What is in this article?:
- Herbicide-resistant weeds continue to bedevil Mid-South.
- Liberty after Liberty a spraying recipe best avoided.
- New research shows importance of time of day when spraying.
A TENNESSEE COTTON field, pictured in early July 2011, is overgrown with resistant pigweeds. Stuck with a bad situation, the producer tilled the field up and replanted.
When to spray?
If Liberty had come out and we’d never seen Roundup, all would think it was an excellent herbicide. But compared to what Roundup used to do, “Liberty is pretty finicky,” said Steckel. “Relative humidity makes a big difference, how cold it is makes a big difference on how well it works.”
Recently completed research also shows that the time of day or night producers choose to spray makes a big difference on how consistently Liberty will control pigweeds. Besides Tennessee, the study was done in Georgia by Stanley Culpepper, in North Carolina by Alan York, in Mississippi by Jason Bond and in Louisiana by Daniel Stephenson.
“Our graduate students were sent out in the wee hours of the morning to spray Liberty. We took a quart of Liberty and sprayed pigweed that was border-line too big -- five to eight inches tall. We sprayed an hour before sunrise, a half-hour before, a half-hour after, an hour after, then two, four and six hours after sunrise. We did the same thing prior to sunset: six hours before sunset, four hours, two hours, one hour, a half-hour as it set, a half-hour after and an hour after.”
Fifteen days later, the students returned and sprayed at the same time. “So, these were sequential applications spaced by 14 days.”
The results proved dramatic on the level and consistency of control. “All of the locations together showed an hour before sunrise there wasn’t 40 percent control. At sunrise, we didn’t get 50 percent control. It was two hours after sunrise before we got good control. That happened at every single location -- an oddity in research.
“So what’s happening? I don’t know if anyone has gone out with a frog-gigging helmet in the dark and checked pigweed, but it’s ‘asleep.’ I never noticed that but the leaves roll up. I think that might be part of the issue -- we aren’t able to get the coverage we would otherwise.”
But that isn’t the whole story. “In fact, research done in Georgia shows that Liberty is a light-activated herbicide. Mechanically, how it works in the chloroplast of the cell is where the rub is, apparently.”
What about spraying around sunset?
“We got a bit more inconsistent results around sunset. In North Carolina and Georgia, as the sun went down so did the level of control. It wasn’t as dramatic as in the morning but it was there.
“In Tennessee and Mississippi, we got 99 percent control even after the sun went down. But after the sun goes down, it’s probably a good time to quit spraying. Of course, when you’re trying to avoid drift, when do you want to spray?”
Steckel says the findings have helped him in field diagnoses. “I’ve been called to fields where farmers have sprayed Liberty on two- or three-inch Palmer pigweed and it’s growing back. First thing I ask: ‘what time of day did you spray?’”
In closing, Steckel offered these tips for using Liberty.
- Spray smaller.
“Of course, that’s easier said than done with something that grows as quick as pigweed. When it gets around eight inches tall, you can figure on a sequential application seven to 10 days apart.”
- Fifteen gallons water minimum.
“With Liberty, Flexstar, Cobra, gramoxone, 15 gallons is clearly better than 10. In my work, I haven’t seen 20 gallons make a big difference over 15.”
- Spray at least two hours after sunrise.
- Use nozzles 350 microns, or less, in size.