By Bill Robertson, Extension Cotton Agronomist
We are quickly approaching the finish line with this cotton crop. It is amazing that many of our fields with a good number of open bolls still have a very active and dark green appearance. This is most likely a result of good soil moisture and sufficient fertility to maintain active growth. While boll maturation is heat unit driven, boll opening is not. This complicates the timing of harvest aid applications.
When to start
There are several ways to determine when to treat cotton with a harvest aid product. An old rule of thumb is to defoliate when 60 percent of the bolls are open. Another method involves counting the nodes above the uppermost first position cracked boll (NACB) and the uppermost first position harvestable boll. When NACB values average four or less, the fields can be defoliated without significant weight or quality loss. Both of these measures of maturity assume a typical level of plant senescence as bolls mature. In situations where conditions for growth are very favorable, plants don’t senescence as rapidly as expected. As a result, the occurrence of boll opening slows while fiber development within the boll continues. Thus, field evaluations involving boll opening can sometimes greatly underestimate maturity by as much as 7 to 10 days.
A heat unit concept of timing defoliation beyond the last effective boll population or cutout as defined by COTMAN allows producers to make this decision with greater confidence as it is much less subjective than other measures of maturity and often allows for an earlier harvest. Initial harvest aid timing of 850 HUs beyond cutout is recommended in Arkansas (Fig. 1.).
Perhaps the most reliable method of determining boll maturity is to slice open bolls with a knife. Mature bolls will be too hard to dent when squeezed and cannot be easily cut with a sharp knife. Lint will string out when a mature is sliced, seed coats will be dark or black in color, and cotyledons will be well formed (Fig. 2.).
Timing of harvest aids can pose a difficult decision to growers since they are often encouraged to use at least two methods to determine maturity of the crop. However, producers are often tempted to wait as long as possible for young immature bolls to open near the top of the plant before defoliating. These last bolls can be insect damaged and often are smaller, which account for little additional yield gains, but the perception of yielding more lint is difficult to overcome.
Avoiding high micronaire discounts
Inconsistencies between boll opening and boll maturity can contribute to undesirable micronaire values for varieties that tend to produce fiber with micronaire values near the discount range. The micronaire estimation method, developed by Dr. Hal Lewis, is an effective tool to avoid discounts from high micronaire values through early defoliation. Preliminary results from this season for varieties that have the potential to produce lint with higher micronaire values indicated that harvest aid initiation should not be delayed beyond 850 HUs for those varieties. Data collected by Dr. Fred Bourland in the Cotton Variety Testing Program are an excellent source to evaluate the micronaire tendencies of varieties (Table 1.). Varieties that have greater micronaire values than the test average should be monitored and managed accordingly.
Harvest aid products generally are not translocated in the plant, therefore coverage is a very important part of the process. Successful defoliation requires uniform canopy coverage. Total spray volumes of 5 to 7 gallons per acre by air or 10 to 15 gallons per acre by ground are typical recommendations to ensure good coverage. Coverage also depends on speed of applicator, spray droplet size, atmospheric conditions and the canopy density. Generally, smaller spray droplets provide better coverage and canopy penetration but are more likely to drift in windy conditions or evaporate in high-temperature, low-humidity conditions. Larger spray droplets experience less drift and evaporation, but provide poor coverage and canopy penetration. A balance must be struck with each application to ensure product delivery to the target while maintaining desired coverage.
A demonstration conducted this season to investigate the interaction of ground application speed, spray volume, and droplet size has provided some useful information. At a speed of 13 mph, coverage and efficacy of products were enhanced using 15 GPA as opposed to 10 GPA as expected. However, the use of a coarse to very coarse spray droplet tended to negate these differences and outperformed the medium or ultra coarse droplet size at both application volumes. If demands on the ground applicator require a decrease from 15 GPA to 10 GPA, the proper tip and pressure to deliver a coarse to very coarse spray droplet is critical to achieve the desired results. Applicators can identify optimal pressure ranges to target droplet sizes by clicking on their nozzle manufacture’s link- Teejet, Hypro, Wilger, Greenleaf, and Deere. Nozzle specific pressure and droplet information can also be provided by contacting Jason Davis, U of A Application Technologist at (501) 749-2077.