Avoid Water Deficit Stress At Critical Cotton Growth Stages
⋅ By Carroll Smith ⋅
The initial bloom that appears near the bottom of the cotton plant is white. After pollination, it quickly turns pink and then red. July is typically the month that a field of healthy cotton puts on this colorful display across many rural landscapes.
Once a cotton plant reaches the bloom stage of development, it’s important to manage its nutrient and water needs to produce the coveted bolls in anticipation of an abundant harvest.
Peak Water Requirement
“If you look at a crop water-use curve for cotton, our peak water requirement is from about the third or fourth week of bloom up to the sixth or seventh week of bloom,” said Wes Porter, University of Georgia precision ag and irrigation specialist. “That’s where we absolutely can’t fall behind. If we cut water during that time, we will significantly reduce our yields.
“Three to six weeks into bloom, we already have a high boll load lower in the plant. However, the plant is still trying to bloom and form bolls higher on the plant while continuing to develop the bolls on the lower part of the plant.
“You need to water at this high stress time to ensure it can do both. My recommendation as we get further into the season is to apply ½-inch today, skip a day, then apply ½-inch the day after — kind of spoon feed the crop. This keeps the soil wet, and you don’t lose moisture. Very large or small irrigation events are not recommended as water can be lost or have little to no effect.”
Although there is always the potential exception to the rule, July is a hot, dry month.
“We typically have temperatures in the 90s every day with no reprieve,” Porter said. “Sometimes this will bring thunderstorms across the South. But we may go a week or two without any rainfall, especially if we don’t get any moisture moving up from the Gulf.
“During that time, it’s easy to fall behind on irrigation because we have so much going on. A lot of people get comfortable irrigating once a week or 1.5 times on average per week during that first 40 to 60 days after planting. But it’s important to make sure we are prepared, watch where our moisture levels are and not allow the crop to fall into a deficit.”
What If You Fall Behind?
Moving into July when cotton hits peak bloom, the above ground biomass is about equivalent to the below ground biomass, Porter said. The root system is extracting water a lot deeper than it was earlier in the season.
“If we have not paid close attention, we are now falling behind because we are using deep moisture and drying out the entire profile,” he said. “When you deplete the deeper soil moisture, you won’t catch up with overhead, sprinkler-style irrigation alone. That’s a scary place to be midseason.”
If a farmer gets to that point, Porter recommends backing off the irrigation rate and making multiple applications, if possible. This “bounce back” option — that includes two lower-rate applications fast enough to stay on top of water requirements — is possible with smaller overhead irrigation systems.
But with a larger overhead system that takes more than 24 to 36 hours to make an application, survival mode becomes a reality. To maintain the water requirement, the system then has to run almost continuously.
Advanced Irrigation Scheduling
Another consideration, according to the UGA Cotton Team Newsletter, “If proper growth regulator is applied, it will prevent vegetative growth as it should. If rain chances are low, irrigation will be required to get the fertilizer to the plant by irrigating it in and allowing the plant to uptake the nutrients.”
However, Porter said, it’s also important not to overwater the crop, which could affect yield. To avoid this scenario, he strongly encourages anyone who is irrigating cotton to use an advanced irrigation scheduling method whether it’s a wet or a dry year.
“Use some type of computer model, soil moisture sensor or the basic checkbook irrigation method,” he said. “Most universities publish a checkbook method for their region. It shows the weekly water requirement for the crop.
“You should keep up with rainfall and supplement the difference. With our sandy soil types in Georgia, two to three days is about all the water we can store.”
Another irrigation scheduling tool is the SmartIrrigation Cotton App. According to UGA AgWET, “The Cotton App uses an evapotranspiration-based model to estimate when irrigation is necessary and provides the user with an estimate of how much water should be applied. It does not require any sensors, sends the user notifications when action is needed and is currently available at no cost.”
If a farmer chooses to use a soil moisture sensing system, it directly measures soil moisture in the field. Some of the irrigation manufacturers have programs as well. Lindsay offers FieldNET Advisor that can be used without sensors. Valley offers Valley Scheduling advanced management software. Reinke has partnered with Crop Metrics to offer the CropX sensors system through most of their dealers.
“Valley Scheduling and the CropX systems do require sensors, but they can be set up specifically to cotton,” Porter said. “And there are a lot of sensor companies that offer aftermarket sensor options you can purchase to put in your fields and utilize, too.”
Weighting Sensor Depths
When using a soil moisture sensing system, Porter said it is a good idea to weight the sensor depths according to the root depth by assigning percentage values to the shallow sensors and deeper sensors.
For example, early in the season, 70% to 80% of irrigation decisions potentially would be based on the shallow sensor. As the cotton grows, the percentage value on the shallow sensor might drop to 50% combined with a 25%/25% split on two deeper sensors.
Using a weighted average that matches root growth instead of using a simple average is typically more accurate when trying to make cotton irrigation decisions based on soil moisture sensor graphs.
“You have to consider where we are actually extracting water from and where the plant needs water replenished,” Porter said. “That’s how I assign a percentage value for the different sensors — that extend to different depths — to match root growth at the time.
“If the roots are extracting water from deeper in the ground at a certain point, make sure that factor is part of your irrigation scheduling decision when using soil moisture sensors. It’s important to make these adjustments throughout the growing season.”
For questions about cotton irrigation scheduling in your specific area, reach out to your local Extension agent.