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Continued development of irrigation strategies that minimize crop yield losses, while increasing water use efficiency are needed in many semi-arid and arid cotton producing areas world wide. Controlled deficit irrigation (CDI) incorporates the knowledge of crop physiology and phenology to identify specific plant growth stages in which water deficits have a minimal impact on crop yield and quality. Previous research in California and other irrigated agricultural regions have documented the severe yield impact when moderate early season water stress is allowed to accumulate in cotton (Gossypium hirsutum L.). Conversely, similar late season water stress following plant cutout has had a minimal impact on crop yield and quality. This paper is a report of recent studies that were undertaken in an effort to apply the concepts of CDI for cotton (Gossypium hirsutum L.) and suggest approaches to farm water managers which enable a greater understanding of deficit irrigation strategies. Studies conducted in the San Joaquin Valley of California from 1991 to 1993 have consistently demonstrated that high yields can be obtained although late season water deficits in cotton were present. The optimum timing of the final in-season irrigation for cotton was shown to be dependent on the cultivar. The determinant plant types tended to have more significant yield reductions as water stress is increased following plant cutout, while the indeterminate types were found to be less sensitive to the timing of late season water stress. The timing of late season water directly alters the stress accumulated in the crop thereby impacting late season boll retention, boll maturation and crop yield.
The monitoring of cotton plant performance characteristics following periods of induced late season water stress can assist in developing deficit irrigation management strategies. To date, very little information is available regarding modern cotton cultivators and their tolerance to late season water stress. By varying the degree and timing of water stress accumulation, we can begin to recognize both general trends for timing the final irrigation and select varieties that suit an individual field managers needs with respect to timing the final irrigation. Moderate plant water stress induced late in the season, can reduce consumptive water use without severely impacting on crop yield or quality. The decision of when to time the final irrigation for cotton, is dependent upon the variety and the degree of water deficits. Delayed scheduling of late season water is preferred for more indeterminate plant types resulting from their improved tolerance to water deficits. Shorter season, more determinant plant types, experienced significant yield reductions when moderate late season water stress was allowed to build. The preferred irrigation strategies for determinant varieties would therefore favor the deliver of available water supplies prior to the development of moderate water stress levels (-21 bars).
The pressure chamber can be an effective tool in evaluating the intensity and duration of cotton water stress. Generally, plants performed well with highest yields obtained when LWP readings were not allowed to exceed -23 bars. Significant impacts on plant growth and fruit retention were observed when LWP readings were allowed to reach the wilting point of -30 bars. At these high water stress levels, decreases in transpiration rate and photosynthesis are likely causes of delayed fruit set and hence the production of unharvestable late season bolls. The production of these late season bolls, although not equivalent to the lost production of lower fruiting positions, does demonstrate a resiliency of cotton to partially recover from severe water stress levels.