Cotton is considered to be among the most drought tolerant crops grown in the Southeast.

More and more growers are finding top yields from adding irrigation water, but when to apply it and how it affects different varieties has been a challenge.

A recent survey by Cotton Incorporated of 885 cotton growers across the U.S. Cotton Belt indicates moisture shortage is the No. 1 cause lack of yield and lack of sustainable yield. The survey also emphasized the need for drought tolerant cotton varieties, which are rapidly being developed by major cottonseed companies.

The water for cotton issue isn’t a new one. Back in the 1940s researchers established that cotton yield is related to cotton plant height, which is related to cell elongation, which is closely related to drought stressed, lower yielding plants.

The Cotton Incorporated grower survey initiated a series of tests across the Southeast and Delta to determine how different varieties respond to irrigation at different stages in plant development.

Tests with varying types of irrigation equipment were conducted in 2011 and 2012 in Arkansas, Mississippi, Tennessee and South Carolina.

Clemson University Cotton Specialist Mike Jones conducted these tests at the PeeDee Agricultural Research and Education Center near Florence, S.C.

He used an overhead irrigation system to apply four inches of water to 15 different cotton varieties, including two Americot varieties, one DynaGro variety, one FiberMax variety, three Stoneville varieties, three PhytoGen varieties and six Deltapine varieties — all commonly grown in the Southeast.

“Moisture stress for nine days during peak cotton bloom seems to be the most critical time,” Jones says.

“During vegetative growth of the cotton plant, root growth out-paces shoot elongation, so we don’t usually see much impact of water stress during the early part of the growing season, he adds.

“If we see moisture stress during construction of the cotton canopy, we can get significant reduction in fruiting sites, which will reduce yield. With increased irrigation, the plant gets greater sunlight interception by the canopy, which leads to increased yields, he adds.

Technically, under limited soil water conditions, the reduction in transpiration is caused by a highly complex feedback mechanism in the plant that tells the stomata to close and thus limit further water loss from the leaves. As the stomata close, plant temperature rises and the plant undergoes water stress. Stress may not be visible initially, but plant processes begin to slow down as plant temperature goes up.

Less technical terms

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.

Some varieties respond better than others

Overall, some varieties respond to irrigation better than others. In general, Stoneville and Americot varieties seemed to have the most positive response to irrigation.

Last year, in the second year of the study, experimental varieties were used, and the researchers got the same kinds of varietal differences as they found with frequently used varieties in testing in 2011.

In the second year of the tests, researchers did a tedious box mapping analysis which gave them a breakdown of which parts of the cotton plant responded most to irrigation water.

In most locations, dryland cotton actually set more bolls on the lower part of the plant. However, increased fruit set on the vegetative branches, or sympodia, especially in the middle part of the cotton plant likely accounted for increased yield from irrigation.

Two experimental varieties from Deltapine and one from Phytogen showed significant yield and quality improvement from irrigation, offering some opportunities in the future for cotton growers contemplating irrigation.

“Location had a big impact on yield response to irrigation, with the biggest coming from Arkansas and the least from Mississippi.

“In South Carolina, we had a consistent yield response of 12-14 percent, and that may be something our growers can hang their hat on when deciding whether or not to irrigate their cotton,” Jones says.

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