Cotton Irrigation Decisions for 2019 (Collins & Edmisten)

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Although many areas have recently experienced rainfall, it is important to understand that we are never more than a few days away from significant drought, especially when temperatures in the 90s prevail. This, of course, varies among soil type (soil water holding capacity), tillage, nematodes, or any other factor that can influence root development. Generally speaking, the early planted crop in 2019 has developed a good taproot which will be beneficial through the remainder of the season. However, that does not mean that it will be immune to drought. Naturally, there are also several fields that have very shallow roots which will exacerbate the effects of dry spells, and the crop will likely be more sensitive to drought, and thus more responsive to rains.

Much of the irrigation research that has been conducted suggests that if you ever get behind with irrigating, it can be very difficult, and sometimes impossible to catch back up. This is especially the case in years when drought is prolonged and/or severe. Timely irrigation can help growers bridge the gap during 7 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, with regard to irrigating cotton, it is important to be more proactive than reactive. This starts by monitoring plant status and soil moisture at first square.

The squaring stage of cotton is an important benchmark in cotton development, marking the point in time in which irrigation management requires more attention. There are many who think that cotton should not be irrigated until first bloom, so that a robust root system can develop prior to bloom. The thought process behind this mentality makes sense, and the cotton plant does not need, nor respond to much water prior to first bloom. However, the cotton plant still does need 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 even during cotton squaring. Therefore, irrigating during the squaring stage SHOULD NOT be neglected.

There are several effective methods used for irrigating cotton, but hopefully this article will provide growers that are either new to irrigating cotton or do not have access to some irrigation technologies with basic irrigation guidelines to 1.) avoid yield penalties due to drought at various stages of cotton growth, and 2.) 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. This method and further discussion of a few other points in this article can be found in the 2019 UGA Cotton Production Guide, along with discussions of sensor-based methods, and software/apps that are valuable in fine-tuning cotton irrigation practices to maximize yields/returns while conserving water. Therefore, it is important to note that there are other methods available to producers that may also be effective at achieving high yields while utilizing less water than this approach. This particular method illustrates very general cotton water requirements per week of development throughout the season, beginning at the first square stage. The following describes the weekly water requirements according to the checkbook method.

Week 1 of squaring (7-8 leaf cotton):                   0.75-1 inch per week

Week 2 of squaring:                                             0.75-1 inch per week

Week 3 of squaring:                                             0.75-1 inch per week

Week 1 of bloom (55-60 days after planting):       1 inch per week

Week 2 of bloom:                                                 1.5 inches per week

Week 3 of bloom:                                                  2 inches per week

Week 4 of bloom:                                                  2 inches per week

Week 5 of bloom:                                                  1.5 inches per week

Week 6 of bloom:                                                  1.5 inches per week

Week 7 of bloom:                                                   1 inch per week

Week 8 of bloom:                                                   1 inch per week

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

  • Notice that this method begins when visible squares appear (7-8 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 (herbicides, nematodes, etc), however irrigating prior to squaring is rarely needed. Also, the length of the squaring period can range from 3 to 4 weeks depending on heat unit accumulation and current growth rate, therefore it is important to observe when blooming begins so that weekly rates can be adjusted accordingly. Lastly, there have been instances in my experience where squaring cotton did not require a full inch of water per week during squaring, however research in 2012 clearly illustrated that withholding irrigation to the point of allowing severe drought stress to occur during squaring could result in significant yield penalties (approximately 300-600 lbs/A) despite later efforts to irrigate appropriately during bloom. During squaring, water demands increase as plant size increases. There are many cases growers could apply lower rates than what is described in the chart above, as the plant does not utilize much water during this time. The approach during squaring should be focused towards avoiding 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 and 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 7-8 leaf stage (40-45 days after planting in most years), and first bloom should occur at 55-60 days after planting. At first bloom, it should be difficult to find blooms as there should only be a few blooms on the lowest fruiting branches (1 bloom every 5 to 6 feet of row). If you can ride by the field and see blooms with little effort, chances are that you are already 7 to 10 days into the bloom period, and therefore making irrigation decisions based on windshield evaluations will likely cause growers to apply the wrong rates at the wrong time. These growth stages are important in determining when to begin irrigating and how much you should irrigate. In my experience, it is very difficult and sometimes impossible to attempt to “catch up” on irrigating if you miss these growth stages by a week or more when hot and dry conditions prevail.
  • It is important to know your soil characteristics and efficiency of your irrigation system. Naturally, 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 you will need to irrigate. A further discussion of these factors can be found in the 2019 UGA Cotton Production Guide. Lastly, your tillage system (i.e. the amount of residue or ground cover present on the soil surface) could theoretically influence irrigation rates and frequency, by influencing how quickly the cotton crop may reach the point of yield-reducing stress.
  • The water requirements listed above are total weekly water rates. These rates should be divided into 2 or more irrigation events. Note that these rates include rainfall that may occur, therefore your irrigation rates should be adjusted for any meaningful rainfall (approximately 0.3 inches or greater) that occurs in a given week. Splitting the weekly rates into 2 or more irrigation events offers several advantages as this: 1.) allows for a better adjustment for rainfall that may occur in a given week which helps avoid over-irrigating or wasting irrigation water and the associated pumping costs, 2.) allows for improved water absorption in the soil and thus utilization by the plant (most soils cannot likely absorb much more than 0.75-1 inch from a single rain or irrigation event, while higher rates tend to runoff or move below the effective rooting zone), 3.) 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, then drought stress could occur during that time if hot conditions prevail without additional rainfall), and 4.) maximizes the efficiency of your irrigation system (most irrigation systems can not 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, therefore 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 8 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 continuing irrigation can increase yields if a large number of upper yield-contributing bolls are not fully developed at that time. However, irrigation should be avoided when a noticeable amount of bolls have opened if at all possible, especially if conditions are suitable for hardlock or boll rot (damp/cloudy/foggy conditions when lower bolls reach full maturity).
  • There are several types of commercially available soil moisture sensors that can be used in concert with this method. There are 2 common types of sensors: sensors that measure volumetric water content or soil water potential. Both offer advantages to producers, however you need to understand the difference in how these sensors work and what they measure. Sensors can help with irrigation scheduling by 1.) accounting for rainfall which would tell you when you need 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 in order to reap the full benefits of these technologies.
  • Lastly, it is important to note that wilting should be avoided at all costs. Many growers take the approach of only irrigating once they see visible wilting, which naturally 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 reaching the point of wilting is necessary for achieving optimal yields, therefore growers should irrigate to prevent wilting from occurring, as opposed to irrigating after this point of significant stress.

Another useful resource for making irrigation decisions in the Southeast can be found at: Cotton Incorporated – Irrigation Management