Alexander M. "Sandy" Stewart, Donnie K. Miller and Joel C. Faircloth

Cotton defoliation, a critical step in cotton production, is the process of removing leaves and preparing the crop for mechanical harvest. Leaf removal facilitates harvest and allows for more efficient and faster picker operation, quicker drying of seedcotton, straightening of lodged plants, retardation of boll rot and faster opening of green bolls. In many cases, chemicals that hasten the opening of green bolls and inhibit juvenile regrowth are included with a defoliant.

Cotton defoliation research has focused on the efficacy of labeled compounds. Environmental and crop conditions play a vital role. Most of these compounds are temperature sensitive, and the closer the plant is to full maturity, the more likely it is to defoliate well. Given the tremendous impact of these variables on defoliation, results from research trials often vary among locations and from year to year within a given location. Therefore, cotton defoliation has sometimes been referred to as more art than science, and only through a large amount of data can clear trends emerge.

Proper timing of application for defoliants is critical. Generally, the more mature and open the crop is, the more efficacious the defoliant. Premature defoliation can result in higher grades, but it also can result in yield loss. Delaying defoliation too long, however, can result in losses in yield and quality caused by weathering and expose growers to the risks of harvesting later in the season when tropical storms and wet conditions are more likely.

Trials in Louisiana and other states have investigated two timing methods. One method is timing defoliation by the percentage of open bolls of the crop. Most recommendations call for defoliation when 60 percent to 70 percent of the bolls are open. Another method is called nodes above cracked boll (NACB). Counting the number of mainstem nodes between the uppermost first-position cracked and the last harvestable boll on the plant determines NACB. Most recommendations call for defoliants to be applied at four NACB.

In 2000, trials were conducted at the Northeast Research Station in St. Joseph and the Dean Lee Research Station in Alexandria. At St. Joseph, the highest yield was obtained at 42 percent open and 3.6 nodes above cracked boll; the highest yield at Alexandria was found at 75 percent open and 1.2 nodes above cracked boll. The range in the findings strongly suggests that the boll distribution on the plant must be considered before applying any recommended timing rule. No recommendation is likely to fit every situation. Researchers are investigating the relationship between the fruit distribution and the timing of harvest aids.

In a three-year, replicated study conducted in North Carolina, defoliation timing was investigated in two varieties of varying yield and quality characteristics. The study demonstrated the overall importance of defoliation timing, especially in varieties that produce relatively high micronaire values.

In a similar North Carolina study, defoliation timing was examined in cotton containing a mid- to late-season fruiting gap (late July to late August) resulting from insect pressure, fertility problems or any environmental stress. The results demonstrated that if no fruiting gap exists, cotton can be defoliated as early as 50 percent open without realizing significant yield loss. If a fruiting gap exists, however, growers should delay defoliation until approximately 75 percent of the bolls are open to allow adequate time for plants to compensate for lost fruit. In studies from other states, early season fruiting gaps (mid-July and earlier) have not been found to alter defoliation timing.

Both aforementioned studies identified a trend of increasing micronaire and yield as the percentage of open bolls increased. Additionally, the node above cracked boll technique was examined in these studies for timing defoliation and was found to be as effective as the percent open technique. This is important for growers, because recording the nodes above cracked boll in a field takes considerably less time than recording the percentage of open bolls.

Another area of defoliation research focuses on its economic return. It is often less expensive for producers either not to defoliate, achieve less than 100 percent defoliation or apply a desiccant to dry, but not actually defoliate, the leaves.

A large experiment was initiated at the Dean Lee Research Station to investigate the effects of three levels of defoliation on cotton quality. The three levels tested were clean 100 percent defoliation, 20 percent desiccation of green leaves and 20 percent green juvenile growth on the plant at the time of harvest. The experiment was replicated three times, and harvested cotton was stored in modules and ginned commercially to mimic commercial production as closely as possible. Results indicate that the final loan value of the cotton was highest where 100 percent defoliation was achieved. The source of discounts for the less than 100 percent defoliation treatments was primarily due to the relative yellowness of the bale samples as classed by the U.S. Department of Agriculture’s High Volume Instrumentation (HVI) system. These data suggest that there is an economic value to defoliation, but further research is ongoing to determine the optimum level of defoliation to make defoliant recommendations as cost effective as possible for cotton producers.

To better understand cotton defoliation and make recommendations for producers, LSU AgCenter scientists continue to investigate these and other areas of cotton defoliation. Future projects will focus on evaluating new products, defoliation timing, application technology and economic returns to defoliation.

(This article was published in the spring 2003 issue of Louisiana Agriculture.)