When is the air temperature too cold to apply a burndown herbicide? Each year we receive a number of phone calls that ask this question. With the recent temperature swings, it is hard to pinpoint when would be the best time to apply a burndown herbicide. In spring 2002, we initiated an experiment to address this question. We wanted, first, to determine the effect of air temperature during herbicide application on control of winter annual weeds and, second, to compare control of these winter annuals from herbicide treatments that had differing "speeds" of activity.
Three herbicide treatments, glyphosate (Roundup UltraMax), paraquat (Gramoxone Max), and paraquat (Gramoxone Max) plus metribuzin (Sencor), were applied at six different application timings based on daytime high air temperatures ranging from 47° to 87°F. The herbicides included one herbicide that was systemic in nature (glyphosate), one that was contact in nature (paraquat), and a contact herbicide combined with a herbicide that had soil residual activity (paraquat + metribuzin). Weeds evaluated included common chickweed and henbit. Overall, temperature did not affect common chickweed control with glyphosate and paraquat plus metribuzin. However, temperature had a significant impact on common chickweed control with paraquat; control increased as temperature increased (Figure 3). Conversely, temperature greatly affected henbit control, which was less than 80% with all herbicides until applications were made when temperatures were above 76°F (Figure 4).
Overall, increases in temperature significantly enhanced weed control and reduced weed biomass. The treatment least affected by temperature was paraquat plus metribuzin. This may be attributed to the soil residual activity of metribuzin. Some weed control provided by this treatment at lower temperatures may have come from this residual activity. Differences also appear to exist in how application temperature affects control of these two species. Temperature had little affect on common chickweed control with glyphosate; however, application temperature significantly affected glyphosate activity on henbit. So when deciding whether it is too cold to spray, make sure to consider what weeds are out there and what type of herbicide you will be using. This research was funded by the Illinois Soybean Program Operating Board.--Christy Sprague and Aaron Hager