Much of the information in this section is adapted from publications included in the 2007 Cotton Resource DVD, available for on-line viewing at http://lubbock.tamu.edu/cottondvd.
Cotton Plant Development
Producing a high yielding cotton crop begins with establishing a good stand. Planting high quality cottonseed is essential. With the current price of most cotton varieties and the increased use of new planting equipment, many producers are reducing seeding rates. Consequently, this places even more importance on planting high quality seed. Cotton seedlings often encounter multiple adverse stress conditions at the onset of the growing season. While high seedling vigor may not mitigate all of these factors, it can definitely help. One of the methods to determine vigor is through the use of the Cool-Warm Vigor Index (CWVI). Information provided by the CWVI test is not required by law; therefore it does not appear on the seed tag. However, some companies perform the Texas Cool Test (which is one of the components of the CWVI), and will provide this information upon request.
In order to achieve good germination and early-season vigor, it is important to plant at an adequate soil temperature. Cotton seed germination is favored by high soil oxygen concentration, adequate moisture, and soil temperatures above 64° F. At temperatures above 64 degrees F at seed level, a cotton seed will require more than 100 hours to emerge. The optimal planting target is to have a 10-day average soil temperature of 65 degrees F at the 8-inch depth. If poor quality seed is planted, then 70 degrees F may be a better target. At a minimum, soil temperatures in the seed and root zone should exceed 60 degrees F and the five day forecast for daytime maximum temperatures should be above 50 degrees F. Emergence will generally occur after accumulation of 50-80 DD60 heat units after planting (see page ??? for heat unit calculation).
The cotton plant has a primary tap root with many branches, called lateral roots. Initially, the tap root will grow downward about 9 inches before branching, which occurs about the time the cotyledon leaves unfurl. Roots grow most rapidly when there is adequate, but not excessive moisture. Other factors that inhibit optimal root growth include: seedling cold temperatures, seedling disease, nematodes, wind, thrips and wireworms. Minimizing the impact of these factors will hasten root growth and help get the plant off to a good start.
The vegetative or non-fruiting portion of the plant produces the energy that sustains development and feeds the fruit. The first structures that unfold upon emergence are the cotyledon leaves (referred to as node zero). These leaves are kidney shaped and occur directly across from one another; the meristematic terminal arise between these leaves. As the terminal grows, alternating new leaves will arises from new nodes which are numbered from bottom to top consecutively. These are the true leaves, and the stem area between the nodes is termed the internodes. A cotton plant will typically develop a new internode every 2.5-3 days. Short internodes indicate the occurrence of a stress factor, while long internodes are indicative of a rapidly growing plant. Along with the true leaves, branches will develop from buds at the base of the mainstem. Early branches are vegetative branches (monopodia) and are essentially equivalent to secondary plants that will produce secondary fruiting branches, but mainly produce leaves. However, fruiting on these branches will increase when plants number less than 3 per row-ft.
Maximizing early vegetative growth is essential to getting the plant off to a good start, and this includes preserving the integrity of early leaves and the terminal growth. However, cotton can withstand as much as 75% defoliation before the third true leaf stage without impacting yield, although maturity may be delayed. Factors that inhibit early leaf development include: cold temperatures, wind or hail damage, and foliage feeding insects. Root damaging organisms can also stunt early leaf development. Maintaining the main stem terminal is important for proper plant structure. Loss of the main stem terminal to insects, hail, or wind will result in cotton with multiple terminals, term “crazy cotton”, which results in delayed maturity and yield reduction.
As the season progresses, excessive vegetative growth is undesirable. Tall vegetative plants are more difficult to harvest, and adequate insecticide coverage is more difficult to achieve. To provide the best plant structure and leaf area, final plant height should be 30-35 inches for cotton on 30-inch rows and 40-45 inches for 40-inch rows. Plant growth regulators are often used to regulate vegetative growth.
Depending on the variety, fruiting branches (sympodia) will typically begin at nodes 6-10. A majority of the cotton plant’s yield component comes from fruiting
branches. Depending on temperature and geographical location, the first square will appear 35-47 days after planting. New fruiting branches appear in the terminal tissue with the appearance of small or pinhead sized squares (one-eight inch diameter). The square is the triangular shaped flower bud. New fruiting branches occur approximately every 2.5-3 days. The squares will progress in size to stages referred to as matchhead, one-third grown square, one-half grown, and then candle (when the bloom is still wrapped, but visually protruding). It takes approximately 21 days for a pinhead size square to develop into a bloom depending on heat unit accumulation. The bloom will start as a white flower, turning pink and closing the next day. It will then dry up and fall from the plant in 3-7 days, exposing the boll. Sometime the dried bloom remains on the bloom for several day, this is termed a tagged bloom.
Square and small boll shedding by the plant can occur due to a number of factors including: insufficient sun shine, plant crowding, deficit or excessive water, high temperatures, insects and disease. Some pre-bloom square loss under good growing conditions can often be compensated by the plant without affecting yield. However, under short-season conditions such as unusually cool late-season temperatures, yield and/or lint quality may be compromised. By the third week of squaring, it is desirable for the cotton plant to retain at least 75% of its squares. For bolls, normally a plant will retain a boll once it survives 10-14 days after bloom.
The boll maturation period from white flower to a fully mature boll requires about 850 DD60s. Accumulation of 750 DD60s will produce an acceptable boll, although these bolls may be slightly lighter and may suffer some fiber quality issues. Reaching 750 DD60s may require as little as 40 days, or as much as 70 days.
Monitoring Cotton Growth and Fruiting
Early fruiting is desirable and facilitates early crop maturity. Frequent monitoring gives a good indication of crop set. Problem fields often can be detected early if growth and fruiting habits are accurately monitored, although the cause of a problem may not be immediately evident.
To monitor fruiting levels, mark a point on a row of plants and count 100 consecutive 1/3 -grown (1/4 inch in diameter) or larger green squares; record the number of row feet required to gain that count. Additionally, record the number of bolls observed. Later in the season, when bolls are more numerous, count 100 consecutive bolls, both green and open, and record the number of row feet required to make the count. To estimate the number of squares or bolls present per acre, divide the number of row feet recorded to gain a count of 100 consecutive squares or bolls into the number of row feet per acre (13,068 for 40-inch rows and 13,756 for 38-inch rows) and multiply by 100:
squares per acre
|=||Enter value for ft in a linear row acre based on row width [13,068 (40-inch row), 13,756 (38-inch row), 14,520 (36-inch rows) or 17,424 (30-inch rows)]
|No. row feet recorded for 100 consecutive bolls or squares|
To monitor the squaring rate, count fruiting sites and all squares on at least ten plants from each of four representative areas in the field. Fruiting usually begins on nodes 6 to 10 depending on variety and environmental conditions. Node 1 is the first true leaf above the cotyle-dons (seed leaves). To calculate percent square set, divide the number of small squares counted by the number of fruiting sites recorded and multiply by 100. In normal fields, 75 percent or more of the small squares are retained during the first 3 to 4 weeks of squaring.
After moisture, the most important factor in development of squares and bolls is temperature. Researchers have devised a way to describe and measure the relationship between cotton development and temperature — the heat unit concept or DD60 (degree days using 60 degrees F). Heat units measure the amount of useful heat energy a cotton plant accumulates each day, each month, and for the season. The plant must accumulate a specified level of heat units to reach each development stage and to achieve complete physiological maturity.
Several systems have been developed to calculate heat units, but the most universal approach is to use the formula:
DD60 = ((Degrees F Maximum + Degrees F Minimum) / 2) – 60
Knowing when a cotton crop is near cutout can help producers make effective end-of-season decisions. To estimate cutout, monitor the number of nodes above white flower (NAWF) during the bloom period. To determine NAWF, count the number of nodes above the upper most first position white flower on a cotton plant. The last node counted on a plant will have a leaf equal to the size of a quarter.
NAWF will range from 5 to 10 at first bloom, depending on the amount of soil moisture available to the plant before bloom. Other factors affecting NAWF include soil compaction, diseases and fruit retention.
When the average NAWF value of 5 is reached, the field is considered to be cut out. However, keep in mind that a crop may hover at 5 NAWF for several weeks, before true cutout occurs. The flowers produced after NAWF is equal to 5 contribute less to yield because the bolls are smaller and boll retention is reduced.
Once the date of cutout (NAWF = 5) has been reached, growers can determine when to cease insecticide applications for the season by calculating the daily heat units (DD60s) from cutout. The termination of insecticide applications depend on the insect pest and the number of DD60s that have accumulated.
Fields that have accumulated 350 DD60s are safe from plant bugs (Lygus and Creontiades species), boll weevils and first and second instar bollworm/tobacco budworm larvae; and fields accumulating 450 DD60s are safe from stink bugs.
COTMAN: Cotton Management Expert System Software
COTMAN uses cotton crop monitoring techniques to summarize crop developmental status, detect stress, and assist with in-season and end of season management decisions. It was developed by the University of Arkansas Division of Agriculture with major financial support from Cotton Incorporated and through collaboration with colleagues at Mississippi State University, Louisiana State University, Texas A & M University, Virginia Tech and Arkansas State University. COTMAN goals are to promote earliness and plant vigor, and to reduce late-season insecticide applications.
Overview of the COTMAN System of Plant Monitoring
COTMAN is a crop information system based on in-season plant monitoring. The COTMAN computer software makes it easy to enter and generate reports used to make management decisions. It is divided into two parts:
SQUAREMAN is used to monitor crop development up to time of first flower. Reports provide feedback on square retention and plant stress.
BOLLMAN is used when the crop is flowering to monitor boll-loading stress and to assist with end-of-season crop termination decisions. BOLLMAN utilizes NAWF data. A non-computer version of BOLLMAN is available.
The benefits of using COTMAN include:
Better Information Means Better Decision-Making
- Each field has its own report.
- Square sheds alert growers to pest problems and augment insect scouting reports.
- A quick comparison to the target development curve indicates if crop pace is too fast or right on track for a high and early yield.
- Crop curves can indicate if stress is occurring.
- Takes out end-of-season guess-work
- When bolls are safe from fruit-feeding insects.
- When to defoliate for optimum yields and quality.
COTMAN is Easy to Use
- At first squares, plant stand counts and average first fruiting node are recorded.
- From first squares to first flowers, ten plants at each of four sites per field are monitored weekly for the presence of absence of first-position squares.
- From first flowers until cutout, nodes-above-white-flower (NAWF) counts are recorded weekly from ten plants at each of four sites per field.
- From cutout until defoliation, daily high and low temperatures are recoded from a local weather source.
COTMAN Provides Timely Information
- Plant and fruit numbers per acre.
- Graph of crop developmental pace.
- Flowering date of the last effective boll population.
- Dates when bolls are safe from damage by fruit-feeding insects such as boll weevils, tobacco budworms, bollworms, stinkbugs, lygus and green mirids.
- Dates when crop is ready for defoliation.
COTMAN is Profitable
- The cost of full-season crop monitoring is more than offset by savings on the late-season insecticide
- Timely feedback on crop development pace and stress give growers potential to…
- Take prompt corrective action.
- Integrate management systems.
- Meet objective of overall profitability.
To obtain a copy of the COTMAN software or for more information visit: http://cotman.tamu.edu.