Day‐degree Units and Time in Relation to Vegetative Development and Fruiting for Three Cultivars of Cotton1

Abstract

In a test of two Upland cotton (Gossypium hlrsutum L.) cultivars and one Pima (G. barbadmme L.) cultivar Planted on five dates, fewer days were required to produce first true leaf, square, and flower stages as the date of planting was advanced from 1 April to 27 May. Days from planting required to produce open bolls decreased as planting temperatures approached an optimum, then increased for the last planting date. Differences among planting dates for cumulative day‐degree units needed to produce flowers and true leaves were non‐significant, whereas those needed to produce the square and open boll stages were significant. The boll period increased with delay of the flower and harvest stages as the season progressed. The numbers of day‐degree units and heat units were negatively correlated with the length of the boll period. The number of day‐degree units gave a better estimate of boll period by harvest week than the number of heat units. Boll period was influenced more by reductions in day‐degree units in ‘Pima S‐4’ than in the two Upland cultivars.