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Scheduling your final irrigation

Timing the last irrigation is a balance between ensuring there's enough moisture to allow the growth and maturity of harvestable bolls with limited stress, and ensuring fields are dry enough to assist defoliation, limit regrowth, and minimise picking delays and soil compaction.
“The objective of the last irrigation is to ensure that boll maturity is completed without any stress” – James Quinn, CSD.

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.

Timing the final irrigation and defoliation

The irrigation requirement for late crop growth is related to the time of defoliation. It's a decision about which bolls you will pick - are you chasing the top bolls? Have you got enough water and, more importantly, time, to mature these top bolls? Water requirement to defoliation and weather forecasts must also be considered.
End of season water requirements can be determined by estimating the number of days until defoliation and predicting the amount of water likely to be used over this period.  At the time of first open boll, crop water use may be 5-7mm/day, but this can decline to only 3-4mm/day during the last 2-4 weeks prior to defoliation.
Measuring Nodes Above (last) Cracked Boll (NACB) is most commonly used to accurately time final irrigation and defoliation.
  1. Measure your NACB and calculate the number of days for the bolls to reach maturity
  2. Estimate your crop water use from now till defoliation
  3. Know how much is already stored in your soil profile (ie how much is left in the bucket) to be sure extra water is required at this late stage to get the crop through to defoliation.
This video provides a great demonstration on the NACB method.

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