Cold temperatures and applying burndown herbicides

Cold temperatures and applying burndown herbicides

What effect does outdoor air temperature have on your burndown herbicide treatment?

It's early April and it's time to apply a burndown herbicide on emerged weeds or cover crops prior to planting. But the weather is colder than you'd like for a burndown treatment to work well.

Farmers have been calling Iowa State University Extension field agronomists this past week asking about how the outdoor air temperature affects herbicides applied as a burndown on weeds and also on cover crops. Farmers ask: "What can we do to kill these weeds and cover crops?"

KILL SMALL WEEDS: If they have enough green tissue present, small weeds are easier to control with a burndown herbicide treatment. And being close to the ground may protect small weeds with radiant heat from soil, to improve burndown treatment results.

If the weeds are small they are easier to control, and being close to the ground that may protect them with radiant heat from the soil. This spring, as of the second week of April, most of the small weeds look like they have enough green tissue to control them pretty easily if temperatures cooperate from here on out.

Many factors to consider when you try to answer this question
"There is no definite answer to that question," says Clarke McGrath, an ISU agronomist at Harlan in western Iowa. "It all depends on what weeds you have, what crop is in the field, what herbicide products you are thinking of using, your time factor, how cold it actually got, your crop residue situation in the field and numerous other factors," says McGrath, a regular columnist each month in Wallaces Farmer magazine. Here are a few of his thoughts on the use of burndown herbicides this spring:

"My scouting has led me to believe that annual weeds are pretty small so far this spring. There are a few hot spots where they are taking off and really growing fast, but that seems to be the exception. Keep in mind if the spring annual weeds in your field are still fairly small, that should help in a couple ways."

Small weeds are easier to control, if they have enough green tissue
If the weeds are small they are easier to control, and being close to the ground may protect them with radiant heat from the soil. Most of the small weeds look like they have enough green tissue to control them pretty easily if temperatures cooperate from here on out. "If we get frost again, you should wait a couple days and go reevaluate the weed situation," says McGrath. "You need good green tissue to take up herbicides."

He adds, "When temperatures get cold like we saw as week or so ago, transport of herbicides in plants is slower and active ingredients are more vulnerable to inactivation in the weeds. Glyphosate in particular has an affinity for organic material and can be more attracted to inert structures in the plant due to the increased transport time to active sites. In general, increasing your application rate 20% to 30% is a common recommendation, and in the absence of any research I can find, this works for me."


This issue is not so much a concern with 2, 4-D, however, says McGrath. "So I would not generally recommend upping the rate of 2, 4-D if used as a corn or soybean burndown herbicide treatment, mostly due to crop safety."

What about the temperature effect on glyphosate herbicides?
It temperatures get below the low 40 degree range at night followed by days that don't get above the low-mid 50 degree range, you should consider either waiting a couple days to treat, or bump up the rate a third or so to mitigate the risk of not controlling some of the larger winter annuals and spring annuals, especially broadleaf weeds, says McGrath. Here are some other key points and recommendations he makes:

• If it is around the mid-40 degree range or higher at night and the low 50's or above in the daytime, "we still worry a little; you can probably spray the next day, but if giant ragweed or other tough weeds are there, you should still consider bumping the glyphosate application rate up, or tankmixing the glyphosate with another product."

• Annual grasses are not too much of a concern with glyphosate. Annual grassy weeds tend to be easy to control with burndown application rates. "But if you have had a heavy frost, you still may want to wait two or three days to spray glyphosate," he says.

Suggestions for using paraquat as a cover crop burndown
What about applying paraquat as a cover crop burndown? Or applying it as a general no-till burndown? Be sure to use the right additives (surfactants, etc.) with Gramoxone (paraquat). As with most burndown herbicides, strong rates of crop oils, methylated seed oils and/or nonionic surfactants are critical, notes McGrath.

While adding UAN to the herbicide tankmix can impede glyphosate activity, it seems to enhance Gramoxone activity, making UAN a great choice for corn burndown treatments when you are applying UAN as the nitrogen source anyway, he says. Also, keep in mind that tank mixing triazines (atrazine for corn, or metribuzin such as Sencor in soybeans) increases the speed and efficacy of Gramoxone in burning down the weeds.

Use flat fan nozzles with Gramoxone and try to get your carrier up to 15 gallons per acre or so, he advises. "The label goes down to 10 gallons per acre, but my experience was that 15 gallons per acre was more consistent on bigger weeds."


Gramoxone isn't nearly as temperature sensitive as glyphosate, an important observation given the weather we are having this spring, says McGrath. The last point is probably the most important, and he gives credit to a friend from Syngenta for reminding him of this—"There is no weed resistance issue with Gramoxone, so using it breaks the glyphosate cycle."

Keep in mind 2,4-D isn't as sensitive to cold temperatures
What about using 2,4-D products? They aren't as cold sensitive as long as the plants themselves are not too damaged by the cold temperature or frost. In general, winter annuals won't sustain much damage from cold temperatures, so you should key off of the spring annuals that are up. If there is around 50% undamaged leaf tissue on the spring annuals, then treatment should be ok with 2,4-D 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, before you spray with 2,4-D as a burndown.

"In corn where you are using a burndown treatment, a concern is any cold, wet soil stress and any cracked open seed trenches we may see if we have a tough spring," cautions McGrath. "Try to avoid applying 2,4-D under these conditions if there is potential of a rain driving acetanilides and 2,4-D into the seed zone soon before and/or after planting, as this can cause seedling damage. With the price of glyphosate being so low, glyphosate herbicide would be a great substitute to use here with an atrazine premix rather than 2,4-D if there is a weather driven risk of corn injury."

If weeds aren't growing actively, there's greater risk this won't work
Bottom line on all of this is— "If a weed is not growing actively, there is a greater risk that the burndown herbicide treatment won't work as well as planned," says McGrath.

"Sometimes to get the majority of early weeds taken out before the crop is up, we have to compromise, so don't hold your chemical dealer's feet to the fire if your burndown herbicide application misses a few weeds in the field. Follow your local dealer's best recommendations. They will be working with chemical company representatives for specific ideas on use of burndown herbicide treatments."

Last key thought: use the adjuvants your dealer recommends
Use plenty of adjuvants/additives such as COC's, NIS's, and AMS— whatever your dealer recommends for the tank mix. Be generous with the amounts, especially if the weather is tough, like cold nights or drought stress.

Also, if the wind will cooperate, use flat fan nozzles (like XR's) for applying your burndown herbicide treatment. Flat fans do a better job on tough weeds than some of the other nozzles. "We always add more NIS and sometimes some COC to our glyphosate even if it is already a "loaded" product," says McGrath. "Experience shows more consistent weed control if we do this."

McGrath has a side note on the cover crops he and his ISU colleagues tested in plots in southwest Iowa this year. They included field pennycress in the plots. "Last week I scouted two sites to see when I'd need to get spring data," says McGrath. "The pennycress rosettes were large and doing quite well. If we can control this weed, we may have found a solid overwintering broadleaf cover crop candidate. Yes, maybe we can use pennycress—now considered just a weed by farmers—as a cover crop. We'll follow up on this and discuss our plot results in the future."

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