March 7, 2006

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HALFWAY – It has been awhile since agricultural researchers discovered and then proved the benefits of crop rotation. Since then, most farmers have embraced the practice of switching a piece of ground from one crop to another to improve yields, reduce erosion potential, and break insect and disease cycles.

Texas Agricultural Experiment Station researchers are now pondering whether crop rotation offers another benefit. Can it add value to irrigation water and help maintain or improve yields in limited irrigation situations?

"The Ogallala Aquifer is a finite source of water," said Jim Bordovsky, Experiment Station agricultural engineer. "Our farmers know we have to conserve water, so they are refocusing on rain-based crop production supplemented by limited, or deficit, irrigation."

There are more than 1 million dryland and more than 2 million irrigated crop acres on the Texas High Plains. As the demand for water grows and available supplies are used, crop producers face increasing competition to share their common, finite water resource with residential, manufacturing and livestock sectors of the economy.

Bordovsky and other scientists are investigating the feasibility of producing cotton and grain sorghum in rotation using dryland production strategies supplemented by very limited irrigation using efficient delivery systems.

"We started this study in 2001," Bordovsky said. "The rationale is to do everything possible to harvest our average 18 inches of annual rainfall for a crop rotation and then add small amounts of irrigation at the right time to stabilize or improve yields.

"Rotating drought-tolerant crops, utilizing crop residue, furrow diking to catch rainfall, reducing or minimizing tillage, adjusting varieties and plant populations to field conditions and using efficient irrigation systems are key to making this strategy work."

The study was conducted at the Halfway Experiment Station on a 30-acre field equipped with a Low-Energy Precision Application (LEPA) center pivot sprinkler.

Each year's crop rotation treatments included cotton-cotton-grain sorghum, cotton-grain sorghum-cotton, and grain sorghum-cotton-cotton. These treatments were compared to a three-year cycle of continuous cotton. The rotations utilized two varieties of cotton and one variety of grain sorghum planted on 40-inch rows.

The study also compared the water-use efficiencies of three irrigation regimes.

"Those three regimes are zero or no seasonal irrigation, irrigation at 1.25 gallons per minute per acre irrigation capacity or approximately 25 percent of the evapotranspiration rate, and 2.5 gallons per minute per acre or 50 percent of peak cotton evapotranspiration," Bordovsky said. "The LEPA pivot has drops in alternate rows and all the drop lines have an 18- to 20-inch sock over the emitter. Each drop line is also equipped with a manual on-off valves to precisely control the amount of water applied."

The researchers used seeding rates, fertility and pest control practices common in the region. They used minimum tillage and left grain sorghum residue in place until spring, when the stalks were either mowed or removed with a stalk puller.

"The timing and quantity of rainfall have a huge impact on crop production in areas with limited irrigation," Bordovsky said. "The crops years from 2001 through 2003 were very dry in July and August, a critical growth period. We also had significant cotton damage in June 2003 from heavy rain, hail and high winds. That weather reduced our cotton populations and caused disease that slowed the growth of surviving plants."

The researchers started irrigation in early June each year and stopped in August. The Low-Energy Precision Application center pivot allowed them to "shift" irrigation between crops within the rotation treatment to partially address plant water requirements during critical growth periods.

Study results to date show the cotton-grain sorghum treatments consistently increased cotton yield compared to continuous cotton.

In the harshest year, 2003, their cotton-grain sorghum rotation yields were 26 percent higher than continuous cotton yields, used 19 percent less water and required less tillage to prevent blowing sand.

"Our seasonal irrigation water-use efficiency was highest in rotations where cotton followed grain sorghum, using the 1.25 gpm/acre irrigation capacity," Bordovsky said. "But there is a real risk with rotations under dryland conditions. In low rainfall years such as 2002 and 2003, we didn't produce any harvestable grain, and we didn't see any real cotton yield benefit from the grain sorghum residue."

And because cotton commands a higher market value than grain sorghum, their continuous cotton resulted in a higher water value than their cotton-grain sorghum rotation.

"We got a higher return for our water with continuous cotton, but cotton grown in rotation with grain sorghum used its limited water more efficiently than continuous cotton," Bordovsky said. "Higher market prices for milo or reducing water input into the sorghum in the rotation treatments might swing the water-value factor in favor of the cotton-grain sorghum rotation."

The researchers plan to continue their studies for at least two more years. They want to test other crops such as drought-tolerant corn or forage sorghums in rotation with cotton. They also are trying to calibrate the CROPMAN software model to help evaluate other crop production and irrigation scenarios.

"CROPMAN simulates crop production and the related economics that could possibly be used to decide when and where you will get the most bang for your input dollars," Bordovsky said.

Their research is funded by contributions from the Experiment Station's Cropping Systems Initiative and the U.S. Department of Agriculture's Agricultural Research Service Ogallala Initiative.