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Irrigation is necessary for supplying water in arid and semi-arid regions of the West and helps supplement rainfall in other regions.1 Supplemental irrigation to reduce soil water deficit stress can improve emergence and nutrient uptake, incorporate herbicides, support plant growth, prevent water stress induced square and boll shedding, maintain fiber quality, and support maximum yield potential. A challenge in providing optimal irrigation is providing sufficient water at critical cotton growth stages to supplement rainfall, without causing plant stress from excess soil moisture.2 Average cotton water use by growth stage is shown in Table 1.
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.1 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 soil borne disease infection.1
Timing of the first post-planting irrigation is critical. Growers must consider well capacity and the irrigation system’s ability to supply sufficient water to meet crop needs at critical growing stages. Starting too early may lead to a shallow root system, weed germination, and rank vegetative growth. Starting too late may lead to stunted plants, early cutout, and reduced yield potential. Optimum irrigation timing in the West occurs when plants are slowing mainstem growth, but before obvious leaf color changes.1 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.1
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 will 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 deficit early in the season can stimulate root production and development of a deeper root system.2
Once cotton plants are emerged and growing, irrigation should be used to supplement rainfall and prevent depleting the soil moisture profile prior to bloom.1 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).
Crop water demand increases from 0.2 to 0.28 inches per day during this period.2 Lack of water 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 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.2
Water demand begins to decrease as bolls mature and open. While severe water deficits 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.
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 can not 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).
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.1 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 reduce yield potential. This strategy requires both close crop monitoring and the ability to apply timely, controlled amounts of water.1
Geography-specific irrigation scheduling tools can be found at: www.cottoninc.com