Ask 10 different people how to effectively use over-head (center pivot) irrigation and you will get 10 different responses. Driving around west Tennessee it becomes apparent that substantial investments have been made to increase irrigation capacity over the last two years.

There have been more pivots installed in west Tennessee during this time period than the totality of all pivots installed since the invention of pivot systems. Periodically throughout the season I will give irrigation tips and updates. UT AgResearch data from the past five years indicates that the availability of 1 inch of water per week from either rainfall or irrigation beginning at or just after bloom will optimize yield potential. The following information is adapted from the National Cotton Council’s Cotton Physiology Today (vol 11:2, 1999 and vol 3:8, 1992) to help understand how a cotton plant uses water.

Cotton Sensitivity to Stress

Water stress from the initiation of squaring to first flower has the highest correlation with lint yield, because stress at this time reduces the number of bolls and boll size. One way to determine crop stress going into flowering is the number of nodes above white flower (NAWF) at first bloom. If cotton has 8-10 NAWF at first flower, there are already 15-18 floral buds present per plant and the crop has high yield potential. However, if water stress restricts plant growth and only 5-6 NAWF are present at first bloom, bolls will form mainly on first fruiting positions and yield will be severely limited.

Boll size is highly genetically controlled, but lint produced per boll is highly correlated with the number of locks per boll controlling the number of seed per boll. A cotton plant with 8-10 NAWF at first bloom can produce 30-32 seed per boll where a stressed plant will average only 24-26 seed per boll or less. Lock number can be determined as early as 32 days prior to a white flower opening. At this time a square is microscopic and cells are differentiating into different reproductive parts.

Irrigation should be used to help relieve stress allowing plants to maintain fruit for two weeks after flowering (time a small boll is sensitive to shed). So how do we handle irrigation with our late planted cotton crop in 2011? We need to strive to maintain an early maturing crop by promoting early boll set and not promote rank growth that can cause boll shed and boll rot. The best advice I can give is to irrigate before plant water stress occurs. Currently, I would only consider irrigating cotton that has been stressed by herbicide or thrips injury. Irrigating this cotton will promote new growth and will help the plants overcome the bad start to this season. If rainfall does not occur in the next week all producers should consider starting irrigation after squaring begins to relieve stress going into the fruiting period. Remember that light frequent irrigations work best on our hill ground. A good target is 0.3-0.35 inches per turn of the pivot with a weekly target of 1.0 inch of rainfall or irrigation to prevent saturated soils, runoff and excessive plant growth. If the crop blooms before irrigation is initiated, do not delay needed irrigations. After blooming occurs water can be applied without as much concern for promoting excessive growth.

Irrigation Termination

Termination of irrigation is even more complicated with a late planted crop in our short season environment. Excessive irrigation promotes rank growth, delays maturity, increases the time need to protect the crop from insects, makes the crop harder to defoliate, can effect fiber quality, and delays harvest. The harvest delay can have the most major impact in our short season environment. Traditionally irrigation is terminated with the appearance of the first open boll. In an early planted crop this is a good rule of thumb. In a late planted crop irrigation may need to be terminated earlier to prevent an excessively late maturing crop. In west TN our last effective bloom date is August 12^th. This is the date when based on our weather history a bloom has a 50% chance to become a boll. In contrast, our planting date trials have demonstrated that cotton planted after June 10^th has its first bloom near this last effective bloom date and has yielded as high as 850 lbs. of lint per acre. So late blooms can make high yields, but Mother Nature needs to cooperate. Perhaps terminating irrigation in a late crop can occur at NAWF=5 (cutout) + 400 heat units. This will prevent stress on the last small bolls until they are safe from stress related shed events. While this is speculation on my part (research is under way to investigate this idea) it potentially could help finish a late planted crop in a timely manner.

More information on irrigation will follow this season on the UT Crops News blog. If questions arise about irrigation, please contact your county UT Extension office or contact me directly.