Integrated Crop Management

Water Management

Cotton's water requirement is determined by the location and environment where it is being grown.  The dryer and hotter the environment, the more water the plant requires.  A desert-like environment with high temperatures and low humidity will result in high water requirements ranging from 40 to 50 inches of water per year.  A more humid and temperate environment often results in lower water requirements anywhere between 20 to 30 inches.

Cotton is a drought-tolerant crop and in many parts of the Cotton Belt where summer precipitation is adequate, it can be grown without supplemental irrigation.  In more arid regions, where irrigation is required to make the crop, growers have several application methods to choose from depending on their location and cultural methods available.  The most common irrigation method is flood irrigation where water is diverted down furrows or the entire surface area is flooded.  Other methods include sprinklers and subsurface drip systems.  Low pressure drip irrigation systems can provide an economic alternative to traditional subsurface drip and conventional irrigation systems (Robertson et al., 2007c).  Water quantity, quality and drainage are important considerations in determining the best method to irrigate cotton.  In some regions and years, having the ability to remove or drain surface water from fields is as important to maintaining high yield and fiber quality as adding water through irrigation.  In arid regions where water quantity and availability is limited more reliance on sprinkler and drip irrigation systems are utilized.

 Having an adequate supply of moisture is critical to establishing and maintaining high yield and quality potential (Table 1).  Avoidance of water-deficit stress, beginning at first square, is critical to establishing adequate plant structure and fruiting forms to set high yield and quality potentials, especially with early-maturing varieties grown in locations with a limited growing season.  Being at or near field capacity at early bloom and maintaining adequate water supplies at least through cutout is recommended.  This requires constant monitoring of crop water use and soil moisture conditions, and irrigating before the crop stresses to maintain high yield and fiber potentials.  A soil profile full of moisture at first open boll will often meet the water requirements necessary to mature the crop (Gerik et al., 1996).

Table 1. Estimated impact of severe deficit moisture stress on fruit and fiber development (Hake and Grimes, 2010).

Fruit Stage	Retention	Impact on: Fiber Quality	Yield
Presquare	Minimal	Minimal	Minimal
Square initiation	Moderate	Minimal	Loss - Fewer and smaller bolls
Boll – 0 to 30 days of age	Severe	Severe	Loss - Short staple and high micronaire
Boll – 31 to 60 days of age	Minimal	Moderate	Loss - Immature fiber
Boll opening	None	Minimal	Hasten maturity

Moisture stress resulting from the lack of or an excess of water early in the growing season will restrict root and crop development (Pace et al. 1999). Cotton is particularly sensitive to moisture stress (deficit) just prior to and during squaring through the end of the effective flowering window.  Abrupt changes in soil moisture will adversely affect growth and cause fruit shed.  Severe water deficits can essentially stop terminal growth.  Growth, both vegetative and reproductive, will resume as moisture levels improve.  The shed of fruit as a result of stress will not usually occur until growth resumes as abscission is an active process (Voloudakis et al. 2002).  Although growth starts again after severe water deficits, a significant delay in maturity can occur as square production must resume replacing shed fruiting forms (Pettigrew, 2004, Karam et al. 2006).  Bolls greater than tens days of age generally do not shed as a result of stress.  Time requirements from square initiation to a ten-day old boll can exceed four weeks.  Square loss during this fruiting period can significantly reduce yield potential or increase the length of time necessary to produce desired yields.  Delays generally result in higher production costs due to the extended fruiting period.

Irrigation scheduling

Using local guidelines for irrigation scheduling is recommended as the foundation for making important decisions for initiating, scheduling and terminating irrigation.  Irrigation initiation and termination decisions are difficult and can greatly impact yield and quality in a positive or negative manner.  Irrigation scheduling guidelines often include criteria based on the soil moisture as a result of actual measurements.  More subjective soil moisture evaluations can be derived from the look and feel of the soil.  Observations of the cotton plant will reveal that a change in leaf color toward a slight bluish tinge occurs before wilting.  In the drier spots in the field, the color appears somewhat darker than the remainder of the field.  These spots can be used year to year as a diagnostic indicator to initiate irrigation.  However, these spots don’t often provide a great deal of advanced warning that yield limiting moisture stress is imminent.  Bookkeeping methods are utilized to keep a running inventory of plant available water by adding effective rainfall and irrigation to account for water entering the soil profile and subtracting soil water losses.  Plant water use based on daily high and low temperatures and estimated evapotranspiration rates is determined by crop growth and other factors accounting for water leaving  the soil profile.  Computer programs allow producers to input data from various measurements to schedule irrigation based on plant, soil and environmental data generally collected onsite.

The thresholds in which irrigation is recommended and amounts to apply will vary during the season based on factors including crop growth stage, environmental conditions, soil characteristics, water quality and quantity.  The plant’s water use and the sensitivity to stress in maintaining high yield and quality potentials vary throughout the growing season.  Monitoring plant development, particularly nodal development from the appearance of the first true leaf to first flower, can be useful in providing feedback to the effectiveness of irrigation scheduling as nodal development is very sensitive to moisture stress.  The increasing demands for energy by the developing bolls significantly impact nodal development rates.  This change causes observations of nodal development rates to be much less reliable for providing feedback for irrigation scheduling after the onset of flowering.  Nodes above white flower measurements reveal much about the levels of stress the plant is experiencing and can provide a benchmark from which irrigation termination can be made.  However, this measure does not differentiate between good stress (heavy boll load) and bad stress (deficit soil moisture stress).  Nonetheless, monitoring plant development is useful in evaluating the effectiveness of irrigation scheduling.  Knowledge of boll retention rates coupled with how the plant is responding vegetatively to reproductive stress can provide the producer a better understanding of the current status and future needs of the plant (Robertson et al., 2008).