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In fields with irrigation as an option, prebloom water management involves providing adequate water in order to obtain maximum growth, but not so frequent as to be wasteful or to reduce root growth. It is important to note that water needs during the first 30 days of growth are minimal, but then increase dramatically until peak bloom.
Depending on the region, cotton acres may receive moisture from only precipitation (dryland) or may receive additional moisture from irrigation by sprinkler, furrow, or subsurface drip irrigation systems. To efficiently irrigate a cotton crop, it is important to understand critical growth stages and peak water use timing.
Monitoring plant growth, moisture availability, and tracking accumulated heat units can help in determining the appropriate time to irrigate. Like many other crops, cotton development is strongly influenced by temperature and the amount of heat units accumulated after planting (HUAP). Tracking the HUAP can help assess cotton development and when to begin irrigation. The growth stage associated with the accumulated heat units can vary according to the variety selected for the region. According to the University of Georgia, the first square typically develops at about 550 HUAP, followed by the first flower at about 950 HUAP. The optimum timing for the first irrigation is typically between the occurrence of the first square and first flower. At this time, water requirements for the plant increase. The most critical time for moisture availability through the life of a fruiting site is during square development, as water stress can cause a cotton plant to shed squares. If moisture is limited, the squares are competing for energy with the demand put on the plant from a growing boll population.
Pivot Irrigation—When using pivot irrigation, rates should be applied equivalent to the evapotranspiration (ET) losses. Approximately 1 inch or less of water should be applied per irrigation. Greater amounts can cause problems if heavy rainfall occurs soon after an irrigation takes place.
Surface (Furrow) Irrigation—Supplying water with surface irrigation has the least upfront costs, but water efficiency can vary as much as 30 to 90%, depending on the crop and soil type. Approximately 2 inches should be applied per application. With surface irrigation systems, it is important to start irrigating soon enough to get across the field before stress occurs. Surface irrigation systems typically need water applied in 8 to 10 day intervals depending on accumulated precipitation. Drainage is especially important when surface irrigation is used, for optimum surface water flow, fields should be sloped 0.15 to 0.3% and drains and ditches should be periodically cleaned out.
Deficit Irrigation—Deficit irrigation is a common practice in areas with limited water supply. Deficit irrigation is a method of applying less water than the daily evapotranspiration (ET) of the plants. Deficit irrigation assumes that the plants will also use any stored water. By limiting irrigation to the right amount, the cotton crop can continue to fruit without the risk of encouraging rank growth.
Low Energy Precision Application (LEPA)—When water is limited many growers are using different types of Low Energy Precision Application (LEPA) irrigation systems to increase water efficiency. LEPA irrigation systems can reduce losses from evaporation by applying water down into the crop and closer to the soil surface. Different LEPA irrigation systems include circular rows, furrow diking, or dropped sprinklers with low pressure nozzles. Modifying older high pressure sprinkler systems to utilize LEPA methods can increase water efficiency as much as 20 to 40%. LEPA irrigation systems can increase efficiency as cotton responds well to high-frequency deficit irrigation even when less water is applied.
This article is from the Cotton Management Guide, a publication with year-round advice on managing high-yielding cotton. Download the Deltapine Cotton Management Guide now or sign up for new content to be delivered to your email each month.