Terminating Irrigation in Cotton Production

Deciding when to terminate irrigation can affect final yields as well as efficiencies in cotton production. Proper irrigation termination in cotton relies on observations of plant developmental stage, not by referring to calendar date. Irrigation termination decisions should be made on a field-by-field basis taking into account: plant developmental stage, soil type and soil moisture status, the type of irrigation system used, geography, boll load, and crop health. Applying too much water late in the season may lead to problems such as unnecessary vegetative growth, delays in boll maturity, and can even reduce defoliant efficacy.

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General Termination Guidelines

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Proper irrigation termination in cotton relies on observations of plant developmental stage (usually boll developmental stage), with some recommendations coordinating plant developmental stage with accumulated heat units. The water holding capacity of the soil (soil type) and the type of irrigation system used will also impact when irrigation should be terminated. In some cases, irrigation may be terminated earlier on soils with a high water holding capacity because sufficient water may remain in the soil to bring the crop through maturity. Optimal cotton irrigation termination occurs when the final irrigation application supplies enough moisture to complete maturity for all bolls with acceptable lint quality.1

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Generally, furrow irrigation systems deliver two to three inches of water per acre each time a field is watered and are more likely to fill the soil to field capacity with an irrigation. Pivot and drip irrigation systems do not deliver as much water per application as furrow irrigation. Application amounts may be substantially less depending on well capacity. Irrigation may need to be terminated later when pivot or drip irrigation is used as less water is applied per application.

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Termination Recommendations

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Under normal growing conditions, cotton usually requires 45 to 55 days from flowering to produce an open boll. This time may be extended for blooms occurring later in the season. Irrigation should be applied so that these bolls are able to reach maturity, followed by decreased soil moisture to deter any re-growth. To produce a “normal�? quality boll in the High Plains, a cotton bloom typically requires 850 heat units. An “acceptable�? quality boll generally requires 650 heat units past bloom. For a typical season, this results in four to eight weeks of adequate soil moisture after cut-out.1 Cutout occurs when the number of nodes above the uppermost, first position white flower is equal to five (NAWF = 5). (NAWF = 5).

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Figure 1
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An irrigation termination study conducted in the Rolling Plains compared three irrigation methods: furrow, 80-inch drip line centers, and 40-inch drip line centers. Irrigation was terminated at 0, 190, and 380 heat units after 5 NAWF. When furrow irrigation is terminated at 5 NAWF, a 50% loss of yield potential can occur (Table 1) as compared to termination at 380 heat units. Yield loss was not as significant for drip line irrigation, but delaying irrigation termination until 380 heat units past cutout resulted in increased yield potential for all three irrigation methods.2

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Late-Season Irrigation Precautions

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Applying a small amount of additional moisture after the recommended termination period may help small bolls mature; however, applying too much water may cause problems such as unnecessary vegetative growth, delays in boll maturity, and reduction of defoliant efficacy. Too much moisture can also increase pest pressure and the potential for boll rot. The additional expense of late-season irrigation may not be realized in final yield.

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Irrigation termination decisions should be made on a field-by-field basis taking into account: plant developmental stage, soil type and soil moisture status, the type of irrigation system used, geography, boll load, and crop health.