With so many effective cotton defoliants available to growers, sometimes optimum use depends more on when defoliants are used rather than which of these products a grower uses.
Clemson Cotton Specialist Mike Jones says, “cotton defoliants from a physiology standpoint are amazing products, which can cause leaves to fall off a cotton plant, but not damage the cotton boll — if used correctly.”
“We have two basic types of defoliants — herbicidal or contact materials and hormonal — both of which cause the release of ethylene in the cotton plant. The extra ethylene release essentially tells the plant it is mature and it’s time for the leaves to fall off,” Jones says.
There are a number of products, which if applied when cotton is ready to be defoliated will provide near 100 percent defoliation. The trick is knowing which product to use, under which conditions and at which time.
Deciding when to defoliate a crop is an important decision from several standpoints. If the crop is defoliated too soon, yields, quality and profits suffer.
On the other hand, depending on the location and the field condition, delaying defoliation may increase likelihood of additional insect problems, or delaying harvest into bad weather which will affect yield and profits.
Jones says cotton that is ready to be defoliated when it has a good boll load, it has used up most of the nitrogen, the potash is gone, most of the new growth is devoted to boll development, and cotton is not rank.
The problem in South Carolina, he says, is that the cotton growing areas of the state are prone to drought and high temperatures in July and August, which make the cuticle of the leaf really thick and the leaf hard. Under these conditions we have problems defoliating cotton, he adds.
There is no one recipe for defoliating cotton, Jones stresses. All the cotton defoliants on the market have strong points and weak points. Some work better in warm weather and others in cool weather, all have different rain or moisture tolerance, some are better as boll openers and some are better on mature leaves, while others are better on juvenile leaves and regrowth.
Most of the time two-way and three-way mixes can cover cotton that is in several different stages of readiness for defoliation. Low rates of these materials will do an outstanding job of defoliating cotton without affecting yield or quality of the cotton, if the right combinations are used.
Jones says his work at Clemson’s PeeDee Research and Education Center has shown promising results using PPO inhibitors for cotton defoliation. These herbicides inhibit the protoporphyrinogen oxidase (PPO) enzyme which is in the pigment synthesis pathway.
The PPO inhibition starts a reaction in the cell that ultimately causes the cell membranes to leak. The leaking cell membranes rapidly dry and disintegrate.
Aim, ET and Blizzard are three PPOs being evaluated by Jones and other cotton researchers around the Southeast. “So far in South Carolina we haven’t seen a lot of difference among the three materials used in tank-mixes with some of the more traditional defoliants. A lot of growers are interested in these materials, because they tend to be a little cheaper,” Jones says.
“Timing is where most of our growers in South Carolina run into problems with defoliation. Some growers wait for all the bolls to open and wait too late to defoliate. Research has shown that when leaving that cotton boll open, it loses about one-half percent per day of its weight,” he adds.
There are several ways to determine when to defoliate cotton. An old rule of thumb is to defoliate when 60 percent of the bolls are open. “In South Carolina, we are pretty safe with 50 percent fully mature bolls,” Jones says.
Another method is nodes above cracked bolls (NACB). Research has shown that cotton with four nodes above the highest cracked boll can be defoliated without significant weight or quality loss. If NACB counts average five or more, defoliant applications should be delayed.
“The best approach is to use both methods. Either one of them can lead to less than optimum timing for defoliation, but used together, they cover most combinations than growers will face in determining when to defoliate,’ Jones says.
Growers can be misled by a late-set top crop that looks like it will add significantly to final yield. However, it has been shown many times that the fruit set during the first four weeks of bloom normally contributes 90 percent to 95 percent of the total yield of the cotton crop.
Additionally, vigorously growing plants with limited boll loads are more difficult to defoliate than mature, heavily fruited plants. Regrowth is especially challenging if late season rains follow early or mid-summer drought.
Across the upper Southeast cotton growers appear ready to harvest a big crop. Making the potential come true on the bottom line depends on efficiency of getting the cotton out of the field and to the gin. Proper timing of defoliation can go a long way toward making that process as efficient as it can be.