The final irrigation is a very important decision to make for a cotton grower.
There are three primary considerations involved; agronomic - stage of the crop; climatic conditions, and insect pest pressures. Keeping these considerations in mind, the following steps can help decide the irrigation termination date.
Know where the crop is in the fruiting cycle. Is it at cutout? In Arizona, cutout is defined as the point at which the crop has an average of five or less nodes above the white flower (NAWF) during the primary fruiting cycle.
Once cutout is determined, a decision to continue to grow the crop in order to harvest a top crop needs to be made. The decision depends upon how well the crop has set bolls and what the overall boll load was after the first primary fruiting cycle.
Research has shown that if the crop has 45 percent or more fruit retention (first two fruit sites on each fruiting branch) the probability that extending the season will result in significant yield gain is low. This probability is reduced even further as fruit retention goes up.
Full season varieties have the best chance for a top crop with significant yield increase.
In order to realize any real significant increase from a second crop, hot and dry weather conditions are required.
Remember it takes about 600 heat units to go from flower to hard boll, and then another 400 heat units to get to open boll.
A good rule of thumb is to identify the last flower for harvest and then irrigate the normal schedule for an additional 3-5 weeks or identify the last harvestable boll and assure adequate soil moisture for the next 2-3 weeks.
Don’t get greedy, be reasonable and practical. Cotton will keep on producing flowers and the season will get well into October before too long.
For defoliant applications, the rule is to defoliate at about twice the normal irrigation interval.
So if the field is being irrigated every 10 days, apply defoliant about 20 days after the last irrigation (assuming no rain has occurred).
This is to assure the plant is adequately stressed, but not too stressed, which can negatively affect the defoliant efficacy.