In less technical terms, water stress causes the plant to grow slower and smaller. The higher the severity and duration of the water stress, the higher the loss of biomass production and thus yield.

Also, the sensitivity of the plant to water stress changes with growth stages, and is usually highest during rapid canopy development and effective flowering stages, which is borne out by the finding of Jones and his colleagues across the South.

In the Southeast, average rainfall is greater than the demand cotton has for moisture. The catch is getting rainfall at the right time.

As Jones’ study in South Carolina emphasizes moisture deficiency at one critical 9-10 day period of cotton growth can cause significant yield losses.

In the region, irrigation has been shown to nearly double the non-irrigated cotton yield from about 750 to near 1,200 to 1,500 pounds of lint per acre during water limited years. 

On the sandy Coastal Plain soils in the Southeast, these large differences in yield are mainly because irrigation supplements rainfall, ensuring adequate water in the root zone to meet crop water needs on a consistent basis.

Lower yields from non-irrigated fields, as the Cotton Incorporated study shows across the South, are due to lack of water at critical times in cotton development.

In the Southeast and Delta, average rainfall is 45-55 inches, about double what a cotton plant needs, but it frequently doesn’t come at the right time in the development of cotton plants.

Cotton is an indeterminate perennial shrub that is somewhat tolerant to drought and soil salinity. Because of its drought adaptations, cotton responds favorably to periods of water stress sufficient to slow vegetative growth; a physiological feature that can be benefited by timely irrigation management.

According to Cotton Incorporated, the majority of U.S. cotton (about 65 percent) is currently produced under non-irrigated conditions.

In the South and the Southeast, non-irrigated cotton systems dominate, with more than 90 percent of the cotton crop grown in some states without irrigation to supplement rainfall.

In addition to yield, fiber quality of cotton can be significantly impacted by periods of dry weather.

Micronaire has been both increased and decreased by irrigation in previous studies, but in the two-year study in the Southeast, it was generally lower with irrigation. Jones points out that Phytogen 499 did not show as much micronaire response to irrigation as the other 15 varieties in the test.

Across the tests in four cotton-producing states staple was increased uniformly across all varieties, except for the DynaGro variety.

Jones says this variety response data is from only one year and may or may not hold up over multiple years with different growing conditions and different soil types.