So far this has been one of those springs where temperatures swing wildly which can be very detrimental to the efficacy of burndown herbicide applications. "I've had calls in recent days from farmers asking about these temperatures, tankmixes and other cold weather issues," says Clarke McGrath, an Iowa State University Extension field agronomist. "Bottom line is…What can we do to kill these weeds? And cover crops?"
There's no definitive answer to that question, he says. It depends on what weeds you have, what crop is in the field, what products you are thinking of using, your time factor, how cold it actually got, your crop residue situation and numerous other factors.
A few thoughts regarding use of burndowns
"My scouting leads me to observe that the spring annuals are pretty small with the exception of a few giant ragweeds in the bean stubble near protected areas like sloughs and riparian areas," says McGrath. "This should help in a couple ways. 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 them look like they have enough green tissue to control them pretty easily if temperatures cooperate from here on out. If we get frost again, wait a couple days and go reevaluate them; we need good green tissue to take up herbicides."
"When temperatures get cold like we saw this week, transport of herbicides in the plants is slower and the 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 the application rate 20% to 30% is a common recommendation, and in the absence of any research I can find, this works for me. This cold temperature issue is not so much a concern with 2,4-D. So I would not generally recommend upping the 2,4-D rate for burndown treatment with corn or beans, mostly due to crop safety."
What about glyphosate herbicides?
If the temperature is below the low 40's at night followed by days that don't get above the low-mid 50's, you should consider either waiting a couple days to treat, says McGrath. Or bump the rate up 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 40's or better at night and 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 stuff is out there, still consider bumping the glyphosate rate up or tankmixing it with another product."
Annual grasses are not too much of a concern with glyphosate. Annual grassy weeds tend to be easy to control with recommended burndown rates but if you have had a heavy frost, you still may want to wait two or three days to spray glyphosate.
Using paraquat as a general no-till burndown
At meetings this past winter, McGrath spent time talking about use of paraquat as a cover crop burndown. What about using it as a general no-till burndown? He lists these key points:
1) 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.
2) While UAN can impede glyphosate activity, it seems to enhance Gramoxone activity, making it a great choice for corn burndown treatments when you are applying UAN.
3) Tank mixing triazines (atrazine for corn, metribuzin, aka Sencor in soybeans) increases the speed and efficacy of Gramoxone in burning down the weeds. Think napalm.
4) Use flat fan nozzles with Gramoxone and try to get your carrier up to 15 gallons per acre or so (the label goes down to 10 gpa, my experience was that 15 gpa was more consistent on bigger weeds).
5) Gramoxone isn't nearly as temperature sensitive as glyphosate, an important observation given the weather we are having this spring.
He adds, "The last point is probably the most important, and I have to give credit to a friend from Syngenta for reminding me of this --There is no weed resistance issue with Gramoxone, so using it breaks the glyphosate cycle."
2,4-D is not as sensitive to cold temperatures
Keep in mind 2,4-D products aren't as cold-sensitive as long as the plants themselves are not too damaged, says McGrath. In general, winter annuals won't sustain much damage, so pay attention to the spring annuals that are up and growing. If there is around 50% undamaged leaf tissue, then treatment should be ok after a couple days of low temperatures that don't go below freezing. Otherwise the weeds will need some time to get new tissue out.
What about using burndown treatments where you have planted or will plant corn? "A concern is any cold, wet soil stress and any cracked open seed trenches we may see if we have a tough spring," he says. "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 in with an atrazine premix rather than 2,4-D if there is a weather driven risk of corn injury."
Bottom line on all of this is, "If a plant is not growing actively, there is a greater risk that the burndown 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 dealer's feet to the fire if your burndown misses a few weeds. Follow your local dealer's best recommendations. They will be working with chemical company reps for specific ideas on the use of burndowns."
Final key thoughts: 1) use plenty of adjuvants/additives—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. 2) If the wind will cooperate, using flat fan nozzles (like XR's) do a better job on tough weeds than AI's or TT's. 3) We always add more NIS and sometimes some COC to our glyphosate even if it is already a "loaded" product -- experience shows more consistent control.