Agriculture Reference
In-Depth Information
require complicated stochastic methods using
data collected from previous harvests (Brooks
et al., 2001).
between 0.5 and 0.7 based on agronomic data col-
lected from a wide range of environments) and
the maximum possible grain yield for each spe-
cifi c environment.
The improved midseason N management
approaches have resulted in documented increases
in NUE exceeding 15% (Raun et al., 2002).
Extensive on-farm evaluation of the Sensor Based
Nitrogen Rate Calculator for wheat and maize has
shown a minimum increase in farmer revenue of
$22.00 ha
−1
when using the recommended N rate
in wheat (Ortiz-Monasterio and Raun 2007), and
over $40.00 ha
−1
in maize.
The majority of this work has focused on deliv-
ering improved N rates at the fi eld scale where
only temporal variability is addressed. However,
spatial variability, as well as temporal variability,
can be addressed with this methodology and is
more profi table (Biermacher at al., 2006), but that
requires increased investment in sensing equip-
ment that can be adapted to virtually any fertilizer
applicator for midseason fertilizer application in
cereals (NTech Industries, www.ntechindustries.
com). Their systems employ the algorithm YP
0
×
RI = YP
N
as discussed earlier in this section, and
they too have delivered increased farmer profi t
using this approach for winter wheat, spring
wheat, and maize.
The combined knowledge of fl uctuating yield
levels spatially within a fi eld and changes in addi-
tional N availability from one year to the next
demands that a N application rate be determined
midway through the crop season. This, coupled
with knowledge of plant stands and vigor obtained
from early season growth, provides an accurate
method of applying a judicious fertilizer rate.
PREDICTION OF N RESPONSIVENESS
INDEPENDENT OF YIELD POTENTIAL
Yield prediction is but part of the process when
arriving at an optimal fertilizer N rate. As noted
earlier the crop's response to N fertilizer changes
radically from year to year as a function of the rate
of conversion of organic N to inorganic N
(Johnson and Raun 2003), as well as from fi eld to
fi eld as a function of soil type and previous man-
agement. Furthermore, the same NDVI sensors
used to predict wheat grain yield potential can
also be used on the NRS and the area surrounding
the NRS (hereafter termed
farmer practice
) to
predict N responsiveness, or what is termed the
response index (RI, estimated as NDVI collected
from the NRS divided by NDVI from the farmer
practice) (Mullen et al., 2003). Mullen et al.
(2003) showed that the RI from NDVI readings
(RI
NDVI
) was highly correlated with fi nal grain
yield in the NRS divided by fi nal grain yield in
the farmer practice, or the harvest response index
(RI
harvest
). This positive and strong relationship
confi rmed that N responsiveness could be
predicted across a wide range of yield levels
from midseason indirect measurements, such as
NDVI.
Once it was recognized that crop response to N
fertilizer could be predicted from early-season
sensor readings, this agronomic component was
combined with yield potential at no additional N
(YP
0
) (Raun et al., 2001) to estimate crop yield
with suffi cient N, YP
N
(Raun et al., 2005). The
fundamental relationship was YP
0
× RI = YP
N
. By
estimating the amount of N taken up by the crop
(yield potential if fertilized, YP
N
, and yield poten-
tial without added fertilizer, YP
0
), the defi cit or
added amount needed to achieve maximum or
near-maximum yields was that amount of N
required to produce the predicted difference in
grain yield (YP
N
−YP
0
). This calculation was
further refi ned by accounting for the expected
effi ciency of the midseason N applied (usually
MIDSEASON N APPLICATIONS CAN
RESULT IN MAXIMUM YIELDS
Provided N is applied later, early-season N stress
seldom results in decreased grain yield in winter
wheat or spring wheat. Thus, midseason N appli-
cations are much more effi cient, and as a result of
this improved effi ciency, lower amounts of N are
required relative to a preplant N application to
produce the same level of yield (Morris et al.,
