When irrigation water becomes limited, many growers try to spread the available supply evenly across the season. It seems the safest approach, and it often costs more yield than necessary. Cotton does not need the same amount of water every week, and it is not equally vulnerable to drought at every stage of growth. Knowing when the crop is most sensitive lets a grower protect yield with less water than a uniform schedule would require.
This article sets out how water demand changes through the season, which growth stages suffer most from water stress, and how to prioritise irrigation when supply is short.
Why timing beats total volume
Research on deficit irrigation across a range of crops shows that the timing of a water shortage often matters as much as the total amount applied. In cotton, cutting irrigation during flowering and boll development costs far more yield than withholding the same water early or late in the season. Reviews of cotton water-use studies also show that a carefully managed deficit can improve water-use efficiency compared with full irrigation.
When water is limited, the question is not how much you have. It is when the crop needs it most. This is the detail behind the general advice in the earlier piece on reducing cotton inputs, which noted that holding irrigation through bloom while easing back elsewhere preserves yield better than an even reduction.
Squaring to first bloom: cotton grows rapidly and begins setting the fruiting sites that later become bolls. Demand for phosphorus and potassium rises, and daily water use climbs from about 0.2 to nearly 0.3 inches. If the profile dries out here, the crop enters flowering already under stress and with less capacity to carry fruit.
First bloom to peak bloom: this is the stage to protect above all others. Field experiments and simulation studies from the Texas High Plains consistently identify flower initiation, early bloom, and peak bloom as the most sensitive to water stress. Shortages here increase the shedding of squares and young bolls, reduce boll size, and under severe stress shorten fibre length and push micronaire outside the desired range.
Peak bloom through boll fill: water use stays high while bolls fill, but the crop becomes less sensitive to moderate deficits as established bolls continue to develop. Simulation work covering hundreds of irrigation scenarios identified cutout, late bloom, and boll opening as the least sensitive stages of the season.
Cutout to harvest: as bolls mature and open, crop water use declines naturally. This is usually where irrigation can be reduced or stopped, letting the crop finish under moderate stress with relatively little effect on final yield.
A practical deficit strategy
The research points to a simple principle. Do not spread a water shortage evenly across the season. Concentrate the deficit in the stages that can tolerate it.
Texas A&M AgriLife researchers modelled four deficit levels across four cotton growth stages, giving 256 combinations run against weather records from 1977 to 2019. The strategy they identified as ideal in a normal year was a 70% deficit in the fourth stage, covering cutout, late bloom and boll opening, with only a 10% deficit through the first three. The same study found flower initiation, early bloom, and peak bloom to be the stages most sensitive to stress, which is what makes that split work.
The exact schedule will depend on soil type, climate, and the water available. The principle holds across most cotton-growing regions. Reserve as much water as possible for squaring through peak bloom, then reduce irrigation after cutout as the crop approaches maturity.
Tracking where your crop actually is
Calendar dates are only a rough guide. Cotton develops according to temperature and growing conditions, so fields planted weeks apart, or facing cool rather than hot weather, reach bloom and cutout at very different times. The plant itself is a far more reliable indicator.
The most widely used field method is the COTMAN system, developed at the University of Arkansas. It counts the main-stem nodes above the uppermost first-position white flower, abbreviated to NAWF. As more bolls begin to fill, the plant shifts resources from vegetative growth to fruit, new node production slows, and the uppermost white flower moves closer to the growing point. When NAWF falls to about five, the crop has reached cutout, which marks the last flowers likely to produce harvestable bolls.
Checking NAWF weekly on a representative sample of plants gives a much clearer picture of development than days after planting, and it tells you which stage of water sensitivity the crop is actually in.
Choosing a scheduling tool you will actually use
Effective scheduling does not require expensive technology. The best system is the one that fits your operation and gets used all season.
The checkbook, or water balance, method: track rainfall, irrigation, and estimated crop water use much as you would balance an account. Combined with your soil's water-holding capacity, that tells you roughly how much water remains in the root zone. It costs nothing beyond careful record keeping in a notebook or spreadsheet.
Soil moisture sensors: tensiometers and granular matrix sensors measure plant-available water directly and can show when depletion approaches about 50%, a common irrigation threshold in cotton. They need regular monitoring and some experience to read correctly.
Free scheduling apps: university-developed tools such as the Smart Irrigation Cotton app from the University of Florida and the University of Georgia combine local weather data with a checkbook-style balance and send recommendations to a phone.
Whichever you choose, start monitoring soil moisture before flowering begins. Waiting until squares are already shedding means the stress has already happened.
Calling the last irrigation
The timing of the final irrigation matters almost as much as the first. Stopping too early reduces boll weight and fibre quality, because the remaining bolls have not finished filling. Continuing too long, particularly under overhead systems, keeps the canopy wet and raises the risk of boll rot and hardlock, while encouraging vegetative growth that delays maturity and defoliation.
Extension recommendations rely on two practical guides.
Percent open bolls: terminate irrigation at about 10% open bolls, provided the profile still holds enough moisture to carry the remaining bolls to maturity.
Heat units after cutout: schedule the final irrigation on accumulated heat units after cutout, with most recommendations falling between 350 and 600 DD60s depending on soil depth and stored moisture.
Check the short-term forecast before deciding. A hot, dry spell may justify one more irrigation, while forecast rain may let you stop earlier and save the pumping cost.
A season checklist
Encourage deep rooting during establishment and early squaring by avoiding unnecessary irrigation. Give highest priority to the period from first bloom through peak bloom, where water stress costs the most yield. Allow moderate deficits during boll filling and especially after cutout, when the crop tolerates stress better. Measure NAWF weekly from squaring onward rather than working to the calendar. Pick one scheduling method and apply it consistently all season. End irrigation at about 10% open bolls, or after the recommended heat-unit accumulation following cutout, adjusting for soil water reserves and the forecast.
Limited water does not have to mean poor yields. The aim is to supply water when the crop needs it most, and protecting the few weeks that determine most of the season's yield generally beats spreading every available irrigation evenly across it.
Sources
Himanshu, S. K., Ale, S., Bell, J., Fan, Y., Samanta, S., Bordovsky, J. P., Gitz, D. C., Lascano, R. J., and Brauer, D. K. (2023). Evaluation of growth-stage-based variable deficit irrigation strategies for cotton production in the Texas High Plains. Agricultural Water Management.
Kothari, K., et al. (2021). Simulated efficient growth-stage-based deficit irrigation strategies for maximizing cotton yield, crop water productivity and net returns. Agricultural Water Management.
Kirda, C. (2002). Deficit irrigation scheduling based on plant growth stages showing water stress tolerance. FAO Water Reports.
Bourland, F., et al. (2002). Determining the optimum timing for the final irrigation on Mid-South cotton. Beltwide Cotton Conferences, and the University of Arkansas COTMAN programme.
Mallard, J., Edwards, P., Hall, D., Poythress, T., Thompson, S. B., and Porter, W. (2025). Current considerations for soil moisture and crop water use for row crops. University of Georgia Extension.
Virginia Cooperative Extension (2024). Irrigation scheduling in humid climates using the checkbook method. Virginia Tech.
Collins, G., and Edmisten, K. Factors and points to consider for irrigating cotton. NC State Extension.
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