2,4-D and Dicamba Burndown in Cotton; When Is It Too Late?
Dr. Alan York
William Neal Reynolds Professor of Crop Science
The weather this spring has not been conducive to getting burndown applications for cotton made in a timely manner. I have been swamped with questions about how long one must wait between 2,4-D application and cotton planting. Labels for 2,4-D products that specify this use (not all labels have this use) clearly say to wait 30 days between 2,4-D application and cotton planting “for residue tolerance”. Most people interpret the labels as saying cotton can be planted 30 or more days after 2,4-D application, but the labels do warn that planting cotton less than 90 days after 2,4-D application may lead to injury. The label for Clarity (several other dicamba-containing products are also registered for burndown on cotton) has the following statement concerning burndown application in cotton: “following application of Clarity and a minimum accumulation of 1” of rainfall or overhead irrigation, a waiting interval of 21 days is required per 8 fluid ounces per acre or less.”
I have attached a manuscript below we published a few years ago. In the introduction, we mentioned several studies that had previously been conducted on waiting intervals between 2,4-D or dicamba application and cotton planting. In our study, we had seven locations over a two-year period. The rates of 2,4-D (530 and 1060 g a.e./ha) correspond to 1 and 2 pints of a typical 3.8 lb/gal product. The rates of dicamba (280 and 560 g a.e./ha) correspond to 0.5 and 1.0 pint of a product such as Clarity. We applied the 2,4-D and dicamba 6, 4, 3, 2, and 1 weeks ahead of planting. The land was strip-tilled immediately ahead of planting (which probably had no effect on the results). We recorded stands, percent of seedlings exhibiting any amount of leaf distortion, and yield. Note in the left column of page 216 we say that the leaf distortion was usually minor, and our percentages of seedlings with leaf distortion was greater than a typical injury rating.
We observed some leaf distortion from 2,4-D at 1 pint applied 2 weeks ahead of planting at one of seven locations or 2,4-D applied 1 week ahead of planting at three of seven locations. There was some stand reduction at three of seven locations where 2,4-D was applied 1 week ahead of planting; stands were unaffected with longer waiting intervals. Yield was reduced at one of seven locations when 2,4-D at 2 pints was applied 1 week ahead of planting; 2,4-D at 1 pint did not reduce yield regardless of time of application. 2,4-D was generally less injurious to cotton than dicamba when the label-recommended waiting period was not met.
What is the bottom line? One should always follow label directions. However, if your growers are asking how much they can “cheat” on the 2,4-D label, the attached publication may give you some guidance. If your growers decide to use 2,4-D at an interval of less than 30 days, make sure they understand that is an off-label application and they are on their own. There appears to be less room to cheat with dicamba than 2,4-D; tell your growers to think twice about cheating on dicamba waiting periods.