It is approaching the time when cotton producers must decide when to terminate irrigation. This decision is generally based upon two primary factors, cotton growth stage and amount of water currently in the soil.
The growth stage of most importance at this time is “Nodes Above White Flower (NAWF).”
As flowering approaches the top of the plant, the plant finally puts most of its energy into boll development, flowering virtually ceases, and plants are considered to have reached the “cut-out” growth stage. This generally occurs at 4 to 5 NAWF. (Sometimes “late” cotton may not reach cutout before a killing frost occurs.)
Depending upon the number of heat units available, it can take 60 or more days for this last white flower to make a mature boll. Ideally, at that time, this boll (and all those below it) would be mature enough to allow timely defoliation/desiccation without significant yield loss.
“Irrigation, if needed to ensure continued boll development, should be scheduled to provide enough water to carry these bolls to maturity, and not have a significant amount remaining in the soil after harvest,” says Dana Porter, Associate Professor and Extension Specialist with Texas Agricultural Extension in Lubbock.
A quick way for the producer to get a good estimation of boll maturity is to use the “knife” technique. A mature green boll will be extremely difficult to cut open, even with a very sharp knife; the seed within the boll will have a tan color, and it will be very difficult to dent the boll by pressing it with your fingers.
An immature boll can be easily cut with a sharp knife, the seed coat will have a milky white color, and the cotyledons will be white instead of green.
“If plants have reached the cut-out stage, and have been adequately watered up to that stage, additional irrigation probably will not be necessary,” Porter said.
“However, if the plants have reached the cut-out stage, have not been adequately watered, have some immature bolls 30 days old (maximum size) and older, a healthy canopy, and it is no later than September 1, then the producer may want to consider the economic aspects of applying additional irrigation to take these immature bolls to maturity,” Porter said.
Instructions for calculating estimated additional irrigation amounts, and the time necessary to apply them can be obtained from “Off-season Management Tips: Pre-plant Irrigation Management,” in Focus on Entomology, S5-02/03, April 11, 2003, available at http://lubbock.tamu.edu/irrigate/usefulPublications/prePlantIrrigation.pdf .
To make these calculations, the producer must have an estimate of amount of water already in soil. Several techniques accomplish this, and these vary in convenience, expense and accuracy. One quick, inexpensive, and often relatively accurate method is the “Feel and Appearance” technique. (For discussion of various methods of estimating soil water amounts, go to http://blm.gov/nhp/efoia/narsc/2003/IB/STIB2003-044-ATT.pdf) 
The website, http://lubbock.tamu.edu/irrigate/pdf/soilfeel.pdf , has “Feel and Appearance” pictures and instructions that can aid producers in estimating water content for various soil types and moisture contents.
Additional information relating to cotton irrigation and plant growth can be obtained from http://lubbock.tamu.edu/ .