Irrigation Starting Time Effects on Cotton under High‐Frequency Irrigation

Deciding when to begin irrigating cotton ( Gossypium hirsutum L.) is difficult because the positive effect of eliminating water stress may be offset by the negative effect of lowered soil temperature. Different starting times for high‐frequency drip irrigation of cotton were investigated in a field having Olton clay loam soil (fine, mixed, thermic Aridic Paleustolls). Early irrigation (EI), started when seedlings had 3 mainstem nodes, was compared with delayed irrigation (DI‐L), started at squaring when there were 7 mainstem nodes. Another delayed irrigation treatment (DI‐H) received an initial large irrigation that equaled the cumulative amount of water applied by that time to the EI treatment. During the early irrigation period, 130 mm of water was applied in 1993 and 120 mm in 1994. Average soil temperature in the top 1000 mm during the early irrigation period was 25°C in the EI treatment, compared with 27°C in the DI treatment. During the late irrigation period, the average soil temperature was 24.5°C for EI and 25.5°C for DI. At first bloom, plants were 30 mm taller in 1993 and 70 mm taller in 1994 in the EI irrigation treatment, compared with the DI‐L treatment. Lint production was greatest in the DI‐L treatment in both years (1548 and 1630 kg ha − ), compared with the DI‐H treatment (1467 and 1460 kg ha −1 ) and the EI treatment (1447 and 1481 kg ha −1 ). Water use efficiency, based on the total of irrigation and rainfall, averaged 3.33, 2.42, and 2.36 kg lint ha‐mm −1 for the DI‐L, DI‐H, and EI irrigation treatments, respectively. Initiating automated irrigation scheduling before squaring in cotton increased seedling height but decreased lint yield and the efficiency of use of total applied water.