"In general, increasing the application rate 20% to 30% is a common recommendation, and in the absence of any research I can find, this works for me," says McGrath. "This issue is not so much a concern with 2,4-D so I would not generally recommend upping the 2,4-D rate in corn or bean burndown treatments, mostly due to crop safety."
What about glyphosate herbicides? Should you do anything different when using glyphosate as a burndown in cold weather?
When the temperature is below the low 40s at night followed by days that don't get above the low-mid 50s, McGrath says you could consider either waiting a couple days to treat, or bump the rate a third or so to mitigate the risk of not controlling some of the larger winter annuals and spring annuals, especially broadleaves.
"If it is around the mid 40s or better at night and low 50s or above in the daytime, we still worry a little; you can probably spray the next day, but if giant ragweed or other tough to control species are out there, still consider bumping the glyphosate rate up or tankmixing in another product," he says.
Annual grasses are not too much of a concern with glyphosate herbicides. "Annual grassy weeds tend to be easy to control with our burndown rates but if we have had a heavy frost, we still may want to wait two or three days to spray glyphosate," he adds.
2,4-D is not as sensitive to cold temperatures as glyphosate herbicide
2,4-D products are not as sensitive to the cold weather, that's been McGrath's experience as long as the plants themselves are not too damaged. In general, the winter annuals won't sustain much damage, so you should pay attention to the spring annuals that are up. If there is around 50% undamaged leaf tissue, then treatment should be ok after a couple days of lows that don't go below freezing. Otherwise the weeds will need some time to get new tissue out, he says.