Considerations for Irrigating Cotton (Collins & Edmisten)

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Some cotton planted in early May has now reached the 7 to 8 leaf stage, with small squares visible in the terminal. The squaring stage is an important benchmark in cotton development, marking the point in time in which irrigation management requires more attention. Recent rains have allowed early planted cotton to enter the squaring stage with adequate moisture in most places across the state, and hopefully this trend will continue, however growers with irrigation capabilities should be prepared to take action if these rains subside.

The purpose of this article is to provide a few basic principles regarding irrigating for high yields. There are several methods used for irrigating cotton, but hopefully this article will help fine-tune some irrigation practices to 1.) avoid yield penalties, and 2.) optimize irrigation water use without much waste.

One method of irrigating cotton is the checkbook method, developed several years ago at the University of Georgia. This method and further discussion of a few other points in this article can be found at http://www.ugacotton.com/vault/file/2015-UGA-Cotton-Production-Guide.pdf. This method illustrates the general cotton water requirements per week of development throughout the season, beginning at the first square stage. During my years in Georgia conducting irrigation research from an agronomic standpoint, I’ve seen several sensor-based methods that yielded as well as the checkbook method while using noticeably less water, however I never had another method to yield better than this method during years when drought stress occurred. The following describes the weekly water requirements according to the checkbook method.

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

Week 2 of squaring:                                                1 inch per week

Week 3 of squaring:                                                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 at first square (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 efforts to irrigate appropriately during bloom.
  • Monitor fields for each of the major growth stages (first square, first bloom, first open boll). First square should occur by the 7-8 leaf stage (40-45 days after planting), 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. 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. 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 is 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 at http://www.ugacotton.com/vault/file/2015-UGA-Cotton-Production-Guide.pdf. 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.
  • 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 (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 wont 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 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 if soil water retention or system efficiency is low, or in prolonged drought periods. 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 beyond first open bolls can increase yields if there are a large number of upper yield-contributing bolls are not fully developed when the lower bolls begin to open. 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 the checkbook 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, what they measure, and how they should be calibrated. 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 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 that 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.