Gene Stevens
State Extension Agronomy Professor

Removing leaves and opening bolls with harvest aids are important components of a management plan for producing high-quality cotton fiber. When not properly managed, harvest aids may reduce yield and quality. Some of the benefits of defoliation include:

  • Increases exposure of bolls to sunlight, which causes faster drying and opening.
  • Picking operations can begin earlier in the day due to faster drying.
  • Reduces trash and green leaf stain (improves grades).
  • Reduces boll rot.

By properly managing the time of defoliation, the crop can be prepared for a timely harvest, which is critical in Missouri's short season environment. The number of suitable hours for harvest operations decreases rapidly during the fall. With timely harvest, earlier stalk destruction can occur as an essential part of insect and disease management. The objective of this publication is to provide information on proper defoliation techniques and the available harvest aids on the market today.

Timing of defoliation

Do not defoliate too early. Micronaire reduction, reduced oil content of seed, lower seed viability and yield reduction may occur. Some of the more traditional methods for determining time of defoliation include:

  • Boll firmness
  • Percent open bolls
  • Seed coat coloration
  • Heat unit accumulation
  • Visual assessment

Defoliation time can be planned earlier in the season by using plant mapping techniques. When plants reach the stage of 4 to 5 nodes above white flower (NAWF), they have reached the end of the effective boll loading period or "cutout." Knowing that approximately four to six weeks or 750 to 850 heat units will be required for boll maturity, a producer can then begin to make harvest aid decisions. Recent research has demonstrated that cotton can be defoliated with no loss in yield or quality when 3 to 4 nodes with harvestable bolls are present above the uppermost, first position cracked boll. Plant mapping techniques will be most useful in fields that have uniform stands, early fruiting initiation and high fruit retention.

These techniques should not replace visual verification of crop maturity status, however. The general practice is still to wait until at least, on average, 60 percent of the bolls are open prior to applying any harvest aid materials. Another excellent check of crop maturity is to cut the top bolls that you hope to harvest. If these bolls have a dark-colored seed coat and the seed is no longer jelly-filled but has developed well-defined cotyledons, then the fiber on that boll is mature and safe for harvest aid application. Use of the 60 percent rule and boll inspection in conjunction with plant mapping techniques will aid a producer in making proper crop termination decisions.

Don’t hurry a harvest aid decision and make a mistake that may cost yield and fiber quality

End-of-season scenarios

For a cotton crop to reach maturity in a timely matter requires more than just good harvest aids. The crop requires management for timely cutout and maturity from the time it is planted. A good date to reach cutout in Missouri is Aug. 10, since blooms formed after that date have a much smaller chance of maturing; not enough heat units are received during most seasons. Blooms formed before Aug. 10 have a much greater chance of maturing. In fact, under average conditions at Portageville, Missouri, blooms that open on Aug. 1 should, on average, mature between Sept. 10 and 20, while an Aug. 10 bloom won't be mature until Oct. 1 or later. The accumulation of heat units in October is very minimal, so little boll growth and maturity is possible during that month in most years.

Managing end-of-season decisions is determined by a combination of crop maturity, temperatures suitable for harvest aids to work, and good field conditions for picking 10 to 14 days after the harvest aids were applied. In general, harvest aids (defoliants and boll openers) work best when the average temperatures stay above 60 degrees Fahrenheit. September is the prime defoliation period, with average temperature remaining above 60 degrees Fahrenheit for most of the month (Figure 1). After mid-October the chances of 10 days of suitable temperatures following application of defoliants and boll openers drops to less than 1 in 3. The rainfall pattern during September and October is variable, but early October appears to be drier than the other weeks. Cotton harvest during early October could take advantage of the slightly drier pattern. When the harvest season continues into November, the days get shorter with less hours of sunlight to dry soil following rains.

Line graph depicting the minimum, maximum and average temperatures in Portageville, Missouri, in September and October 1995 to 2020.
Figure 1. Daily average maximum and minimum temperature averaged from 1995 to 2020 in September and October at Portageville, Missouri (Source: University of Missouri Extension Agricultural Weather Station at MU Fisher Delta Research Center).

Picking the best harvest aid material

Environmental conditions at the time of defoliant application and for three to five days following can have a tremendous influence on the effectiveness of these chemicals. Under conditions of high sunlight, temperature and relative humidity, defoliants are most active. A night temperature of at least 60 degrees Fahrenheit is particularly important. Early research indicates that defoliation proceeds twice as rapidly at 95 degrees Fahrenheit than at 77 degrees Fahrenheit. Most harvest aids need minimum temperatures above 55 to 65 degrees Fahrenheit for optimal performance. If temperatures drop below this level, then leaf drop and boll opening will be slowed. High relative humidity is important for lower evaporation and transpiration will allow better absorption of the chemical defoliant into the leaf.

Most harvest aid chemicals are distributed by more than one company and have several brand names. Table 1 provides a description of the active ingredients and formulations common in Missouri. Always follow label rate and tank mix instructions.

Table 1. Description of harvest aid materials.

Active ingredient(s) and formulationDescriptionRates
Tribufos (6 pounds a.i. per gallon)Defoliant used over a broad range of conditions. Reduce rates if temperatures are warm (85 degrees Fahrenheit or higher) to avoid "stuck" leaves. Is very effective when used in combination with Prep.1.33 to 2 pints per acre
Dimethipin (5 pounds a.i. per gallon)Defoliant that removes leaves while still green, but can also help dry up morningglory vines is the field. Always use with crop oil concentrate.0.5 pint per acre + 1 pint crop oil concentrate
Thidiazuron (50 percent a.i. WP)Defoliant that works best in warm weather (average temperature near 70 degrees Fahrenheit). Effectively suppresses regrowth. Cool weather activity can be improved when used in combination with Prep. Activity may also be increased by using ammonium sulfate or silicone-based spray adjuvant.0.2 to 0.4 pounds per acre
Ethephon (6 pounds a.i. per gallon)Boll opening compound that also causes some defoliation. This compound typically should be applied after defoliants or tank-mixed with defoliants. Should not be applied until all harvestable bolls are mature to avoid reducing yield and fiber quality. Allow 14 to 21 days between application and harvest for complete boll opening.1.33 to 2.67 pints per acre
Ethephon+ Cyclanilide (6 pounds a.i. + 0.375pounds a.i. per gallon)Accelerates opening of mature bolls, defoliates leaves, and provides some inhibition of terminal regrowth.1.3 to 2.6 pints per acre
Sodium chlorate (6 pounds a.i. per gallon)A desiccant that can defoliate cotton, but causes some "stuck" leaves.4.5 pound per acre or 3 quarts per acre

Picking the right harvest aid strategy

Defoliation is as much an art as a science, since many variables of crop and weather conditions interact in the final performance measured in terms of leaf drop, regrowth, boll opening, yield and fiber quality. To simplify the decision somewhat, we will discuss only the variables that can be readily recognized or controlled.

Canopy/crop size

One of the first considerations is how large a canopy of leaves needs to be removed. If the crop is taller than 40 to 45 inches tall, some difficulty may be experienced in removing all the leaves with one application of a defoliant. If a tall crop is not ready to defoliate until late September or October when the temperatures are not conducive for quick leaf drop and boll opening, then higher rates and spray volume may be needed. These conditions are also conducive to boll rot. Managing for final plant height to equal row spacing has been shown to result in less yield loss to boll rot. A plant of manageable size should result in better spray coverage and more effective defoliation.

Leaf/crop maturity

If the leaves have remained green or new young leaves have been added to the plant after cutout, then care should be given to avoid "sticking" leaves by applying too high a rate of defoliant. The greatest potential problem is experienced with warm temperatures (greater than 85 degrees Fahrenheit) for areas of the field that have remained green. It may also be possible to reduce the rate of harvest aid if the canopy has matured and already experienced some natural leaf aging with warm temperatures.


The activity of defoliants depends largely on the temperature experienced during the period three to five days after the material is applied. Average temperatures above 60 to 65 degrees Fahrenheit greatly enhance defoliant activity. Some materials such as Tribufos, Dimethipin, and Sodium Chlorate can defoliate cotton leaves under cool conditions, while Thidiazuron is extremely sensitive to cool conditions. When cool conditions are encountered, a combination of materials is recommended, typically a defoliant and a boll opener. In field trials, the effectiveness of all defoliant and boll opening chemicals was reduced 10 to 30% in cool conditions (less than 60 degrees Fahrenheit) compared to warm conditions (more than 60 degrees Fahrenheit).

Yield potential

Under conditions of above-average yield potential and profitability, using a combination of a defoliant and boll opener can provide a rapid, thorough defoliation and opening of bolls for a large first harvest. Using a combination approach is more expensive, but if the potential return of high-quality lint is in the field, this approach will permit the greatest return. In contrast, if the potential yields are not great for a crop that has experienced water stress, then using a less expensive harvest aid strategy should provide the greatest economic return.

Picking once or twice

Complete defoliation and boll opening generally permit quicker harvest of cotton with less trash. The cost of a combination of Ethephon and a defoliant that can produce once-over harvest is $30 per acre less expensive than a second harvest operation. The fiber harvested with the once-over strategy is typically of higher quality and value.

Original authors
David W. Albers, Fred Fishel and Jill B. Mobley
Publication No. G4253