What does the research say?
Once a boll is 10–14 days old, the abscission layer responsible for boll-shed cannot form. So, late water stress (beyond cut out) does not significantly reduce boll numbers and has less impact on yield when compared to water stress during peak and late flowering (Table 1).
However, late season irrigation management decisions still need to take into account the impact of late water stress on boll size and fibre quality. Water stress primarily affects fibre quality due to changes in staple length and micronaire. Staple length is impacted during the first third of the period a given boll is developing, while micronaire is impacted during the middle third. It is important to note that fibre quality is really only impacted when a crop experiences significant stress prior to defoliation (60 percent to 70 percent open – 4 nodes above cracked boll). This is in part because cotton is an indeterminate plant, so if a crop experiences water stress in this period the overall effect will be diluted by the bolls that have developed under more favourable conditions.
As a result, the timing of the last irrigation should ensure fewer harvestable bolls are still within the first two-thirds of the duration of boll development when the drying soil moisture profile induces water stress.