EDITOR’S NOTE: The section titled “A few more thoughts on burndown herbicides and the cold weather” was inserted into this article as additional information on April 23. This update and the original article are provided by Clarke McGrath, an Iowa State University Extension field agronomist.
A few more thoughts on burndown herbicides and the cold weather
Another strong burndown option is the use of Gramoxone (paraquat) products. This is a fast and consistent burndown herbicide. A few key points about Gramoxone:
* Be sure to add the right additives (surfactants, etc.) to Gramoxone. As with most burndown herbicides, strong rates of crop oils, methylated seed oils and/or nonionic surfactants are critical.
* While UAN (28% or 32% nitrogen) can impede glyphosate activity, it seems to enhance Gramoxone activity so UAN is a great choice for corn burndown treatments when you are applying liquid N.
* Tank mixing triazines (atrazine for corn, metribuzin, aka 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 (the label goes down to 10 gpa, my experience was that 15 gpa was more consistent in providing an effective burndown on bigger weeds).
* Gramoxone isn’t nearly as temperature sensitive as glyphosate, an important observation given the weather we are having this spring.
* The last point is probably the most important, and I have to give credit to my friend for reminding me of this--“There is no weed resistance issue with Gramoxone, so using it breaks the glyphosate cycle.”
This could be one of those springs where temperatures swing wildly which can be detrimental to the effectiveness of any burndown herbicide treatment. What can you do to kill these weeds?
"There's no definitive answer to that," says Clarke McGrath, Iowa State University Extension field 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 and numerous other factors."
McGrath, who writes the "Corn-Soybean Insight" column in Wallaces Farmer magazine each month, offers the following observations and recommendations. If you have specific questions or need more information contact him at email@example.com.
"My scouting has led me to think that spring annuals are pretty small with the exception of a few giant ragweeds in the bean stubble near protected areas like slews 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 that 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 temps cooperate from here on out. If we get frost, wait a couple days and go reevaluate them--you need good green tissue to take up herbicides."
In cold weather you may want to increase application rate of glyphosate
When temps get cold like Iowa has seen recently, 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," 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.
"In corn, when applying burndown herbicide treatments, there is concern regarding any cold, wet soil stress and any cracked open seed trenches we may see if we have a tough spring," says McGrath. "Try to avoid applying 2,4-D under these conditions if there is potential of a rain driving acetanilide (or other residual herbicides for that matter) 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 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."
If a weed is not growing actively, there is greater risk that a burndown herbicide treatment won't work as well as it usually does
Bottom line on all of this: If a plant is not growing actively, there is a greater risk that the burndown won't work as well as planned. "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," says McGrath. "Follow your local dealer's best recommendations. They will be working with chemical company reps for specific ideas on burndown herbicide treatments and how to use them."
A couple other key thoughts on the subject of using burndown herbicides
* Use plenty of adjuvants/additives such as Concentrated Oil Concentrates, or COCs, Nonionic Surfactants, or NIS, and Ammonium Sulfate, or AMS surfactants -- 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.
* If the wind will cooperate, flat fan nozzles (like XRs) do a better job on tough weeds than AIs or TTs in general.
* "We often add more NIS and sometimes some COC to our glyphosate even if it is already a "loaded" product," says McGrath. "Local experience and many of our local agronomists tell us this helps get more consistent weed control."