Meeting the basic needs of the cotton plant is the goal of producers regardless of their location or the production strategies employed. Getting the crop off to a good start is an important step in managing the crop. However, a good start does not guarantee high yields or profitability. A timely, well managed approach to meeting the in-season needs of the crop can help overcome a less than optimal start and help improve yield potential.
Some cotton growing regions present greater challenges to the producer and crop than others. These challenges include, but are not limited to, one or a combination of pests, soil structure, soil chemistry and environmental conditions. Regardless of the nature of the challenges, the crop’s response to the growing conditions becomes apparent and often interpreted by evaluation of the roots, rate of node production, internode length, size of fruit abscission scars and fruit present on the plant at the end of the season.
Producers can benefit from guidelines for in-season management to establish criteria for making decisions. Expert recommendations of best management practices such as those found in the publication, “The First 40 Days™ and Fruiting to Finish™” can be a valuable resource for cotton producers (National Cotton Council of America, 2007). Additionally, guidelines can often help producers evaluate feedback from the plant in response to cultural practices. Successful producers have learned to read the plants and respond to the interactions of practices with one another and with the particular limitations inherent to specific fields.
Monitoring the Crop
By quantifying several growth parameters of the cotton plant, producers can identify potential problems and reconsider their management decisions while there is still time to address issues (Landivar and Benedict, 1996). This management approach requires growers to set aside the time and effort to collect and interpret data. Early approaches to plant mapping were very labor intensive and often required every fruiting site on the plant to be recorded. Plant mapping does not have to be complicated to provide useful information. In fact, the simpler it is, the more likely it will be used.
From emergence to flowering, the producer’s greatest concerns often center on having an acceptable growth rate and adequate fruit retention once squaring begins. After flowering, NAWF values provide producers insight to boll-loading stress and assists with end-of the-season crop termination decisions (Oosterhuis et al., 2008). Plant mapping offers producers the ability to track and summarize this information and identify fields that may need additional attention.
Figure 1. The COTMAN computerized cotton management program instructs users to collect presence or absence of first position squares beginning at first square which generally occurs 35 days after planting. Prior to flowering all fruiting nodes are squaring nodes. At first flower, 60 days after planting, users collect node above white flower (NAWF) data. This data also represents squaring nodes on the plant. Cutout, NAWF=5, generally occurs 80 days after planting. The graph shape of squaring nodes from first square to cutout represents the Target Development Curve of COTMAN (Teague and Danforth, 2008).
A simple approach is to break the growing season down into stages. When mapping cotton it makes sense to break the season into presquare (from emergence to first square), squaring (from first square to first bloom), flowering (from first flower until cutout) and cutout (from cutout to harvest). After dividing the season, it is important to prioritize the parameters to measure during each stage. Experienced field scouts that use plant monitoring as a tool are reluctant to invest time in collecting data that is difficult to interpret or not useful in making management decisions for specific stages. However, when monitoring tools are useful in making more efficient decisions at the farm level, these techniques are readily used by more progressive field scouts. The benefits of monitoring generally outweigh the costs (Hogan et al., 2008).
The number of sites and sampling locations is an important and sometimes overlooked decision. Sample locations for mapping should be representative of the field conditions. The number of sites and the number of plants sampled at each site should be sufficient to obtain reliable data to base management decisions. The objective of plant mapping is different than that of sampling or scouting a field for pests as insect pests may first enter or build to damaging populations in field margins or in areas not always representative of the field. As a result, sample site selection for mapping may need to differ from scouting site locations.
Easily measured plant growth parameters include plant population, plant height and the number of nodes. Additional observations that can be noted are root health, herbicide damage, wind or hail damage and poor drainage areas. Depending on location, more or less factors can be included. These parameters can be measured very quickly. The data can give indications of plant stand, stand uniformity and growth rate or vigor. The rate of plant growth, as measured by the production of new nodes, over time can be a very sensitive measure of stress, which can delay the production of new nodes. Moisture stress is generally the dominant factor impacting plant structure at this stage of growth. Regardless of the cause of stress, further investigation into the cause and potential alleviation of this stress should be undertaken, especially in growing regions where the length of the growing season is limited.
Many measureable parameters related to vegetative and reproductive development exist and can become overwhelming. It is important to consider parameters that can be rapidly and accurately measured. Some of the most important plant parameters to be considered for measurement during this growth stage are plant height, number of nodes, node of first fruiting branch and square retention at the first position. Stress during this stage of growth can be easily detected by evaluating nodal production over time. Plant structure prior to flowering is negatively impacted by stress. Fertility and moisture are generally the dominant factors impacting plant structure prior to flowering (Foshee et al., 1999).
Square retention is not generally impacted by stress during this stage. However, larger than expected squares in the terminal and upper branches is a clear sign of stress and is a signal pointing to a slowdown in terminal growth. Percent retention for squares prior to bloom should remain very high, since squares only need small quantities of carbohydrate for their survival. Physiological square shed generally doesn't occur until the demand for carbohydrate has peaked by growing bolls. Square shed prior to first bloom may be caused by insect feeding. If square loss before bloom is noticed, a closer look at insect pressure is advised.
Excessive vegetative growth can also occur under the most favorable conditions. The length of the upper five internodes can be a direct measure of the current state of the plant as these are the only internodes on the main stem where elongation is occurring. The length of the third internode from the terminal or the combined length of the top five internodes can be used to gauge vigor. Plants in which the third internode exceed 3 to 4 inches or if the top five internodes exceed 7 to 9 inches may be experiencing excessive vegetative growth. These conditions may require the application of a plant growth regulator. Square size can also be a sign of vigorous growth if smaller than expected squares are observed in the first positions of the upper fruiting branches on the plant (Bourland et al., 1992).
During squaring, it is important to maintain good square retention and to develop the plant structure necessary to achieve yield goals. A realistic goal is to achieve a range of square retention from 80 to 85 percent and maintain nine to ten nodes above the first position white flower.
Producers may benefit the greatest from plant monitoring during the flowering stage. During the effective fruiting period, or the time between first flower and cutout, important parameters to monitor include plant height, number of nodes, the number of nodes above the uppermost first position white flower (NAWF), first position squares retained above the white flower and first position bolls retained below the white flower. If detailed records are being collected, then plant height should be taken from the same plants from which nodes and fruit counts are made (Kerby and Hake, 1996; Gwathmey et al., 2004; MSUCARES.com, 2009)
Young bolls, 10 days or less beyond white flower, are very sensitive to physiological shed when carbohydrate supplies are limited (Fig. 2). Factors limiting carbohydrate supplies may include cloudy weather, high temperatures, water stress, leaf damage or a heavy boll load. During this stage, management options should include practices that help the crop maintain an optimum carbohydrate supply to the boll load to ensure best conditions for growth. Avoid problems that can reduce carbohydrate supply by timely irrigation, good fertility, pest control and avoiding any stress that would reduce the canopy size (Holman and Oosterhuis, 1999).
Figure 2. Sensitivity of various fruiting forms to shed (NCC, 1996).
Another major factor in boll retention is varietal sensitivity to heat-stress. Even under irrigated conditions where soil moisture levels are not a limiting factor, high temperatures can adversely affect boll retention due to pollen sterility (Radin et al. 1994, Gerik et al. 1996, Kakani et al. 2005). Breeding for heat stress tolerance has improved the world selection of available cultivars classified as tolerant, moderately tolerant, and susceptible to high temperatures (Liu et al., 2006). Identification of specific heat stress indicators will continue to advance the development of heat tolerant varieties for planting in arid and hot climates (Voloudakis et al. 2002, Kosman et al. 2006, Wu et al. 2007). Even thought the optimum temperatures for pollen germination is defined (Burke et al. 2004) during the flowering period, day and night temperatures are out of the control of cotton producers. In production areas that experience excessive temperatures selection of heat tolerant varieties will be of greater importance (Singh et al. 2007).
Perhaps the simplest single measurement that can provide an overall status of plant well being is NAWF. The NAWF measurement is an indication of the available energy of the plant. The concept of NAWF coupled with a measure of fruit retention, is a powerful tool when conducting plant monitoring and is one of the components of COTMAN, a computerized program for cotton management (Oosterhuis et al., 2008). 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. Identifying the last effective boll population is essential for making end-of-season decisions (Gwathmey et al., 2004).
The first position white flower present at cutout represents the last effective boll population or the last cohort of bolls that will contribute significantly to overall yield and profit. It is this group of bolls that growers should base their decisions for terminating the crop.
Tracking the progression of NAWF values across time can be useful in predicting the date of cutout. Many end of the season termination guidelines are based on heat unit accumulation or DD60s beyond cutout. With COTMAN, historical weather data used in conjunction with projected cutout dates can be very useful in targeting insecticide and irrigation termination dates while also projecting harvest aid application and harvest dates (Robertson et al., 2008). These projections can be updated periodically with actual temperatures to establish more accurate dates for crop termination of harvest aid initiation dates. Having target dates for harvest aid applications and harvest dates can be a very useful planning tool. Careful evaluation of the time needed to harvest the crop in conjunction with target harvest completion dates can help identify a date that harvest must be initiated (Supak et al., 2001).
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).
Evaluation of nodes above cracked boll (NACB) has been developed to help determine harvest aid application timing. This simply tracks the progress of the first position cracked boll relative to that of the uppermost harvestable boll. The percentages of open bolls, NACB, and DD60s beyond cutout are useful tools to time harvest aid applications (Bynum and Cothren, 2008).