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.