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 firstname.lastname@example.org.
"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.