Defoliating a cotton field is often more of an art than a science, but there are some techniques to help cotton producers time the application, according to Sandy Stewart, Extension cotton specialist, LSU AgCenter.
Eyeballing the field to see if there are enough open bolls to proceed with defoliation can be deceiving, Stewart told producers attending a Monsanto-sponsored field day in Union City, Tenn. “If I scan a field, usually what I think is 65-70 percent open bolls, is more like 80-85 percent when I do the counts. It's usually a little higher than you think.”
In the nodes above cracked boll method of defoliation timing, growers choose their topmost harvestable boll, then count down. “The consensus is that a harvestable boll three to five nodes above the uppermost cracked boll is a good timing for defoliation, sometime you can go down to six.
“The problem with this method is choosing your last harvestable boll, and that's subjective.”
Stewart explained that in a lot of fields, “especially with the taller, ranker varieties that extend into the season, it's hard to choose that last boll you're ready to pick.”
Another method under study is to count 12 nodes from the bottom to the top of the plant. “Start at the bottom with a boll you want to harvest, then count up. When you get to 12, you normally have time in the season to mature the boll on the 12th node. On most cotton varieties, whether it's DP 555 BG/RR or ST 4892BR, about 95 percent of our yield comes off a 12-node range on the plant.
“This can take some of the guesswork out of which boll at the top is going to be the last one you can harvest. I'd like to see 12 nodes in most fields. If you get into some really good conditions, you could get 14 nodes. So that could help you time your defoliation.”
Nozzle selection and carrier volume are crucial decisions at defoliation time, noted Stewart. “It's very difficult to get good coverage with a defoliation application with 5 gallons per acre. I'd like to see producers go with 10-15 gallons. Once you go above 15 gallons, it doesn't make a lot of difference.”
While air induction tips “are very good for Roundup applications, we do see some problems when they are used for defoliation. It's simply a coverage issue, especially when you have a herbicidal defoliant like DEF or Folex in the tank.”
Stewart said flat fan and hollow cone tips can provide better coverage than the air induction tips. “You can make an air induction tip work if you run the pressure up high enough.
“Most recommendations are somewhere between 90 pounds and 120 pounds. That's where you begin to get enough fines to blow it down into the canopy. I don't know how many people want to run that much pressure. You can do it, but when you get pressure up that high, you've turned that air induction tip into a flat fan.”
Stewart noted that cotton producers will have one more choice in their defoliation toolbox this season. “We do have another PPO inhibitor that is going to be labeled for defoliation in cotton this year called Resource. It will be labeled for 4-8 ounces in two applications. The PPO inhibitors are very good for a second application where you have to clean up the skirt on a plant.”
Stewart said that Pix management is going to be increasingly important as cotton producers transition into new varieties. Stewart said producers must first learn the growth habits and plant architecture of new varieties.
For example new cotton variety ST 4554B2RF, with 8 ounces of Pix applied at six-leaf to eight-leaf on the second Roundup application, followed by 24 ounces of Pix at early bloom, can produce a completely different plant architecture than one with only one 24-ounce application at early bloom.
“On average the height difference between 8 ounces early, followed by 24 ounces, versus 24 ounces only, is about 11 inches. This year, there is nothing wrong with the plant with an early application, but had we had a much drier season in west Tennessee, the differences may have been much more dramatic.
“The hazard with a mepiquat application pre-bloom is that you always run the risk of dry conditions after the application. Dry conditions after the application with some of these pre-bloom applications can translate into the possibility of early cutout. If you get into a wet year, the pre-bloom applications are fine.”
Stewart noted that many of the new cotton varieties containing Roundup Ready Flex technology “are going to be early-medium maturities and will not produce a huge stalk, like DP 444 BG/RR and DP 555 BG/RR,” the two most predominant varieties currently grown in the Mid-South.
“In my state (Louisiana) and to a certain extent in Tennessee we need to re-evaluate the plant growth regulator systems and the mindset we have going in. A lot of these new Flex varieties are going to respond very well to early-bloom applications of around 16 ounces. A lot of them are not the big plant types that we've dealt with in the past.”