5 MIN READ
Irrigation Scheduling for Cotton
January 26, 2022
Cotton Water Demands
Key Points:
- The sensitivity of cotton to water stress varies by growth stage.
- Irrigation may improve cotton emergence and is important to establish a good plant stand, nutrient uptake, prevent water stress induced square and boll shedding, help maintain fiber quality, and support maximum yield potential.
- Once cotton plants emerge, irrigation should be used to supplement rainfall and prevent depletion of the soil moisture profile prior to bloom and throughout most of the growing season.
- First square to first bloom growth stages is a critical time for avoiding drought stress in cotton.
- Cotton irrigation strategy in the arid and semi-arid regions of the West may be significantly different from irrigation strategy in the rain belt.
Cotton is an indeterminate perennial shrub that is somewhat tolerant to drought and soil salinity. Because of its drought adaptations, cotton responds favorably to periods of water stress enough to slow vegetative growth; a physiological feature that can be benefited by timely irrigation management.1 Irrigation is necessary for supplying water in arid and semi-arid regions of the West and helps supplement rainfall in other regions.2 The benefits of supplemental irrigation to reduce soil water deficit are many. The challenge in optimizing irrigation is providing enough water at critical cotton growth stages to supplement rainfall, without causing excessive plant stress from excess soil moisture.3 Regardless of the region, the goal of water management is to meet crop demand and optimize yield with the resources available.2
Irrigating in Arid Regions
Irrigation is the dominant influence on cotton yield in arid climates. Growers who expect to produce top yields must have the capacity to irrigate their entire acreage over 7 to 10 days to allow precise applications of water.2 Pre-irrigation is often necessary in arid regions. This helps provide a soil profile that is full of moisture to the rooting depth, drained of excess surface water, and warmed by the sun prior to planting. Filling the soil profile delays the need for the first post-planting irrigation and helps prevent potential problems such as: evaporation loss of valuable water, plant cooling, nitrate leaching, and potential soil borne pathogen infection.2
Timing of the first post-planting irrigation is critical. Growers must consider well capacity and the irrigation system’s ability to supply enough water to meet crop needs at critical growing stages. Starting too early may lead to chilling injury (when temperatures remain below 80o F)2, potential development of seedling diseases, a shallow root system, weed germination, and rank vegetative growth. Starting too late may lead to stunted plants, early cutout, and reduced yield potential. Cutout is the point when the terminal is five nodes above the uppermost first position white flower.4 Optimum irrigation timing in the West occurs when plants are slowing mainstem growth, but before obvious leaf color changes.2 Once irrigation is started, most growers in arid regions will need to continue supplying water throughout the growing season to prevent loss of yield potential from water stress (Table 1). While water requirements are higher in the West, so are yields.
Table 1. General cotton growth and water use.5
Irrigating in Humid Regions
Use of irrigation has been increasing across the humid areas of the Cotton Belt for the last 20 years.3 While there is a large collection of information for irrigation management related to cotton in arid regions, information specific to management under humid conditions is not as well developed. With rising production costs and the devastating effect of drought on yield, adopting irrigation to supplement rainfall in the humid areas is becoming increasingly essential to help keep a cotton crop profitable. Irrigation offers safeguards against poor crop performance and/or failure due to insufficient and/or untimely rainfall. Being short an inch of water at the wrong time can easily result in the loss of 75 pounds of seed and 50 pounds of fiber.3 In the sandy Coastal Plain soils in the Southeast, irrigation has been shown to nearly double the non-irrigated cotton yield from about 750 to near 1,200 to 1,500 lbs. of lint per acre during water limited years.3 Yearly rainfall in the humid parts of the Cotton Belt is about 45-55 inches, or almost twice the amount needed for seasonal cotton water use. The problem is that the occurrence of rainfall is random, with no guarantee that the right amount will come at the right time in the growing season relative to the crop demands. The risks associated with yield instability can be partially removed by irrigation, which leads to a more predictable season-ending yield (and thus return) year after year.
Planting to First Square
Cotton water use at planting and through emergence is low. If the seedbed is too dry to support germination, pre-planting irrigation is preferable to irrigating after planting. Irrigating shortly after planting can cool the soil and may encourage development of seedling diseases. Once seeds germinate, young cotton plants put much of their energy into developing roots. Unless soil moisture is extremely low at this time, irrigation contributes little to yield. Some water deficits early in the season can stimulate root production and development of a deeper root system, especially in sandy soils.3
First Square to First Flower
Once cotton plants have emerged and are growing, irrigation should be used to supplement rainfall and prevent depleting the soil moisture profile prior to bloom.2 For approximately 21 days from first square to first bloom, cotton vegetative growth is very rapid; the number of fruiting sites is determined (especially in short-season environments), and plants should be rapidly taking up phosphorus (P) and potassium (K) at this growth stage.
First Flower to Peak Bloom
Crop water demand increases from 0.2 to 0.28 inches per day during this period2 Water stress at this growth stage can reduce plant growth, reduce the number of fruiting sites, cause shedding of young bolls, reduce boll size, and result in loss of yield potential. Severe water deficit at this stage can also lead to shorter staple, higher micronaire, and lower cotton lint fiber quality. Lack of water does not typically result in square shed between first flower and peak bloom. If square shedding is observed during this time, other causes should be investigated.3
Peak Bloom to Open Bolls
Water demand begins to decrease as bolls mature and open. While severe water deficit can lead to square and young boll shedding, loss of late fruit has less effect on yield potential than loss of early-season bolls. Growers should reduce or stop irrigation and allow plants to experience some water stress after bolls start opening. This stress reduces regrowth, makes defoliation easier, and encourages boll opening.
Irrigation Strategy
When to initiate irrigation depends largely on how much irrigation water is available and the cost of application. In areas where the irrigation system and rainfall cannot keep up with peak water demand during the summer, irrigation should be started early to avoid depleting the soil moisture profile prior to bloom. Under these conditions, growers may start irrigating as soon as the soil can hold moisture. This strategy may result in overwatered cotton in the spring, possibly requiring the application of more plant growth regulator to keep plant height under control, but it should reduce water stress during peak bloom (Table 2).
Table 2. Benefits and detriments of irrigation at various cotton growth stages4
Growers who have adequate water and the irrigation capacity to meet peak water use demand may elect to deplete the subsoil moisture before initiating irrigation.2 The most efficient irrigation strategy in the “rain belt” may be to irrigate before severe plant water stress occurs with an amount of water that, even with subsequent rainfall, would not waterlog the soil and impact plant growth. This strategy requires both close crop monitoring and the ability to apply timely, controlled amounts of water.2 A useful relationship between yield produced per unit “evapotranspiration” or ET or crop water used is water use efficiency (WUE). Modern, high water use efficiency (WUE) cotton products tend to provide at least 60 pounds of lint and 90 pounds of seed for every inch of water used. Irrigators should strive to increase yield per total water applied by employing water management practices that reduce losses due to deep leaching and runoff, and by improving irrigation system efficiency and application uniformity through system upgrades. While only about 35% of the cotton acreage in the U.S. is irrigated, for those acres that are irrigated, it is important to practice wise use of water and ensure that in water-limited regions growers employ practices aimed at getting the “most crop per drop.” Geography-specific irrigation scheduling tools can help predict when to irrigate for your area.
For additional agronomic information, please contact your local seed representative.
Sources:
1 2022. Why Irrigate Cotton? Cotton Incorporated. Irrigation Management
https://www.cottoninc.com/cotton-production/ag-resources/irrigation-management/why-irrigate-cotton/.
2 Hake, K., Ayers, V., Hutchinson, B., Lyle, B., Pringle, L., and Thomas, J. 1992. Cotton irrigation scheduling. Cotton Physiology Today. Vol. 3 (9).
3 Farahani, H., Munk, D., Vories, E., Barnes, E., Fisher, K., Udeigwe, T., Bauer, P., and Faircloth, W. 2012. Cotton irrigation management for humid regions. Cotton Incorporated. Irrigation Management https://www.cottoninc.com/cotton-production/ag-resources/irrigation-management.
4 Byrd, S. 2018. Defining cutout in cotton, Oklahoma State University. Extension Fact-Sheet https://extension.okstate.edu/fact-sheets/defining-cutout-in-cotton.html.
5 2021. Cotton water requirements. Cotton Incorporated. Irrigation Management. https://www.cottoninc.com/cotton-production/ag-resources/irrigation-management/cotton-water-requirements.
Web sources verified 11/24/2021.
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