Season Length and Cultivar Determine the Optimum Evapotranspiration Deficit in Cotton

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Abstract
In areas where the growing‐season length limits cotton (Gossypium hirsutum L.) production, irrigation practices that meet the maximum evapotranspiration (ET) demand have been shown to promote excessive vegetative growth and to reduce yield. Past experiments using limited irrigation in cotton do not provide generalized information on the magnitude of ET deficits that optimize lint yields, however. A study was conducted to determine how season length and cultivar maturity affected the response functions to ET deficits in cotton. Four cultivars (Acala SJ‐C1, GC‐510, Coker‐310, and Jaen) were studied at Cordoba (southwestern Spain) on a Typic Xerofluvent soil under variable irrigation. In 1985 and 1986, ET levels ranging from 40 to 100% of maximum ET (ETmax) were generated with a sprinkler line source. In all cultivars, biomass production and number of bolls set decreased as ET decreased, but earliness was hastened by ET deficits. The water production function of Jaen was linear; seed yield was 5300 kg ha− at ETmax (820 mm). In contrast, the production function of the three other cultivars was linear up to 85% of ETmax, but levelled off as ET approached ETmax (830 mm) because a fraction of the bolls set did not open by harvest at high ET levels. Sequential harvests up to the final date showed that, for a given genotype, the ET deficit giving maximum yield decreases as length of season increases. Thus, it is possible to define an optimum ET deficit for cotton based on cultivar earliness, growing‐season length, and availability of irrigation water.

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