How to save more water and N

Agricultural Research Service scientists at Fort Collins, Colo., are ushering farmers into an age of limited water supplies for irrigation.

“With the increasing complexities of modern agriculture due to environmental concerns and more frequent droughts, we need to optimize the use of limited water, as well as nitrogen and other inputs, for varying weather conditions,” says Laj Ahuja, research leader of the ARS unit.

Key Points

• Studies focus on when to irrigate using growth stages or moisture.

• Use 20% of water at vegetative stage and 80% for flowering, grain-filling stage.

• Irrigate when water in the top 18 inches of soil is depleted by 20% to 40%.

Save water and nitrogen

Ahuja has been involved in studies with computer models backed by field experiments in areas of increasingly limited rainfall and irrigation water: Two studies were in wheat-corn double-cropping systems in the North China Plain area, and one was in northeastern Colorado corn fields.

The studies’ computer models used long-term local weather data to provide recommendations for optimal water and nitrogen, or N, use in each area. Researchers focused on irrigations triggered either by growth stages or by declining levels of soil moisture.

For the North China Plain studies, Ahuja teamed up with Quanxiao Fang at China’s Qingdao Agricultural University and the Chinese Academy of Sciences. They used four irrigation levels based on the Fort Collins crop growth-stage method; the scientists found that with limited irrigation, it was best to skip the traditional wheat preplanting irrigation.

Crop simulations in China relied on local 1961 to 1999 weather data. Simulations showed that there would still be adequate water in wheat fields from heavy rains the previous corn season.

“We also found it best to use 80% of the water for the two critical wheat growth stages, and only 20% at corn planting,” Ahuja says.

Stem extension and booting are critical for wheat. Booting is when the wheat head is about to emerge from the top leaf sheath of the stem. The stem-extension stage is crucial because that is when the last leaf emerges, the leaf that contributes the most nutrients to the developing grains and yield.

Irrigate later, fertilize less

North China Plain farmers can delay their first irrigations until the water in the top 18 inches of soil is 50% depleted, they found. And when they do irrigate, it’s best to recharge the land to no more than 70% capacity.

The long-term simulation (38 years of weather data) showed that North China Plain farmers should apply only 180 pounds of nitrogen fertilizer per acre for best winter wheat, followed by corn yields. Farmers usually use about 270 pounds per acre. The results indicate that cutting back N fertilizer by one-third would reduce nitrate leaching by 60% without affecting yields.

Colorado experiments

In Colorado, ARS researchers used the DSSAT model to look at corn produced with or without irrigation.

As in China, results favored crop-growth stages with irrigation; when simulated irrigation water supplies were limited, it was best for yields, water-use efficiency, and minimizing nitrogen losses, when 20% of the water was used during the vegetative stage and 80% for critical flowering and grain-filling stage.

At the higher irrigation levels, they found it is best in northeastern Colorado to begin the reproductive-stage irrigation on June 22, when tassels initiate but before pollen is shed.

With very limited water, farmers would do best to fully irrigate only half a field.

And farmers could also wait until the plant-available water in the top 18 inches of soil is depleted by 20% to 40% before starting irrigations, depending on soil type.

Comis writes for ARS Information.


FIELD TESTS: ARS agricultural engineers Tim Green (left), Walter Bausch and Laj Ahuja adjust the height of solar radiation instruments to 1 meter above the corn canopy. Wind is measured at 2 meters. ARS photo by Stephen Ausmus

This article published in the January, 2010 edition of CALIFORNIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.