How close are Nebraska irrigators to the corn yield potential on their farm? In the three-county Tri-Basin Natural Resources District of south-central Nebraska, at least, the answer is pretty darn close.
Irrigators in Gosper, Phelps and Kearney counties are quite good at “getting more crop per drop,” and the remaining irrigators in the state likely aren’t far behind, according to Ken Cassman.
Cassman is a University of Nebraska-Lincoln professor of agronomy who, along with postdoctoral research associate Patricio Grassini, examined corn yields, water use and management practices within the Tri-Basin NRD. They found positive results in achieving high yields with high water and nitrogen fertilizer efficiency.
“Rising demand for food, livestock feed and biofuel, and limited water supplies, require greater yields on existing irrigated land without using more water,” Cassman says. “Assuming we need to produce corn somewhere in the world, and assuming we need to produce as much as possible with minimal environmental impact, Nebraska irrigators are some of the best to do it.”
The research refers to “water productivity,” defined as the ratio of corn yield to total water supply, which encompasses plant-available soil moisture at planting, plus rainfall and irrigation during the growing season.
They estimated yield potential for individual fields using a crop simulation model in combination with actual weather records, soil properties and detailed data on crop management from 123 farmers’ fields in the NRD. Those 123 fields were among 777 fields in the district from which data on yield, irrigation and nitrogen fertilizer were collected over three years — 2005 through 2007.
The Tri-Basin has maintained that information for years as part of its water-supply and water-quality programs. The more detailed information about crop management came from 123 fields because those farmers responded to a survey sent out by researchers.
“Average yields among the 777 fields were quite high, ranging from 199 to 218 bushels per acre over the three years,” Cassman says. “Comparison between actual yield and simulated yield potential for each of the 123 surveyed fields showed that irrigated corn producers in this region achieve yields, on average, 11% below the simulated yield potential of 234 bushels an acre.”
Based on total water available — planting-time soil moisture, rainfall and irrigation — to the 777 fields and the resulting yields over three years, the researchers determined that the “attainable corn yield per unit of water supply in well-managed commercial production fields” was 7.8 bushels per acre-inch of water. Average on-farm actual water productivity in those fields was 5.7 bushels per acre-inch of water, 73% of the 7.8 figure.
About half the fields had a water supply in excess of 36 inches, the amount of total water required for maximum yield of 246 bushels an acre. That meant a slight majority of fields received more irrigation water than needed to achieve highest attainable yield, Cassman says. Over-irrigation was more common in fields with surface irrigation.
One way to narrow the gap between simulated yield potential and actual yields, Cassman says, is to plant a full-season hybrid early, although that poses risks in some years.
High yields and water productivity occurred when corn followed soybeans, was irrigated by a center pivot and was in a no-till or ridge-till system. “There are substantial opportunities to improve yields and input efficiencies by using pivot instead of surface irrigation, reduced tillage in a corn-soybean rotation, and better nitrogen and irrigation management,” Cassman says.
Reduced-tillage systems save 2.5 inches of irrigation water per acre annually.
Future water savings
Thirty-three percent of the acreage was irrigated by gravity systems and the other 67% by pivot. Cassman and Grassini estimate that if all acres were converted to pivot, 38,000 acre-feet of water could be saved within the district.
Although pivots apply 5 to 6 inches less water than gravity systems, yield was the same under both irrigated systems due to higher-efficiency pivots. Yield increased 13 bushels per acre for every inch applied by pivot, vs. 8 bushels per acre for every inch applied in a gravity system.
If soil moisture sensors and evapotranspiration figures were used in irrigation scheduling on all acres, another 21,000 acre-feet could be saved.
Employing a limited-irrigation strategy, except during the critical reproductive stages of corn, could produce another 32,000 acre-feet of water savings.
“These practices combined can increase yield and lower water and energy use,” Cassman says.
Looking to the future, Cassman says management and regulatory systems that lower production are not the answer, considering world food demand. “We need to use science and technology to increase production while lowering agriculture’s environmental footprint.”
Cassman and Grassini say that their methodology of simulating yield potential and comparing it to actual records and management practices can be applied elsewhere. “For instance, if a farmer comes to us and tells us his yields and where the farm is located, and some of his production practices, we can go to our database and identify ways to increase yield with the same water. Or if he’s under some type of water-limited regulatory program, we can suggest how to protect his current yield.”
Efficient use of N
Tri-Basin irrigators were also efficient users of nitrogen. In fact, they are better than the national average.
For every pound of nitrogen applied to corn following soybeans, they produced 1.4 bushels of corn. “The average for the United States is 1 bushel per pound of nitrogen,” Cassman says.