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Tips For Irrigating Cotton In The Southeast

• By Guy Collins,
North Carolina State University •

cotton plant

The length of the squaring period can range from three to four weeks, depending on heat unit accumulation and current growth rate — photo by Carroll Smith

Due to prolonged rainy weather during the second half of May, the 2018 North Carolina cotton crop can be categorized into distinctive groups, at least for now.

A relatively high proportion of our acres were planted during the first couple of weeks of May, with very few acres planted from May 15-June 1. A few, yet noticeable, acres were planted in early June and don’t account for replanted cotton. This is the case for essentially all parts of our state and is not isolated to a specific region.

Therefore, our crop will be variable for the forseeable future, with individual fields reaching particular growth stages at different times. Water demands during particular growth stages will be different from field to field and influenced by rainfall. Likewise, the timing and severity of episodic drought will likely affect some fields differently than others.

Most folks probably do not want to hear about watering cotton at this point due to the excessive rains many areas received. However, it is important to note that our sandier soils do not hold moisture for long. Under hot, dry conditions, we are never more than a few days away from drought stress.

Fields that experienced excessive rains to the point of reaching water-logged conditions may develop a relatively shallow-rooted crop. If not drained quickly so that root growth can revive before blooming begins, water-logged conditions that result in a permanently shallow- rooted crop will likely cause cotton to be more drought sensitive later in the season after blooming begins.

At this point, most established young cotton looks good. With adequate moisture and heat, growth will soon accelerate, especially once sidedress fertilizer applications are made. Below are a few points growers should keep in mind if they have the ability to irrigate some of their fields.

Be Timely With Inputs

In previous articles, we’ve suggested the best way to manage a later-than-normal crop is to be timely with all inputs.

These include fertilizer, weed management, plant growth regulators, lygus control, caterpillar control and irrigation if rains subside.

Most folks think irrigation can delay maturity. This is true if cotton is irrigated excessively and not managed well with PGRs, especially on varieties with greater potential for excessive growth.

However, timely irrigation using appropriate rates can be used to promote and manage early maturity by avoiding drought stress that may cause earlier-set fruit to abort and improving retention and development of those earlier-set bolls. We again emphasize the word “timely.”

Much of the irrigation research that has been conducted suggests if you ever get behind with irrigating, it can be very difficult, and sometime impossible, to catch up. This is especially the case in dry years.

Timely irrigation can help growers bridge the gap during seven- to 10-day dry spells. But most of our irrigation systems are not equipped to handle prolonged drought when hot, dry conditions prevail for an extended amount of time. Therefore, it is important to be more proactive than reactive with regard to irrigating cotton. This starts by monitoring plant status and soil moisture at first square.

Irrigation At Squaring Stage

The squaring stage of cotton is an important benchmark in cotton development, marking the point in which irrigation management requires more attention.

Many think cotton should not be irrigated until first bloom, so a robust root system can develop prior to bloom. The thought process behind this makes sense, and the cotton plant does not need, nor respond to much water prior to first bloom.

However, the cotton plant still needs some water during squaring, and under the right conditions — hot, dry, shallow root systems, etc. — it can respond significantly to irrigation during squaring.

Although irrigation is most important during fruit development, yield penalties have occurred if drought (to the point of wilting) occurs during the cotton squaring stage. Therefore, irrigating during the squaring stage should not be neglected.

High Yields, Less Water

There are several effective methods used for irrigating cotton. I hope this article will provide basic irrigation guidelines to help growers avoid yield penalties due to drought at various stages of cotton growth, and generally understand when and how much water a cotton crop needs to achieve high yield potential.

One method of irrigating cotton is the checkbook method, developed several years ago by other faculty at the University of Georgia and modified over time. This method and further discussion of a few other points in this article can be found at http://www.ugacotton.com/vault/file/2018-UGA-COTTON-PRODUCTION-GUIDE-1.pdf.

The publication also talks about sensor-based methods and software/apps that are valuable in fine-tuning cotton irrigation practices to maximize yields and returns while conserving water.

It is important to note there are other methods available to producers that also may be effective at achieving high yields while using less water than this approach. This particular method illustrates general cotton water requirements throughout the season, beginning at first square stage.

Tips Based On Experience

checkbook method chartThe checkbook method rates can be adjusted once you gain experience irrigating your farm on your soils. With these general rates in mind, the following are a few points based on my experience with agronomic irrigation research with this and other irrigation methods during my time in Georgia that could help optimize your irrigation program:

➤ This method begins when visible squares appear (seven- to eight-leaf cotton). Occasionally, irrigation may be needed prior to squaring during periods of extreme heat or drought or if some other factor adversely affects plant growth (herbicide injury, nematodes, etc.).

However, irrigating prior to squaring is rarely needed. Also, the length of the squaring period can range from three to four weeks, depending on heat unit accumulation and current growth rate.

Therefore, it is important to observe exactly when blooming begins so weekly rates can be adjusted accordingly.
There also have been instances in my experience where squaring cotton did not require a full inch of water per week during squaring.

However, 2012 research clearly illustrated that withholding irrigation to the point of allowing severe drought stress to occur during squaring could result in significant yield penalties (about 300-600 pounds/acre) despite later efforts to irrigate appropriately during bloom.

During squaring, water demands increase as plant size increases. In many cases, growers could apply lower rates than what is described in the chart, as the plant does not use much water during this time. During squaring, avoid wilting by irrigating in a timely manner. The actual rate is less important. However, rates less than 0.2 to 0.3 inches are sometimes negligible and probably don’t do much good if hot, dry conditions prevail for more than a week.

➤ Monitor fields for each of the major growth stages (first square, first bloom, first open boll). Squares should be visible by the seven- to eight-leaf stage (40-45 days after planting in most years….likely later this year in North Carolina), and first bloom should occur at 55-60 days after planting.

At first bloom, there should only be a few blooms on the lowest fruiting branches (one bloom every 5 to 6 feet of row). If you ride by the field and see blooms with little effort, chances are you are already seven to 10 days into the bloom period. Making irrigation decisions based on windshield evaluations will likely cause you to apply the wrong rates at the wrong time, or at least increase weekly irrigation rates later in the bloom period.

These growth stages are important in determining when to begin irrigating and how much to irrigate. In my experience, it is difficult and sometimes impossible to attempt to “catch up” on irrigating if you miss these growth stages by a week or more when hot, dry conditions prevail.

➤ Know your soil characteristics and irrigation system efficiency. Heavier soils retain water longer than sandier soils, and there is variation in the efficiency of various
irrigation systems (traveling gun, center pivots with and without drop hoses, etc.). These factors influence how much and how often to irrigate.

Find further information at http://www.ugacotton.com/vault/file/2018-UGA-COTTON-PRODUCTION-GUIDE-1.pdf. Lastly, your tillage system — the amount of residue or ground cover present on the soil surface — could theoretically influence irrigation rates and frequency. This can influence how quickly the cotton crop may reach the point of yield-reducing stress.

➤ The water requirements listed are total weekly water rates. These should be divided into two or more irrigations. These rates include rainfall; therefore, irrigation rates should be adjusted for any meaningful rainfall (about 0.3 inches or greater) that occurs in a given week.

Splitting weekly rates into two or more irrigations offers several advantages. It allows for better adjustment for rainfall in a given week, which helps avoid over-irrigating or wasting irrigation water and the associated pumping costs. It also allows for improved water absorption in the soil and use by the plant. Most soils cannot absorb much more than 0.75-1 inch from a single rain or irrigation, while higher rates tend to run off or move below the effective rooting zone.

Third, it allows for a continuous source of moisture throughout the season and helps prevent depletion of soil moisture. If you irrigate the total weekly rate in one event and don’t irrigate again for a week, drought stress could occur during that time if hot conditions prevail without additional rainfall, especially on sandier soils. Fourth, it maximizes the efficiency of your irrigation system. Most irrigation systems cannot apply the higher weekly rates in a single event and/or in a timely manner.

➤ This method can be adjusted once you gain a little experience with it. Some fields may require slightly higher weekly rates or more frequent irrigation if soil water retention or system efficiency is low. Additionally, there may be situations when some fields enter the bloom period with insufficient stalk height or nodes above white bloom.

Higher irrigation rates may be needed towards the front end of the bloom period.

Lastly, the length of the bloom period often needs to be adjusted if open bolls are present before eight weeks of bloom have elapsed or if a higher-than-normal proportion of smaller harvestable bolls are located on upper nodes of the plant.

Although this method suggests that irrigation should cease once lower bolls begin to open, research has also shown that, occasionally, continuing irrigation can increase yields if a large number of upper yield-contributing bolls are not fully developed at that time.

However, avoid irrigation if possible when a noticeable amount of bolls have opened, especially if conditions are suitable for hardlock or boll rot.

➤ Several types of soil moisture sensors are available to use with this method. Two common types are ones that measure volumetric water content or soil water potential. Both offer advantages, but you need to understand the difference in how they work and what they measure.

Sensors can help with irrigation scheduling by 1.) accounting for rainfall that would tell you when to resume irrigating following a rainfall event, 2.) adjusting for differences in soil type and soil water retention, and 3.) quantifying the point in which drought stress is likely to occur (i.e. sensors provide a soil moisture threshold for which to trigger irrigation). Frequent monitoring and/or maintenance of soil moisture sensors is necessary to reap the full benefits of these technologies.

➤ Lastly, note that wilting should be avoided at all costs. Many growers only irrigate once they see visible wilting, which indicates the plant is encountering drought stress. While this approach may be better than not irrigating at all, in my experience, some yield has already been lost if you see wilting in a cotton field.

Irrigating before wilting occurs is necessary to achieve optimal yields. Growers should irrigate to prevent wilting, as opposed to irrigating after this point of significant stress.

Another useful resource for making irrigation decisions in the Southeast is http://www.cottoninc.com/fiber/AgriculturalDisciplines/Engineering/Irrigation-Management/.

Guy Collins is a cotton specialist at NCSU. Cotton specialist Keith Edmisten also contributed to this article.