Proper use of cotton harvest aids in the Texas high Plains can result in earlier harvest, preservation of fiber quality, and fewer seed quality reductions due to field exposure.
Weathering losses in the High Plains can result in considerable reduction in dollar value of the crop, unless measures are undertaken to protect yield and quality potential. This is especially true for open boll picker-type varieties and less storm-proof stripper types.
Timing of harvest-aid chemical applications is critical and growers have several options to determine the correct time to apply these materials.
Product selection, tank mix partners and rates vary with environmental and crop conditions. What works best in one year is not necessarily the best for the next season. Efficacy is always a concern and several factors affect performance of harvest-aid chemicals.
Factors that increase performance include:
• Warm, calm, sunny weather.
• Soil moisture relatively low but sufficient to maintain the cotton plant in active growth condition without moisture stress.
• Soil nitrogen levels relatively low.
• Leaves active and uniformly expanded on plants.
• Little or no secondary growth evident on plants.
• Plants with a high percentage of open bolls that have shed some mature leaves.
Factors that negatively affect harvest-aid chemical performance include:
• Applications made under cool (below 60 degree F), cloudy conditions.
• Prolonged periods of wet weather following treatment.
• Plants in vegetative growth state with low fruit set.
• Plants severely moisture stressed with tough, leathery leaves at time of treatment.
• High soil moisture and nitrogen levels that contribute to rank, dense foliage and
• Plants exhibiting secondary growth (re-growth) following a "cutout" period.
• Improper calibration and poor spray coverage.
Crop maturity determination is critical for a successful harvest-aid program. Premature crop termination reduces lint yield, seed quality, micronaire, and fiber strength.
Desiccants generally abruptly terminate fiber and plant development. Harvest-aid chemicals cannot increase the rate of fiber development. Only additional good growing weather including open skies and adequate heat units combined with functional leaves can mature cotton bolls.
Maturity can be determined by using a sharp knife to cut into the bolls. If the boll is watery or jelly-like on the inside, it is immature and needs more heat units. If boll development is such that the knife cannot slice through the lint, the boll is nearly mature. Close inspection of the seed will give further indication of boll maturity. If the seed coat is turning tan and the seed leaves (or cotyledons) are fully developed, the boll is mature.
When determining boll maturity of adjacent fruit, one can consider the following: When moving up the plant from a first position boll that has just cracked to a first position unopened boll on the next fruiting branch, about 60 additional heat units (DD60s) are required to obtain similar boll maturity.
If moving out from a first position boll to a second position boll on the same fruiting branch, about 120 heat units will be required to reach the same level of maturity. For an individual boll, a total of about 800 to 850 heat units are required after pollination to produce normal size and quality. However, bolls obtaining fewer heat units may still make productive lint of lower micronaire that may contribute to final yield.
Nodes above cracked boll (NACB) can be used to time harvest aid application. A Beltwide cotton harvest aid project was conducted over multiple sites determined that if the uppermost first position-cracked boll is within three nodes of the uppermost harvestable first position boll no lint weight will be lost if a defoliant-type harvest aid is applied at that time. However, if the uppermost harvestable first position boll is four or more nodes above the uppermost first position cracked boll, then potential for some lint loss exists.
Lint loss potential increases as the NACB increases. Micronaire reduction generally follows a similar pattern when using the nodes above cracked boll criterion. When defoliant type chemicals are applied, some slight subsequent fiber development may occur before defoliation. If applying desiccants, more bolls must be mature in order to reduce the risk of fiber weight loss or reduction of micronaire, thus two to three NACB would be a better target.
Chemical selection decisions for stripper harvested and picker harvested cotton will vary. For stripper type, lower yielding cotton (generally less than 500 pounds per acre lint yield) a paraquat-based desiccant should be considered because of reduced expense. Two applications of a low initial rate followed by a higher rate may be appropriate. If the plants are large and have considerable green leaves remaining, sequential applications of low rates of desiccants are sometimes used to promote defoliation and reduce leaf sticking.
Use of paraquat-based desiccants should be discouraged when seedling wheat or other crop species are in close proximity to targeted cotton fields. Drift from paraquat can cause severe damage to developing small grains plants grown for cover or harvest. Unlike paraquat, drift from desiccant rates of PPO inhibitor products (such as Aim, Blizzard, ET and perhaps Resource) should not injure small grains.
For cotton yielding in excess of one bale per acre, growers may justify other chemicals and the higher cost. Ethephon-based products result in an increased rate of boll opening and defoliation that generally reaches a maximum within 14 days. Tank mixes of ethephon and defoliants (such as Def or Ginstar) are effective in higher yielding cotton to open bolls and drop leaves. Warm temperatures (80 degrees F) are normally required to obtain the maximum boll opening response, although higher rates of ethephon can still be effective under cooler temperature conditions.
Finish 6 Pro has 6 pounds of ethephon per gallon combined with a proprietary synergist cyclanilide (0.375 pounds per gallon). Cyclanilide is reported to be an effective inhibitor of auxin transport and binding, which should result in increased abscission activity. In order to obtain desirable levels of defoliation with Finish 6 Pro, tank mixes with defoliants are often required.
FirstPick is another ethephon-based product (2.28 pounds of ethephon per gallon or 18.3 percent active ingredient) with a synergist identified as urea sulfate (58.6 percent a.i.).
Sixteen to 21 ounces per acre of ethephon (when using the 6 pounds per gallon product, equivalent to 0.75 to 1 pound per acre a.i.) when tank mixed with low rates (3-5 ounces per acre) of Ginstar typically result in good defoliation, boll opening response and in many instances good re-growth control. Ginstar is a good defoliant that is also one of the most effective products for controlling re-growth, and it works over a fairly wide range of environmental conditions. Tank mixes of ethephon and Ginstar are fairly expensive, and can be used for boll opening and defoliation of cotton with higher yield potential.
When boll openers and defoliants are used, a follow-up application of paraquat (or other product with desiccant activity) is often required to condition the cotton for stripper harvest in the High Plains region. Although this adds more expense to the overall harvest-aid program, it is sometimes necessary in order to complete the season-long earliness investment the producer has made.
For high yielding, picker-type varieties, spindle picking may be a good option for some producers. Some recent trials indicate that micronaire values of harvested lint may increase by about 0.3 units when spindle picked versus stripper harvested. Harvest efficiency may be somewhat lower with spindle picking, but many fiber properties and gin turnout are generally improved when harvested in a timely manner.
Seed cotton remaining in the field after spindle picker harvesting is generally of poor quality, including low micronaire. Selecting harvest aid chemicals for picker harvesting is similar to selecting for stripper harvest in higher yielding cotton. Differences, however, do exist and include reduced necessity to remove all green leaves from the plant and eliminate the need for sequential applications of paraquat for crop conditioning. Some immature unopened bolls may not be a concern, as these bolls will most likely not make it to the harvester basket and those that do could contribute to lower micronaire.
When spindle picking high yielding cotton, growers can justify greater expense for harvest aids because of greater returns.
Rapid boll opening and defoliation are the objectives when considering harvest aid chemicals for spindle picking. This will allow quicker harvesting with reduced risk from High Plains meteorological events. Tank mixes of ethephon (including enhanced ethephon products such as Finish 6 Pro and FirstPick) and defoliants (such as Def or Ginstar) are effective in higher yielding cotton to hasten boll opening and drop leaves.
Excerpted from a report by Dr. Randy Boman, Extension Agronomist-Cotton;
Dr. Mark Kelley, Extension Program Specialist-Cotton, Texas AgriLife Extension Service;
Dr. Wayne Keeling, Systems Agronomist, Texas AgriLife Research;
Dr. John Wanjura, Agricultural Engineer, USDA-ARS; Lubbock, Texas and Dr. Todd Baughman, Extension Agronomist, Texas AgriLife Extension Service, Vernon, Texas