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Crop sensors better at choosing N rates

November 16, 2011  By American Society of Agronomy (ASA) Crop Science Society of America (CSSA) Soil Science Society of

November 15, 2011 –
Choosing how much nitrogen (N) to put on crop fields isn’t something farmers
take lightly. Many factors go into the decision, including past experiences,
the timing of application, yield goals, and results from soil tests.

November 15, 2011 –
Choosing how much nitrogen (N) to put on crop fields isn’t something farmers
take lightly. Many factors go into the decision, including past experiences,
the timing of application, yield goals, and results from soil tests.

Nevertheless, crop sensors
can select N rates for corn that outperform those chosen by farmers, according
to more than 50 on-farm demonstration projects conducted in Missouri from 2004
to 2008. Compared to producers’ nitrogen rates, sensor-selected rates increased
yield by almost two bushels per acre, on average, while reducing by 25 per cent
the amount of excess nitrogen that was applied to fields but not removed in


As concerns about nitrogen
pollution continue to mount, the sensors offer a way to cut fertilizer inputs
without hurting yield or profits.

“The most important thing,
I think, is that we were able to make progress on both fronts: The technology
slightly improved production and slightly improved environmental outcome,” says
the study’s leader, Peter Scharf, a University of Missouri Extension
agronomist. “There has been talk about win-win, but really there have not been
a lot of approaches that have actually [achieved] that.”

Funded by the USDA-NRCS
Conservation Innovation Grants
program, the Missouri DNR Nonpoint Source
Pollution Control
program, and the USEPA Special Grants program, the study
appears in the November-December issue of Agronomy Journal.

Scharf explains that
although optimal N rates can vary substantially within and between fields, most
U.S. corn growers still apply the same rates to entire fields or even entire
farms. Many farmers in Missouri and elsewhere also spread N fertilizer months
before planting, often the November before.

As fertilizer and seed
costs keep climbing, however, corn producers are feeling financial pressure to
apply N more precisely – in amounts that satisfy crop requirements but don’t
exceed them. To help farmers with this, in 1997, Scharf began studying methods
for predicting where to put more N in fields and where to put less before
sowing crops, since that’s the system most people use. But he and his
colleagues eventually turned to crop sensors, employed after plants start
growing, as a more accurate means to diagnose N deficiency and sufficiency.

The sensors take advantage
of what farmers know already from experience and common sense, Scharf says:
Crops with enough nitrogen are darker green and taller, while
nitrogen-deficient crops are lighter and shorter. After developing a technique
for translating sensor output into a suitable N rate within a few seconds –
work that was published in 2009 – Scharf and his collaborators began taking the
technology to farms.

Fifty-five demonstrations
were eventually conducted across a broad swath of Missouri’s corn-growing
region. In most cases, two or three sensors were attached to N applicators
already owned by farmers or their service providers, and then used to
side-dress N at variable rates to corn in growth stages ranging from V6 and
V16. At the same time, fixed N rates chosen by farmers were applied in other
areas, allowing comparison of the two techniques.

An average of 14
pounds/acre less N was applied when sensors chose the rates, the researchers
found, without affecting yields. In fact, during the exceptionally wet spring
of 2008, sensor use actually boosted grain yield by 8 bushels/acre, on average,
over what producer rates achieved – a significant bump that brought the overall
yield gain with the sensors to 2 bushels/acre over all 55 fields.

Scharf believes yield
increased significantly in 2008 because the sensors actually chose higher N
rates than farmers did that year, better compensating for fertilizer lost
through heavy rainfall. And this yield bump, coupled with an overall reduction
in N fertilizer from 2004-2007, ended up increasing partial profit by an
average of $17/acre across all farms.

Despite the sensors’
benefits, however, “the adoption numbers are still quite small,” Scharf says.
Complete systems currently range in price from $10,500 to $16,500, and learning
to use them involves time and expense, as well. Still, these aren’t the main hurdles
to wider adoption, he adds.

The bigger one is getting
farmers to side-dress nitrogen during the growing season, rather than
fertilizing in spring before planting or even the fall before.

The unusually heavy rains
of the past four years may change that. Because applying N months in advance
gives it more time to leach and run off, many farmers have lost loads of it –
and, therefore, money and yield – in recent rain-soaked years. That leaves one
option: applying the nutrient during the growing season

“If this weather keeps up,
I think we’ll see more people going toward in-season N application,” he says.
“And that will be a big obstacle out of the way to using the sensors.”

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