Alabama Cotton: 10 Non-Dicamba Weed Control Considerations

Poorly controlled pigweed following dicamba applications. Photo: University of Tennessee

Poorly controlled pigweed following dicamba applications. Photo: University of Tennessee

The dicamba ban from Ninth Circuit Court caught many growers off guard. Some of them may have booked enough Xtendimax, Engenia and FeXapan for planned applications but many of them did not.

Many dealers sold out a year’s dicamba inventory within less a day after the ban was issued. Many growers may have questions about the future of Xtend technology as they heavily rely on it to control resistant Palmer amaranth (Palmer pigweed).

After a lot of conversation and field visits in the last 10 days, I have no doubt that a good amount of illegal generics have been used due to the ban, and partially due to the cheaper cost ($5 or less vs $10/A for approved formulations) and low commodity prices.

Here are a few Palmer control options for growers to consider after the court ban:

1. For those who did not get enough approved formulations (Xtendimax, Engenia, FeXapan ) to spray field twice, spray Palmer fields ASAP with dicamba + Roundup before they are too big, follow up with Liberty + a group 15 herbicide (Warrant, Dual or Outlook) ~7-10 days later with liquid AMS 3 lb ai/A and a NIS under hot sunny day, to release the full potential of Liberty.

2. For those who cannot get any approved formulations, spray Liberty tank mix stated above ASAP before Palmer gets too big. Then follow up another shot of Liberty + liquid AMS + NIS about 7-10 days later before significant regrowth occurs. Hot, sunny and humid day is the best condition for Liberty.

There are a few generic Liberty on the market (e.g., Scout and Interline) that are marketed competitively which provides similar efficacy as name brand.

3. If big Palmer infestation is sporadic, spot spray Liberty with ATV or backpack sprayer repeatedly is an option. However, the challenge is you don’t know if you have any small Palmer escapes inside the crop because they are hard to see. So, a broadcast application is usually done with spot treatment to take out big escapes.

4. Xtendimax, Engenia and FeXapan have cut off date of 60 DAP or mid-bloom stage, which ever happens first. Therefore, spraying them on Palmer escapes, curl stem and put Palmer close to ground, then use hood sprayer to dump Gramoxone on top of them is an option on relatively flat ground where drift is not a big problem.

A drop nozzle sprayer or layby rig spraying Liberty + Valor, Cotoran or Diuron is also an option. If some of the escaped Palmer did not get killed by Gramoxone or Liberty, hope they are twisted low enough and hurt bad enough to the point that cotton canopy can shade them out. If this does not take them all, go to the next bullet point.

5. If a Palmer escaped all the options stated above because they are too big, get a hoe or chainsaw to finish them. So far, they have not evolved steel resistance yet.

6. With both Xtend and Enlist technology under special attention of Ninth Circuit Court, we need to prevent herbicide drift as best as we can.

A few thoughts from recent farm visits:

7. Escaped Palmer remains the biggest concern. Many fields received really good PRE, activated by rain timely. They kept Palmer free for quite some time but did not get POST 1 timely, then Palmer is taking over the field after PRE runs out.

I have visited crop fields recently where POST 1 has not been sprayed even at 50 DAP, some Palmer were over 1.5 ft tall. You cannot blame your PRE did not do its job in this case.

8. Leaf burn. Lots of questions about leaf burn these days. Growing condition is better than last year, so we have many beautiful looking cotton if thrips were controlled well. Growers have concern to burn cotton when it looks pretty. I get that, but the tradeoff is you may need to deal with overwhelming Palmer, grasses and late season weed flush later.

Many growers have kissed goodbye with their lay rig and hood sprayer, which may be required to control late season flush if no Group 15 herbicide (Dual Magnum, Warrant, Outlook for cotton) is used over the top.

I guess what I am trying to say here is, between hoe pigweed, live with some leaf burns that usually recovered within 14 days, or spending time to use hood sprayer or layby rig, growers will need to pick one out of the three options in a weedy field with Palmer and grass pressure.

9. Dead grass: alive grass weeds are not good in crop, but dead grass is still bad because they will stay there with very slow decomposition due to high content of lignin and lower C:N ratio than broadleaf weeds. Dead broadleaf weeds decompose very quickly in general. See pictures after this article. Do not let grass get a head start against cotton!

10. Liberty on large weeds: effectively burn off the top as a defoliant, symptom shows up very quickly. However, penetration is usually a problem so bottom half of large weed usually survive the treatment, unfortunately. Repeated application is surely required in this case.