Harvest management is more than applying harvest aid products. Fertility, water management and weed control play an important role in the success of a harvest aid program. The goal should be to supply the crop with adequate fertility and moisture levels to meet realistic yield goals. Any excessive fertility or moisture remaining in the soil at the end of the season, beyond what is necessary to maintain an active plant, represents more potential costs or inputs the producer could have avoided. Excess fertility and moisture coupled with favorable temperatures for plant growth at the end of the season will complicate harvest aid programs as well as increase costs of both producing the crop as well as preparing it for harvest (Gwathmey et al., 2001). On the other hand, failure to meet the plants nutritional or moisture needs will negatively impact yield potential and plant activity. The success of a harvest aid program is highly dependent on having adequate plant activity as leaf defoliation and boll opening is an active process. Inadequate weed control programs will present challenges with harvest and possibly impact fiber quality. This is in addition to loss of yield potential as a result of weed competition during the season. Having relatively weed-free fields at the end of the season coupled with low residual fertility and moisture levels will help lower production costs while providing great flexibility in designing economical harvest aid programs to help maintain existing yield and fiber quality (Supak et al., 2001).
Harvest aid programs often include the use of compounds that result in leaf defoliation, boll opening or tissue desiccation. Components of a harvest aid prescription are often dependent of the production strategy and harvest method employed. A universal goal of cotton producers is to harvest as early as possible to minimize environmental risks without sacrificing lint yield or fiber quality. Guidelines for harvest aid products, rates and timing can often be obtained locally. These guidelines generally take normally expected plant status and environmental conditions for a specific region into account in developing recommendations. Products and use rates can vary greatly from region to region and even within a region or country based on grower preference and experience. However, the basic concepts apply to improve efficacy of pre-harvest preparations prior to harvest.
The two primary categories of harvest aid products, hormonal and herbicidal, are based on their mode of action. These products provide a broad range of results and are often tank mixed. Hormonal materials generally fall into the categories of defoliants or boll openers. As a general rule, these tend to be more temperature sensitive than herbicidal defoliants. Increasing rates of herbicidal products, in turn, makes them desiccants. Select products and rates appropriate to the local field environment and crop condition. Previous experience and confidence in treatments should be a major factor in determining choice. Consider weather forecasts when selecting treatments. Some products are more temperature sensitive than others. The yield and fiber quality potential of the field will often dictate the harvest aid budget.
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 does not. Thus, field evaluations involving boll opening can sometimes greatly underestimate maturity. Perhaps the most reliable method of determining boll maturity is to slice open bolls with a knife (Fig. 1). 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.
Figure 1. Boll slicing of first position bolls beginning with a mature cracked boll on the far right to the least mature boll in this image on the far left (NCC, 2007).
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 in an effort to time applications. 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. A heat unit concept of timing defoliation beyond the last effective boll population as defined by COTMAN would allow producers to make this decision with greater confidence as it is much less subjective than other measures of maturity and possibly allow for an earlier harvest (Fig. 2) (Gwathmey et al., 2001; Helms et al., 2007).
Figure 2. Identification of cutout based on boll retention rates and number of flowers a producer must protect to produce a pound of seed cotton (Bourland, 1992).
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 12 to 15 gallons per acre by ground are typical recommendations to ensure good coverage. Coverage also depends on 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. Medium-sized droplets are generally recommended (Fig 3). Increased spray volumes help enhance coverage thus improving defoliation, especially on rank plants with lush foliage (Bader et al., 2001).
Figure 3. Water sensitive spray cards illustrating droplet size and coverage of various spray tips, spray volumes, and pressures (NCC, 2007).
Harvest aid applications should be coordinated with harvest progress. Applications should be timed in a manner so fields will be harvest-ready to meet a defined schedule. It is common for producers who mechanically pick cotton to treat only as much acreage as can be harvested in 7 to 12 days. Early treatment of excess acreage can decrease yields, expose lint to weather and increase the likelihood of significant regrowth. Moisture entering the boll just as it begins to crack open can allow fungal pathogens to enter the boll colonizing on the locks of seedcotton. These locks often fail to fully open and are referred to collectively as a hardlock boll. Boll openers promote and synchronize the opening of bolls regardless of boll age or maturity. Cotton bolls are much more tolerant to wind, rain or other forces which can cause seedcotton to fall to the ground when the leaves are on the plant.
Figure 4. A typical hardlock boll. Seed and fiber quality of a hard lock boll is generally poor
Producers who mechanically strip cotton sometimes have the option to wait on a killing frost to desiccate plant tissue as opposed to using chemical desiccants. Stripper harvest of cotton requires that minimal green tissue be present in the field for efficient operation of the harvester.
Harvest aid products generally work better on mature cotton under warm, humid, sunny conditions. Cool temperatures at the time of application or immediately afterward can retard the activity of defoliants and boll openers often resulting in less than desirable results. If possible, harvest aids should not be applied during or immediately preceding a significant cooling or drop in temperature. More desirable results are often obtained if treatment is delayed until temperatures are allowed to warm or stabilize for at least three to four days (Gwathmey et al., 2001).
Defoliation is not always required for mechanical picking. Cotton that is completely cutout with “tough” leaves and little regrowth present may not need defoliation if harvested and ginned quickly. In this situation, it is important not to pick too early or late in the day as green leaf in the seedcotton provides an additional source of moisture. It is also recommended that the producer contact the gin prior to harvest to ensure that timely ginning of the seedcotton can be achieved. Ginners can often make adjustments in the ginning process to help preserve fiber quality if they are aware of issues that may require special attention (Roberts et al., 1996).