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Pinpoint wheat irrigation yields most for least

Colorado and North China,
although on opposite sides of the globe, have one thing in common: They are
areas of increasingly limited and erratic rainfall and irrigation water.
Building on the commonality, field tests of wheat-corn double-cropping systems
in China and cornfields in Colorado were created. Researchers in both locations
teamed up to see how far they could stretch water resources, as well as
nitrogen, without cutting yields.

Stateside, the research team
included USDA Agricultural Research Service engineers Laj Ahuja, Walter Bausch,
and Gerald Buchleiter based at Colorado State University. At Fort Collins, they
tried four irrigation levels and found that in limited irrigation, it was best
to skip the traditional preplanting irrigation for wheat. “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. “This promises the highest yields, the best
water-use efficiency, and the least water drainage overall.”

The team completed two years
of experiments with four irrigation levels in China. “While the combination of
models has been used in other experiments to test alternative water- and
nitrogen-management practices, this is the first time the models have been used
to evaluate crop responses to lack of water across critical crop-growth stages
and the first to use long-term weather data,” Ahuja explains.

93 Years Of Data, Eight
Years Of Field Tests

The long-term simulation
with 38 years of weather data shows that North China plain farmers should apply
no more than 180 pounds of nitrogen fertilizer per acre to get the best yields
for winter wheat followed by corn. Those farmers usually use about 270 pounds
per acre. The model results indicate that cutting back nitrogen fertilizer use
by one third would reduce nitrate leaching by 60% without affecting crop
yields.

In Colorado, ARS researchers
used the Decision Support System for Agrotechnology Transfer (DSSAT) model to
look at corn produced either with or without irrigation. They used local
historical weather records from 1912 through 2005. Field data came from eight
years of experiments.

The results are similar to
those in China in terms of favoring crop-growth stages with irrigation water.
When simulated irrigation water supplies are limited, it is best for yields,
water-use efficiency, and minimization of nitrogen losses when 20% of the water
is used during the vegetative stage of development and 80% is used for the
critical flowering and grain-filling stage.

Best To Wait Until Tasseling

At the higher irrigation
levels, they found it is best in northeast Colorado to begin the
reproductive-stage irrigation on June 22, when tassels initiate but before
pollen is shed. Also, when water is very limited, the simulations show that
farmers would do best to fully irrigate only half a field.

An alternative is for
farmers to 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.

“Crop-simulation models
(utilized in the ARS research) enable faster and cheaper transfer of technology
from research labs and experiment stations to farmers’ fields,” Ahuja adds.

Ahuja believes that computer
models coupled with field experiments are an excellent tool for making the best
use of limited water.

“The concepts developed in
these studies can potentially be adapted to other locations, climates, and
crops,” Ahuja says. “But site-specific recommendations need to be developed for
each soil-climate zone using models validated by local experiments. And we will
have to do studies based on the various technologies available for triggering
irrigations.”  

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