Irrigation Scheduling and Plant Population Effects on Growth, Bloom Rates, Boll Abscission, and Yield of Cotton1

Techniques are needed for conserving water and shortening the cotton (Gossypium hirsutum L.) fruiting season in irrigated areas. We conducted field tests at Phoenix, Arizona during the summers of 1978 and 1979 to determine the influence of delaying the first irrigation on subsequent growth, fruiting, and yield of cotton grown in an Avondale clay loam (fine-loamy, hyperthermic, Typic Torrifluvents). In 1978, the first post-plant irrigation was applied when the first flower bud (square) became visible (E) or 14 days later (L). Two levels of irrigation during the remainder of the season, normal (N) or 0.6 normal (D), were used as subplots. Delaying the first irrigation slowed growth, square production, and leaf area development, and slightly decreased the number of bolls set by any given date. Despite these apparent adverse effects the LN treatment produced heavier bolls, fewer damaged bolls, and the highest yield. Although LN received 13 cm less water than EN, it produced an average of 606 kg more seed cotton per ha. Water deficit during the remainder of the season (D) greatly decreased growth and yield. In 1979, the first post-plant irrigation was applied at the time of first visible square (E), 7 days later (M), or 16 days later (L). Two plant populations, 94,000 (Hi) and 52,000 (Lo) plants per ha were also variables. The final irrigation was applied 13 August to shorten the season. Despite a 16-day delay in the first irrigation, one less irrigation, and a short-season (early termination), L produced about the same yield as E. The low population produced slightly fewer blooms, but retained more and produced slightly higher yields than the high population. Treatment MLo produced the highest yield, and LLo gave the greatest water-use efficiency. Delaying the first irrigation conserved water and gave satisfactory yields despite early termination in 1979. Although water deficit during the bloom stage hastened cutout, delaying the first irrigation had little effect on earliness. Likewise, low plant population caused no appreciable delay in setting the crop.