As we progressed through the second week of August and white flowers continued to climb cotton stalks throughout the state, several calls came in regarding irrigation termination in cotton. My general recommendations are as follows:
Terminate irrigation when the lowermost first position bolls begin to crack open across the field. Some folks prefer to continue irrigation a week to 10 days after this point.
Little current data exist to confirm or refute this practice, but keep in mind this is a risk/reward situation. When bolls begin to crack open, they become susceptible to a number of plant pathogens that can cause hardlock and or boll rot.
The example that I have always used is that of a loaf of bread. If you keep a loaf of bread in a less humid environment it will not mold nearly as quickly as bread that is kept in a high humidity environment. When cotton is furrow irrigated and has a thick, dense canopy, a microclimate is created within that canopy that is very humid, which may result in increased incidence of hardlock and/or boll rot due to the proliferation of a number of plant pathogens.
If you choose to water after bolls begin to crack open, at the very least give some consideration to these points.
Terminate irrigation a week to 10 days after lowermost first position bolls begin to crack open across the field. The reason for the difference in timing between the two irrigation methods revolves around the volume of water delivered in each irrigation application. While it is not uncommon to see 2+ acre inches applied in a furrow irrigation event, 0.5 to 1.0 inch is more commonly applied using sprinkler irrigation.
However, the same concerns exist when watering after bolls begin to open. Given that less water is delivered using sprinkler irrigation, the level of humidity in the lower canopy will likely not persist as long, which may reduce the potential for hardlock and/or boll rot.