Cotton has been part of the fabric of human existence for thousands of years. This crop’s long and colorful past has shaped world history as countries expanded its production to fuel demands of the industrial revolution. The diverse and challenging conditions of cultivating this crop have provided a strong thread to the modernization of farm equipment and practices shared today by many other important commodities. The uniqueness and diversity of cotton ensures this crop’s enduring importance and consistency in world markets well into the 21st Century.
Cotton not only produces the natural fibers used in textiles and clothing, but also yields a high grade vegetable oil, multiple cellulosic byproducts, and whole seeds used as a primary source of fiber and protein in animal rations. Botanically, cotton is a perennial shrub and in tropical regions, cotton grows year around. In more temperate regions such as the US Cotton Belt, cotton is grown and managed as an annual crop (Fig. 1). There are two major types of cotton grown that meet the needs of global fiber markets. The most extensively grown types are the “Upland” cottons (Gossypium hirsutum). The second group is the Extra Long Staple (ELS) types (Gossypium barbadense) also know as Pima, Sea Island and Egyptian cottons. Upland type cottons are more adaptable to growing conditions, whereas the ELS types are associated with production areas which have longer growing seasons. In the United States, ELS production has been predominantly in the irrigated western states of Texas, New Mexico, Arizona and California.
The perennial nature of cotton allows producers to manipulate its growth and development to optimize seed and fiber production. This basic principle applies to all cotton producers regardless of their location and the production strategies or technologies utilized. Strategies used to manipulate the crop can vary greatly and often allow the producer to be adaptive to local and regional conditions. However, some circumstances force decisions that can limit a producer’s options. Certain decisions, besides the basics of seed selection and planting sites, must be effectively evaluated and addressed. Growers should carefully consider prices (current and future), seasonal water availability, nutrient requirements, pest control options, harvest and ginning as major production components. A good assessment of these steps prior to planting will greatly enhance the success for a given season. Errors or misjudgments in these key decisions will linger the entire season and limit potential yields (Hake et al., 1996).
Figure 1. The basic structure of a cotton plant includes the main stem, which is made up of a series of nodes and internodes, and two types of branches, vegetative and fruiting branches (NCC, 1996).
The season can be divided into specific phases each offering different management challenges that can impact subsequent growth and final results (Fig. 2). Early-season phase is characterized by planting conditions, which is extremely important in establishing the stand. This phase represents early seedling and root growth and is entirely vegetative. Next is the reproductive phase that begins with the initiation of fruiting structures, called squares that develop into blossoms or flowers and then into bolls. This phase normally begins 35 days after planting.
Figure 2. Seasonal development of cotton in the Mid-South with a May 1 planting date, showing typical production patterns of squares, bolls and open bolls (Oosterhuis, 1990, with permission ASA).
The reproductive phase is influenced by accumulating boll load as squares develop into mature bolls. Maturing bolls have a strong demand for photosynthate and they compete directly with vegetative growth. Important phenological distinctions that occur during this development period are “first flower”, “peak bloom” and “cutout”. First flower is determined when over 25 percent of the field’s plant population has a first position bloom. Peak bloom occurs approximately 20 to 30 days following first flower and represents the stage of growth where the plant is flowering at first, second and even third fruiting positions on the main stem branches making up the top third of the plant. Cutout is the point following peak bloom, where the plant’s photosynthic energy, going into developing bolls, exceeds that necessary to maintain vegetative growth. Following cutout, most of the plants energy is directed to maturing bolls (Kerby and Hake, 1996). The final late-season phase is characterized with the opening of mature bolls and the crop being prepared for harvest. This is often facilitated with the use of harvest aid products which enhance leaf drop and boll opening.
Establishing a stand and getting the crop off to a good start can be challenging during the early-season when above-ground growth may get off to a slow beginning. This slow establishment makes cotton a poor competitor through much of the early vegetative growth stage. However, with favorable growing conditions, vegetative growth can become excessive as the plant begins to square if growth is not managed. Maintaining the proper balance between vegetative and reproductive growth is essential for high yields, especially in situations where the length of the growing season limits production. Plant monitoring and field scouting the entire season is essential to ensuring management strategies are implemented in a timely manner (Landivar and Benedict, 1996; Oosterhuis et al., 2008). Failure to accomplish the execution of many cultural practices by as little as two or three days can make the difference between a great cotton crop and a good one. During the reproductive phase it is important to maintain good square retention and vegetative growth in order to develop the plant’s structure necessary to achieve optimum yield goals. At first flower, a common management goal is to have first position square retentions above 80 percent and nine to ten nodes above first position white flower (NAWF) (Robertson et al., 2008). Properly managing early square retentions at this level and potential reproductive nodal development has been closely associated with higher yields at the end of the season (Mauney, 1986; Kerby and Hake, 1996,). Square retention values prior to first flower are generally most impacted by insects (Leigh et al., 1988). Plant squaring and nodal development which contributes to NAWF, prior to flowering is negatively impacted by stress. Soil fertility, moisture, and early-season pest damage are generally the dominant stress factors impacting plant structure prior to flowering (Kerby and Hake 1996; Roberts and Rechel, 1996). Square retention values less than 80 percent at first flower can often result in delayed maturity and excessive vegetative growth due to the lack of adequate fruiting forms during boll development. Boll weevil eradication efforts and insect-related transgenic technologies in some regions have helped to reduce the occurrence of low retention rates throughout squaring as well as into the flowering cycle. Retention rates of 90 percent or greater can present logistical challenges to producers because margins of error for input requirements are small. High retention values coupled with poor plant structure can result in premature cutout, which significantly impacts potential yields (Robertson et al., 2008). Physiological cutout is the condition where the plant’s total photosynthetic production is being allocated to developing bolls and vegetative growth temporarily stops or slows significantly. Square loss as a result of environmental stress can be extenuated in situations where retention rates are very high (Mauney, 1986).
Managing inputs to achieve nine to ten NAWF at first flower will result in the plant having the growth capacity to avoid premature cutout in most instances (Oosterhuis et al., 2008). Fields in which NAWF values are in a range of six to seven often require more immediate action to alleviate stress to avoid premature cutout. These NAWF differences can be translated into 12 to 15 days of mid-season growth (Constable, 1991; Kerby and Hake, 1996). To optimize yields, high retention values will magnify the urgency to relieve the stress in this situation. As a rule, early or more determinate varieties are more sensitive to having adequate growth capacity or “horsepower” at first flower to achieve desired yield potential than later maturing varieties. Being on track at first flower, or taking corrective actions to get back in line shortly thereafter, is necessary to achieving high yield goals. Beginning at first flower, NAWF counts recorded weekly can help establish the last effective boll population or the last group of bolls that will contribute significantly to yield and profit (Fig. 3) (Bourland et al., 1992: Oosterhuis et al., 2008). Identifying the last effective boll population is essential for making end-of-season decisions. Cutout is reached when NAWF counts become less than five or when the probability of accumulating sufficient heat units to mature a flower falls below a user defined threshold (Kerby et al., 1987). Crop termination guidelines may be keyed on heat unit accumulation beyond cutout based on when bolls can be considered safe from insect damage and when terminating irrigation and the initiation of harvest aids do not significantly impact yield and quality (Helms et al., 2007; Leonard et al., 2008). It is vital for producers to continually strive to stay current with the latest research concerning the growth and development of cotton to better understand and predict the needs of the plant to produce seed and fiber more efficiently and profitably.
Figure 3. Identification of cutout (NAWF=5) based on boll retention rates and number of flowers a producer must protect to produce a pound of seed cotton (Bourland, 1992).