Managing Irrigated Cotton in the Mid-South

By Carroll Smith, Senior Writer

The Mid-South is a humid region, which typically has abundant rainfall during the cotton-growing season. Even so, producers have continued to increase their irrigated cotton acres and are curious about how to manage irrigation to realize the most benefit from it.

Several years ago, FSA numbers put Arkansas at about 75 percent irrigated cotton acres, according to Arkansas Extension cotton specialist Bill Robertson.

“Although the number of cotton acres in the state has dropped, I think it would be conceivable that our percent of irrigated acres may be higher because we tend to plant cotton on our best ground,” Robertson says. “Another thing to keep in mind is that the numbers can be a bit deceiving. For example, if you have a 40-acre pivot, the corners are dry, but the entire 40 acres is considered to be irrigated.”

As far as irrigation system types, Robertson says Arkansas is a mix of pivot and furrow irrigation. Northeast Arkansas has a high percentage of center pivots, whereas row watering with polypipe is more popular in southeast Arkansas.

One advantage that the pivot has over furrow irrigation is that a producer can quickly put out a quarter of an inch of water to activate herbicides.

“This really helps in our resistance management programs when we are trying to overlay residuals,” he says. “Using the pivot to help activate residuals really is a valuable tool. We can’t do this with furrow irrigation.”

Another consideration when using a pivot for irrigation is what to do about the dry corners. Different people have different philosophies, and some farmers choose to plant a more drought-tolerant crop in the corners. For example, Robertson recalls one producer’s approach of planting soybeans in the corners. In Texas, some farmers will plant grain sorghum in the dry corners of their cotton fields.

Another option is to manage the cotton in the dry corners differently than the cotton under the pivot, especially when it comes to fertility and insect management.

“The dry corners are going to cut out a lot earlier,” Robertson says. “If you are having plant bug issues and the corners are mature or drought stressed, the plant bugs will avoid those areas and migrate to the lush cotton. In this case, it’s not necessary to spray the dry corners.

“Some producers also are adjusting plant populations in the dry corners to cut costs,” he adds. “They know that their income is not going to be the same as it is under the pivot. So they are farming cotton based on what the dry corners need as opposed to what the cotton under the pivot needs. By doing this, they can be profitable on the corners as well as in the center of the field.”

Precision ag tools, such as variable-rate seeders, fertilizer and pesticide applicators, play a big role in these types of strategies.

In trying to determine when to terminate irrigation, Robertson says the “350-heat-unit rule” seems to work well for Arkansas.

“Based on the experiments that we have conducted, we really don’t see a yield advantage in irrigating after we’ve reached 350 heat units beyond cutout or NAWF-5,” he says.

A Louisiana Perspective

In Louisiana, LSU AgCenter Extension cotton specialist Dan Fromme estimates that about 30 percent of the cotton acres are irrigated. The most popular system used by Louisiana cotton farmers is furrow irrigation.

“The main questions that I get from farmers regarding irrigation are when to start, how much water to apply and when to terminate,” Fromme says.

When to start irrigating and when to stop irrigating depends on how much soil moisture is available, Fromme explains. There are a couple of different ways to determine how much moisture is in the soil profile.

A basic approach is the “feel and appearance” method in which a producer inserts a probe into the ground, pulls some soil and simply feels and observes it. Another, more advanced option, is the soil sensor.

“The soil moisture sensor is a popular method right now,” he says. “These sensors are installed in the field and read twice a week to determine how much moisture is there. From those readings, a producer can determine whether it’s necessary to irrigate at that time.”

The tough question for Louisiana farmers, Fromme says, is how much water to apply.

“Most of our farmers use furrow irrigation, which makes it hard to regulate the amount of water going out,” he says. “Most of them try to put out about three inches at a time.”

Fromme notes that the main frustration farmers in his area face is not knowing how much rain to expect following an irrigation application.

“The Mid-South is a humid, high rainfall area,” he explains. “We get a lot of rain, but it often doesn’t come in a timely manner. It’s frustrating to irrigate, then get a five- or six-inch rain the next day.

“When that happens, the soil becomes waterlogged, causing the plant to shed young bolls and become vegetative,” Fromme adds. “And when you get a lot of vegetative growth, tall, rank cotton is often the result. That’s always a problem in humid regions.”

The Louisiana specialist also notes two important agronomic guidelines for managing irrigated cotton.

“First,” Fromme says, “producers need to be more aggressive in applying plant growth regulators (PGRs). Apply more and apply often. Producers also may want to increase nitrogen rates on irrigated cotton since yield goals may be somewhat higher because they have a reliable source of water.”

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