Agriculture Reference
In-Depth Information
the number of days from planting to sensing in
which growing degree day values are positive
[GDD = (T
min
+ T
max
)/2 − 4.4 ºC]:
2006). In winter wheat, the life cycle from plant-
ing to harvest can exceed 240 days. Unlike spring
wheat and hybrid maize production cycles that
seldom exceed 110 days (planting to physiological
maturity), the growth cycle of winter wheat is
long enough that corrections can be made in-
season, if a defi ciency exists. Raun et al. (2002)
showed that N applications delayed until Feekes
growth stage 5 (Large 1954) could still result in
maximum or near-maximum grain yields over
four sites and two years in winter wheat. It is
important to note differences between wheat and
maize, since maize will remove almost double the
amount of total N in the grain in one-half the
time. Because of the shorter growth cycle and
increased total N uptake, getting behind early in
the growth cycle of maize can lead to decreased
grain yields even if additional N is applied (Varvel
et al., 1997). In addition, wheat can better recover
than maize from midseason N applications
because wheat can produce later tillers to com-
pensate, whereas maize cannot set additional
ears.
YP
0
= 0.590e
258.2INSEY
(10.1)
In spring wheat, INSEY is equal to NDVI
divided by the number of days from planting to
sensing, since with few exceptions all days in the
spring wheat cycle will have a positive value for
GDD. For winter wheat, many days may occur
in which the mean temperature does not exceed
4.4 ºC, and as such there is no plant growth.
Parameters for equation 10.1 are crop-specifi c
(e.g., winter wheat, spring wheat) and are pub-
lished at http://www.nue.okstate. The following
equations predict the potential yield with
additional N fertilizer, or YP
N
, for two sets of
conditions:
YP
N
= YP
0
× RI
NDVI
,
if NDVI
FieldRate
≥ 0.25 and YP
N
< YP
max
(10.2)
or
DETERMINATION OF MIDSEASON
N RATE
YP
N
= YP
max
, if YP
0
× RI
NDVI
≤ YP
max
Our observations over several years indicate that
values of NDVI < 0.25 occur on bare soil or
on soil with wheat stands so poor at Feekes
growth stage 5 that they will not produce appre-
ciable yields. The second set of conditions
for YP
N
simply state that YP
N
cannot exceed
YP
MAX
, where YP
MAX
is an agronomic optimum
identifi ed by farmers and scientists in specifi c
regions.
For wheat, the top-dress N requirement can
then be calculated by:
The response to fertilizer N is dependent on the
supply of nonfertilizer N (e.g., mineralized from
soil organic matter, deposited in rainfall) in any
given year. This is attributed to the extensive dif-
ferences in annual rainfall and temperature and
associated change in crop need (temporal vari-
ability), which directly infl uences how much non-
fertilizer N is used by the crop.
The NRS, where N is not limiting, defi nes the
sensor NDVI value at which N fertilizer is no
longer limited. The highest NDVI measurement
along the NRS can be used to calculate the
maximum potential yield. Thus when N is not
limiting for the year of measurement, YP
max
is the
maximum yield that can be expected within the
most productive area in a fi eld.
Three equations are used to calculate N fertil-
izer rates. Yield potential or YP
0
(Mg ha
−1
) in
winter wheat can be calculated directly by equa-
tion 10.1, where INSEY equals NDVI divided by
R
= 2.39 (YP
N
− YP
0
)/h
(10.3)
in which
R
is the N application rate in kg ha
−1
,
2.39 is the percentage N contained in wheat grain,
and h is the expected effi ciency from top-dress N
application, generally between 0.5 and 0.7. This
expected use-effi ciency can vary widely, but
should fall within this range for midseason N
applications based on our observations.
