By Guy Collins And Keith Edmisten
North Carolina State University
The squaring stage is an important benchmark in cotton development, marking the point in time in which irrigation management requires more attention. Although most of the 2016 cotton crop in North Carolina is noticeably delayed, several fields are now at the squaring stage or will be soon due to recent favorable weather in places. Growers with irrigation capabilities should be prepared to take action if rains subside and hot, dry conditions prevail. Although irrigation is most important during fruit development, yield penalties can occur if drought (to the point of wilting) occurs even during cotton squaring. Therefore, irrigating during the squaring stage should not be neglected.
Here are a few basic cotton irrigation principles to help North Carolina growers achieve high yields based on my personal research experience while in Georgia. There are several effective methods used for irrigating cotton. This information should provide growers who are either new to irrigating cotton or do not have access to some irrigation technologies with basic irrigation guidelines to avoid yield penalties due to drought at various stages of cotton growth. Farmers should have a general understanding of when and how much water a cotton crop needs to achieve high yield potential while conserving water.
One approach to irrigating cotton is the checkbook method, developed several years ago by other faculty at the University of Georgia. Further discussion can be found at http://www.ugacotton.com/vault/file/2016-UGA-Cotton-Production-Guide.pdf. It is important to note that there are other methods available to producers that also may be effective in achieving high yields while utilizing less water. This particular method illustrates general cotton water requirements per week of development throughout the season, beginning at the first square stage.
Figure 1 describes the weekly water requirements, according to the checkbook method. These are general rates that can be adjusted once you gain some experience with irrigation and soils on your farm. With these general rates in mind, the following points based on my experience with agronomic irrigation research in Georgia could help optimize your irrigation program.
Consider Timing Adjustments
The checkbook method begins at first square (7-8 leaf cotton). Irrigation may be needed prior to squaring during extreme heat or drought or if another factor, such as herbicides, nematodes, etc., adversely affects plant growth. Irrigating prior to squaring is rarely needed. The length of the squaring period can range from three to four weeks, depending on heat unit accumulation and current growth rate. Observe when blooming begins so weekly rates can be adjusted accordingly. There have been instances in my experience where squaring cotton did not require a full inch of water per week. However, 2012 research illustrated that withholding irrigation to the point of severe drought stress occurring during squaring could result in significant yield penalties (300-600 lbs/acre) despite later efforts to irrigate appropriately during bloom.
Keep Up With Growth Stages
Monitor fields for each of the major growth stages. First square should occur by the 7- to 8-leaf stage (40-45 days after planting in most years….likely later this year in North Carolina). First bloom typically occurs 55-60 days after planting. It should be difficult to find blooms at first bloom as there are only a few on the lowest fruiting branches (one bloom every 5-6 feet of row). If you see blooms with little effort, chances are that you are already seven to 10 days into the bloom period. Making irrigation decisions based on windshield evaluations can cause growers to apply the wrong rates at the wrong time. 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 “catch up” if you miss these growth stages by a week or more when hot, dry conditions prevail.
Be aware of 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. These factors influence how much and how often you will need to irrigate. Further discussion can be found at http://www.ugacotton.com/vault/file/2016-UGA-Cotton-Production-Guide.pdf. Lastly, your tillage system could theoretically influence irrigation rates and frequency by influencing how quickly the cotton crop may reach the point of yield-reducing stress.
Split Weekly Rates
The water requirements listed in Figure 1 are total weekly water rates, including rainfall, and should be divided into two or more irrigations. Adjust irrigation rates for any meaningful rainfall (0.3 inches or greater) that occurs in a given week. Splitting the weekly rates helps avoid over-irrigating or wasting irrigation water and the associated pumping costs. It also allows for improved soil water absorption and thus utilization 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 runoff or move below the effective rooting zone. Another reason to split the weekly rates is to have a continuous source of moisture throughout the season and help prevent soil moisture depletion. 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. Splitting rates also maximizes irrigation system efficiency.
The checkbook method can be adjusted once you gain experience with it. Some fields may require slightly higher weekly rates or more frequent irrigation if soil water retention or system efficiency is low. Some fields enter the bloom period with insufficient stalk height or nodes above white bloom, which may call for higher irrigation rates towards the front end of the bloom period. 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 irrigation should cease once lower bolls begin to open, research has shown that continuing irrigation can increase yields if a large number of upper yield-contributing bolls are not fully developed. Irrigation should be avoided, if possible, when a noticeable amount of bolls have opened, especially if damp, cloudy, foggy conditions exist when lower bolls reach full maturity. This scenario is conducive to hardlock or boll rot.
Soil Moisture Sensors
There are several types of commercially available soil moisture sensors that can be used in concert with the checkbook method. Two common types are those 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 accounting for rainfall, which would tell you when you need to resume irrigating following a rainfall event; adjusting for soil type differences and soil water retention; and quantifying the point in which drought stress is likely to occur. Sensors provide a soil moisture threshold to trigger irrigation. Frequent monitoring and/or maintenance of soil moisture sensors is necessary to reap the full benefits.
Avoid wilting at all costs. In my experience, if you see wilting in a cotton field, some yield has already been lost Irrigate before reaching the point of wilting to achieve optimal yields. Another useful resource for making irrigation decisions in the Southeast can be found at: http://www.cottoninc.com/fiber/AgriculturalDisciplines/Engineering/Irrigation-Management/.
Contact Guy Collins, Extension Cotton Specialist Crop Science, at firstname.lastname@example.org. Email Keith Edmisten, Professor of Crop Science and Extension Cotton Specialist Crop Science, at email@example.com.