Gordon Johnson and Bill Raun
of Plant & Soil Sciences
Agricultural Sciences & Natural Resources
CARES? The largest purchased input for wheat farmers
is usually nitrogen (N) fertilizer.
Costs vary, but are about $20 per acre.
An average of 65% of the applied fertilizer is lost by volatilization
and leaching, equivalent to about $13/acre per year. In
nitrogen fertilizer based on the relationship of the crop requiring 2 pounds of
available N per bushel of yield may be wrong this year. For
about the last 35 years, specialists at
often calculated but seldom experienced. The problem of using the traditional approach
of 2 lb N/bushel of wheat is that this “rule of thumb” was based on the average
response of wheat over many years and several locations. At the time it was first recommended (about
1969) it was a big improvement over any other information farmers had for
figuring out how much fertilizer to apply.
However, a graph of data from the Lahoma Experiment Station long-term
trial (502) shows that actual N fertilizer required for maximum yield sometimes
varied almost three-fold from one year (1990) to the next (1991).
The graph also shows that for the 31 years of data, 60
% of the time a 2 lb N/bushel N rate would have been wrong by more than 10 %. Being a little bit wrong is not too bad, but almost
one-half of the time (42 %) the error was at least 35 %.
the wrong rate of N fertilizer. The result of all this is that fields will
often receive the wrong rate of fertilizer.
For the Lahoma research data, the yield goal based rate (80 lb N/acre
for a 40 bu yield goal) was wrong two-thirds of the
time, and half of the years the cost for being wrong was greater than $5/acre
(average loss over 31 years was $9/acre/yr).
to the problem. Instead of using a yield goal, which assumes
the year will be “average or slightly better”, evaluating the crop yield
potential during the season leads to a more accurate fertilizer N rate. This means N fertilizer is not applied to the
field, except for the N-Rich Strip (see PT 2003-07), until after the crop has
been growing and the farmer can develop an idea of what kind of year it will
be. Fertilizer is applied during normal
benefit. As a result of applying the N during the
season, instead of preplant, the nitrogen use efficiency is improved, leading
to decreased loss of N to the environment and improved profitability. The net economic benefit from in-season estimates
of yield and topdressing all fertilizer N for grain production is estimated at
$17/acre/year for the 31 years of data from the Lahoma research trial.
change. Not applying preplant N fertilizer, usually
anhydrous ammonia, is a change not easy for most farmers to put into practice. Tradition is the biggest obstacle. It is often easiest to do things the way they
have always been done, even if it is not the most profitable or best for the
environment. Cost of N is another
factor. Preplant N applications are
often justified on the basis of lower cost of N as anhydrous ammonia. This difference has usually been about $0.05
per lb N, but is less of a factor when an extra cultivation is made to apply
pasture is managed differently. Wheat being managed for pasture is more likely to
respond to an early, low rate (20 to 40 lb N/acre) of fertilizer. An N-Rich Strip should still be established
to help determine later topdressing needs.
related PTs and other information go to:
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