Defoliating drought stressed cotton is a losing proposition for growers, and a problem many will face with the 2007 crop across the Southeast. Likewise choosing the right defoliant usually comes down to the one that hurts the least, not helps the most.
The problem will be compounded by the low yield potential of drought-stressed cotton and low prices for the crop.
Early fall rains may add some to the quality components of cotton, but may add significantly to regrowth of the crop, creating even greater harvest problems.
North Carolina State University Cotton Specialist Keith Edmiston says a good deal of cotton around his state was ready to defoliate by early September — 3-4 weeks earlier than usual due to the drought.
Producers will not want to spend much money on defoliation with most of the crop having poor to average yield potential, Edmiston contends.
For the 2007 cotton crop in much of the Southeast, Edmiston says the key harvest concern is re-growth. A poor drought-stressed crop has little bolls to fill now and residual nitrogen due to the lack of moisture. Because of this, cotton will likely have tremendous re-growth potential if the rains return, he explains.
In most years the combination of Def plus Prep has proven to be the optimal treatment for defoliating cotton in the Southeast. In the 2007 drought year, this may be the worst treatment, because of its weakness in controlling re-growth.
PPO products, commonly used as herbicides, may be a better base for cotton defoliation treatments in this year's drought-stressed crop. PPO products, including Aim, Blizzard, ET, and Resource are herbicidal and do not have the hormonal mode of action, unlike products containing thidiazuron (Dropp, Ginstar, Freefall etc.).
When compared to Def, the PPOs, appear to have less regrowth. This appears to be because the PPOs do a better job of taking off small existing regrowth and not due to hormonal inhibition of future regrowth, according to Edmiston.
PPO products used for defoliation will buy growers some time, compared to DEF plus ethephon.
Products containing thidiazuron (the active ingredient in Dropp, Ginstar, Freefall and a number of other generic defoliants will provide even more time for growers — the higher rate of these products used is directly related to the length of regrowth, Edmiston explains.
When using thidiazuron products, plant coverage is critical. A characteristic of herbicidal defoliants is to burn upper leaves and do little for regrowth lower in the plant. In these cases, the grower has to pick through dessicated leaves and green leaves. If a grower can't get plant coverage with his sprayer, it is probably better to leave the regrowth.
PPO's tend to act very quickly and burn all regrowth that is contacted with the material. This tends to reduce the uptake of thidiazuron into the plant and can result in a shorter period of regrowth control than you might expect for a given rate of thidiazuron.
Virginia Tech Cotton Specialist Joel Faircloth recommends Dropp, even high rates of the defoliant, for growers who cannot get into the fields as quickly as needed. Higher rates of Dropp tend to lengthen the period of control for regrowth.
“We have drought in a lot of our cotton and the fertilizer sat there and was not taken up. The result is a lot of regrowth in the top of cotton plants. If we get rainfall, we expect a problem with regrowth,” Faircloth says.
At least part of the problem with managing regrowth in cotton is getting proper spray coverage up and down the plant. With any of these defoliants, but especially with the PPOs, it is critical to get good coverage of the cotton plant. Upping the pressure, using a 15 gallon per acre spray volume and using hollow cone nozzles can help insure better coverage from top to bottom, but also can increase drift problems, Faircloth emphasizes.
“In a dry year cotton is set up for high micronaire, so growers don't want to wait too long to defoliate or to get cotton out of the field — to manage micronaire. “Plus, in Virginia, we want to get our cotton out before the picking hours get shorter and the dew gets heavier,” he adds.
Micronaire tends to occur in dry years because less mature fruit is shed during stress. Less mature fruit has lower micronaire than older fruit, leaving a higher percentage of old fruit tends to create higher micronaire.
The best thing to do to prevent high micronaire is to defoliate on time and not allow the least mature bolls to become overly mature (black seed coats). The down side of controlling micronaire in drought-stressed cotton is that it can promote regrowth, leaving growers with some tricky management decisions
“We hear a lot about needing special adjuvants on drought stressed cotton to get the material into the leaf. I think this is primarily a concern with thidiazuron. I would use recommended adjuvants with herbicidal defoliants and avoid the temptation to “soup up” the mixture with some adjuvant not recommended by the company. A lot of our defoliation will likely take place in high temperatures and “souped up” mixtures may cause excessive desiccation,” Edmiston says‥
“The take home is that nobody has ever invented a defoliant that fits all situations and all possible conditions of cotton. I think a good way to go is to base your defoliation around a PPO if you are going to harvest quickly and around thidiazuron if you are not and use boll openers where needed,” Edmiston says.