AL SUTHERLAND Oklahoma Mesonet left and Randy Boman research director and Extension cotton program leader at the Oklahoma State University Research Center in Altus discuss the importance of evaluating evapotranspiration to determine crop water needs

AL SUTHERLAND, Oklahoma Mesonet, left, and Randy Boman, research director and Extension cotton program leader at the Oklahoma State University Research Center in Altus, discuss the importance of evaluating evapotranspiration to determine crop water needs.

Efficient cotton irrigation demands systems approach

In recent years, growers, even with irrigation, have had a tough time providing enough water to meet a cotton plant's peak demand.

Wringing the most possible benefit out of a drop of water has been a high priority for Oklahoma cotton farmers for the past four years. Even producers with irrigation systems have been left high and dust dry for most of those four growing seasons as reservoirs dropped below levels that permit withdrawal for irrigating cropland.

“We’ve had no runoff into reservoirs for the past four years,” said Randy Boman, research director and Extension cotton program leader at the Oklahoma State University Research Center in Altus.

Boman closed out the inaugural Oklahoma Irrigation Conference recently with an update on water issues for cotton producers.

Boman said cotton water peak demand begins about 75 days after planting and runs to about 115 days after planting. During that stretch, cotton plants may require from one-fourth inch to nearly one-half inch of water per day.

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In recent years, growers, even with irrigation, have had a tough time providing that much.

Improving water use efficiency, he added, may require a handful of strategies that include using a more effective delivery system; increasing irrigation system efficiency; including evapotranspiration rates in irrigation scheduling; and reducing non-water production limitations—better variety selection, fertility management, weed control and proper start-and-stop time for irrigation applications.

 

The weed issue has become more problematic in recent years as herbicide-resistant weeds escape typical management efforts and compete with cotton for limited moisture.

Irrigation timing

Reducing the amount of irrigation is another option and in some cases producers can do that with limited, if any, adverse effect on profit potential. He urged growers to analyze irrigation capacity to determine if water sources can meet crop needs. If capacity is greater than crop demand, producers may be able to cut back.

Also, cotton farmers may delay irrigation following a decent rainfall event. And they can cut back late in the growing season. “After bolls are open, irrigation is questionable unless the evapotranspiration rate is high and the field moisture profile is depleted,” Boman said.

Altering nozzle packages also may improve irrigation efficiency. “Adjust nozzles as water declines,” he said.

“We learned in 2011 that sometimes we have to increase the amount of water we apply through irrigation,” Boman added. Concentrating water on smaller plots – half circles, for instance—helped farmers save at least part of their planted acreage.

Repairing leaks in the system also improves water use efficiency. “Producers also need to determine the cost of pumping per inch of water delivered.

In 2011, farmers who had very high irrigation capacity and got their cotton to canopy by early August  made “strikingly good yields. Most could not, however.”

Deficit irrigation—applying less than the maximum amount of irrigation the cotton crop needs—can be an effective way to irrigate and reduce water use, Boman said. In fact, applying 75 percent of the ET rate may produce close to the typical yield possible with 100 percent ET irrigation. Studies with low energy precision application (LEPA) irrigation applying 75 percent of ET has shown “a wash in terms of yield (compared to 100 percent ET application) with 25 percent less water.” In some cases, yield with 75 percent ET applied through LEPA was equal to 100 percent ET applied with a spray irrigation system.

“Cotton can respond well to moderate levels of deficit irrigation, but water salinity can be an issue, and salinity in this area is highly variable.”

System comparisons

Irrigation system type also makes a difference.

Flood and furrow irrigation offer the least efficient options with efficiency ratings from 40 percent to 80 percent. Labor issues, poor water distribution, and difficulty in applying small amounts of water are limitations. “Also, land must be level, and uniform nutrient application is almost impossible,” Boman said.

A sprinkler or spray system offers from 65 percent to 90 percent efficiency but adds chemigation potential. “It’s more expensive than flood or furrow systems and requires more energy to operate.” These systems also do not fully utilize all land areas. Adding an end gun does not provide an efficient option, Boman added.

With LEPA, producers farm on circle rows with drag hoses and socks on alternate rows. Some growers use diked furrows to hold moisture. Water is less exposed on the soil surface.

LEPA systems are considered 85 percent to 95 percent efficient.

Disadvantages, Boman says, include increased level of management, a switch to farming in a circle, topographical limitations (terraces, slopes, etc.). “Also, farmers must be precise with tillage. Auto-steer helps with that.”

Subsurface drip irrigation gets top billing for water use efficiency at 85 percent to 99 percent. Producers use more frequent irrigation applications at smaller amounts. Drip systems also offer “unique potential for chemigation.” Disadvantages include initial cost of installation, concern over seed germination and limitations for foliar application. “Also, we can’t move it,” Boman said.

Water quality

With all these systems, water quality is a crucial issue, Boman says. Salinity, in particular, may cause significant problems. “We’ve had better moisture for part of the growing season this year,” Boman said, “but Southwest Oklahoma remains in severe drought status, and we have pumped a lot of saline water. It is a problem.”

 

He recommends farmers sample the soil and the water to determine salinity. “Salinity can affect a lot of things in cotton production, especially if producers want to grow a cover crop. Look at soil salinity and then look at crop options.”

He also recommends farmers check nutrient levels, especially nitrogen, in irrigation water. “Cotton does not need too much nitrogen. The crop will be harder to defoliate, and too much nitrogen may reduce mic. A short-season area is especially vulnerable to overuse of nitrogen.”

Boman said farmers may find more nitrogen in irrigation water then they expect, enough to affect fertilizer demand.

With 15 inches of irrigation water applied and 10 parts per million of nitrogen in the water, producers are adding 30 pounds of nitrogen per acre. With 20 inches of irrigation water, that rate increases to 70 pounds of nitrogen per acre.

“Farmers may add nitrogen without knowing it”’ Boman warned. “We also find boron and sulfur in irrigation water.”

Testing is the only way to know, Boman says. 

Cotton farmers in the cotton-production region of Oklahoma typically need about 15 inches of irrigation water to supplement average rainfall. For the past four years across most of the area, farmers have had neither.

With reservoirs at historic low levels and drought status still rated severe or worse, Oklahoma farmers who irrigate must do all they can to improve irrigation efficiency, Boman says. And accomplishing that probably will require a handful of techniques, technologies and a bit of system tampering to get the most out of every drop of water.

 

Boman Credits Texas A&M AgriLife irrigation researcher Jim Bordovsky and Extension engineer Dana Porter for their work in developing information included in the cotton water demand presentation

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