Deciding when to defoliate a crop is an important decision from several stand points. 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 effect yield and profits. That is why defoliation decisions must be based on the crop and the crop environment. Plant maturity is perhaps the most important factor, but other factors such as picking capacity, custom harvesting, and weather are also important.

Defoliation is an important management practice associated with high yields and high quality cotton. The decision as to how and when to remove the leaves and open the bolls appears to be one of the more difficult tasks confronting a cotton grower. There are so many variables involved that the results of harvest aid applications are often unpredictable and sometimes even undesirable. You would think that after more than 40 years of research in this area we could obtain desirable results under all circumstances. However, this is not always true, and we often have failures. Thus, defoliation has come to be considered as much art as science.

There are many benefits that can be expected from a good defoliation job. Many experiments have shown that defoliation improves picker efficiency in fields with large green plants. Additionally, defoliated fields tend to dry out faster, permit more picking hours per day and allow picking sooner after rain. Defoliation also cuts off the food supply to late season insects that are entering diapause. Under certain conditions, defoliation has reduced boll rot by creating better drying conditions in the field. This is especially true in rank cotton.

Defoliation may also have some disadvantages and limitations. When plants are defoliated, the fiber and seed development essentially stops. Therefore, if too many bolls are immature at the time of application there can be a reduction in yield and quality associated with the treatment.

Cotton leaves have a mechanism that causes them to fall from the plant after they become senescent. This physiological process which involves the separation of living tissue from the living plant occurs in a specialized zone located near the base of the leaf petiole referred to as the "abscission zone". Abscission occurs due to enzyme activity within this zone and is regulated by hormones which are produced in the leaf. This regulation occurs as a resulting gradient in hormone concentration between the main-stem side and the leaf side of the abscission zone. The enzymes that are in this zone dissolve the cell walls, and the weight of leaf. Wind will cause the leaf to drop from the plant. Young, actively growing leaves produce an abundance of hormones which are translocated down the petiole to the main-stem. This tends to keep the leaf on the plant. As the leaf becomes older and more mature, hormone production decreases and the leaf becomes more susceptible to senescence factors such as hormone imbalances and defoliation. Factors other than age may also upset the hormone balance in the leaf. Some of these factors are injury, insect damage, disease, nutrient stress, water stress, cold stress or chemical injury. These injuries may upset the hormonal balance and initiate the abscission process and cause the leaves to fall from the plant much like an application of defoliant.

Basically, a defoliant chemical is designed to inflict sufficient injury to the leaf to upset the hormone balance at the abscission zone and allow enzymes to begin the abscission process. If too much chemical is applied, the leaf may be killed before the hormone imbalance occurs resulting in "stuck" leaves. If too little is applied the abscission process may not be initiated and result in a complete failure.

There are several factors which will directly impact the effect a defoliate chemical application may have. These are the condition of the plant, weather conditions at the time of application and for a couple of days after, and the chemical itself. The first two of these are by far the most important, and both must be within a reasonable range of limits in order for any of the chemicals to work.

Plant Effect: Plants are more easily defoliated when the cultural practices followed throughout the growing season are designed to promote well-fruited plants that mature evenly and early. This includes uniform stands, adequate but not excessive moisture and fertilization, and proper, well-timed insect, disease, and weed control. Proper management of the plant canopy with a Plant Growth Regulator may be beneficial in many cases. Generally speaking, defoliation is more easily accomplished when the plants have stopped both vegetative and reproductive growth, or if they have completely cut out. The ideal situation would be for the plant to reach maturity and run out of nitrogen and water at the same time. That rarely occurs in a field situation so we have to do the best we can.

Weather Effect: Weather conditions at the time of application, and for a few days afterwards, have a tremendous influence on the effectiveness of any of the harvest-aid chemicals and/or combinations. Weather conditions are perhaps the most important factor directly effecting the efficiency of defoliation attempts. As mentioned before, defoliation is a physiological process, and the physiological processes within the cotton plant are regulated by temperature. As the temperature is lowered, processes are slowed down and will completely halt at about 60F. Therefore, it is very difficult to defoliate cotton when the temperature gets down around 60F, plus or minus, for a few days around the time of application.

High humidity at the time of application is helpful to insure the defoliants work well. This permits the chemical to stay on the leaf in a liquid state for a longer period of time, giving more time for absorption of the chemical into the leaf. Also, when atmospheric moisture is high, evaporation and transpiration are reduced, and thus the moisture content of the leaf remains high, which aids in movement of the defoliation chemical.

Cotton plants are more responsive to defoliant chemicals when the sun is shining than on a cloudy day. The reason for this is not fully understood. It is assumed that the temperatures are a little bit lower under cloudy conditions. However, some research in Arizona has shown that even if the temperatures remain high, the response is slower under dark (relative) conditions than under light conditions. Therefore, sunlight does have some effect on response.

Chemical Effect: There are several chemicals available for defoliation, and all are equally effective when plant and environmental conditions are favorable for defoliation and they are applied in accordance with their label. For material information CLICK HERE.

There are other materials on the market for use in defoliation and countless combinations for numerous situations. You should select the material which will best fit the field situation. Note: Any mention of a product does not constitute an endorsement, nor does the omission of a product mean that it will not work in harvest aid applications.

Application: The application of these products is very, very important. None of the defoliant chemicals are considered to be very systemic, and thus none will translocate within the plant. In order for any leaf to fall from the plant, it must receive a sufficient amount of material to initiate the abscission process. For this reason the spray volume should be higher with defoliants than with insecticides. With ground equipment, at least 10 gallons per acre should be used and at least 5 gallons per acre with aerial equipment.

Preconditioning: Often a grower may be tempted to do what may be refer to as "precondition" the crop for defoliation. This can be very risky, and I do not recommend this practice. What some people refer to as "preconditioning" is the application of a low rate of defoliant chemical a week or 10 days before the regular defoliant treatment is to be applied. If conditions are right, it is possible that this may cause too much leaf drop and reduce both yield and fiber quality. Sometimes a grower may get by with this, but due to the variables involved it is very, very risky.

Timing: Deciding when to apply the chemical is often a tough decision to make. Above all else, the decision should be based on the maturity of the plants and field. Harvest schedules and prevailing weather conditions and forecast are also prominent considerations. For all practical purposes, the maturation processes stop when the leaves are taken off a cotton plant. Anytime that the decision is made to apply a defoliant or harvest aid chemical, there will probably be some immature bolls on the plant. However, a grower cannot wait until 100% of the bolls are mature; some will have to be sacrificed. As a general rule of thumb, the last boll to be consistently picked will probably be the first position boll on the 4th or 5th node down from the terminal. The maturity of this boll should be used as the key for timing an application of defoliant. The yield and quality of the bottom crop and middle crop is far more important than those last 2 or 3 little tiny bolls in the terminal. This decision is often hard to make. For more information on timing,

In closing, let me make one last point. Defoliation chemicals and boll openers do not contribute to maturity; they simply take the leaves off and crack bolls open at whatever stage of development the crop is in. A cotton crop should be at the desired stage of maturity when a harvest aid chemical is applied. Field examination is critical for defoliation decisions.