respond the same way to applications made at high and low air temperatures, and all weeds are not going to respond the same way to different temperatures regimes at the time of application. Although there have been very few studies published on this topic in the weed science literature, weed scientists in Illinois did an experiment on this a few years ago and ultimately found that lower temperatures (<60 F or so) at application had a significant impact on glyphosate activity on henbit, but had very little influence on common chickweed control with glyphosate. In this same study, they reported that glyphosate seemed to be more sensitive to low air temperatures at application than paraquat (Gramoxone).
So what can you do about all this? The simple answer is to watch your forecasts closely and to wait for more favorable temperatures to arrive before you make your burndown herbicide application. 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. 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.
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REVISED: October 1, 2015