Figure 1. Wheat grain yield from 1971 to
2003 from longterm experiment #502, near Lahoma, OK from the check plot
where no N has been applied since 1971 (04060), and the nonN limiting
plot that has received 100 lb N/ac every year (1004060). Note that
in 2002, near maximum yields were produced in the check plot, a product of
an extremely warm and wet winter resulting in significant quantities of N
mineralized from soil organic matter.
Definitions
INSEY
In Season Estimated Yield = NDVI (Feekes 4 to 6)/days from planting
to sensing (days with GDD>0) = YP_{0
}_{
}
GDD
Growing Degree Days = (Tmin + Tmax)/2 – 4.4°C
RI_{NDVI
}= NDVI from plots receiving adequate but not excessive
preplant N, divided by NDVI from the farmer check (specific to that field)
where preplant N may or may not have been applied.
RI_{HARVEST
}= Maximum observed grain yield (treatment average with N
fertilizer) divided by the observed grain yield from plots where no N was
applied either preplant or topdress
RI_{SV}
= Estimate of RI (spatial variability), using the average NDVI
value from a random sensor sample (100 m in length) collected in a farmers
field + 1 standard deviation, divided by the average NDVI value minus 1
standard deviation.
YP_{MAX
}
= Maximum obtainable yield level for a specific environment
determined by the farmer, or previously defined as a biological maximum by
research agronomists for that crop, and for that region (units: Mg/ha)
YP_{0
}
= Predicted or potential yield based on growing conditions up to
the time of sensing, that can be achieved with no additional (topdress) N
fertilization (units: Mg/ha)
YP_{N
}= Predicted or potential yield that can be achieved with
additional (topdress) N fertilization based on the inseason response
index (RI_{NDVI}) (units: Mg/ha)
= (YP_{0})/ (1/RI_{NDVI})
=
(YP_{0})*(RI_{NDVI})
PNG
= Percent nitrogen in the grain estimated from multiple
observations, over years, using various different wheat varieties (likely
to be region specific) (units: %)
GNUP
= Predicted grain N uptake (predicted percent N in the grain
multiplied times YP_{N}/1000) (units: kg/ha)
FNUP
= Predicted forage N uptake estimated from the calibrated
relationship with NDVI (units:
kg/ha)
FNR
= Fertilizer nitrogen requirement estimated by subtracting
predicted forage N uptake (FNUP) from predicted grain N uptake (GNUP)
(units: kg/ha)
Restrictions
YP_{N}
cannot be > YP_{MAX
}_{
}
Figure
1. Predicted yield achievable
with added N fertilization (YP_{N}) based on RI_{NDVI }as
a function of predicted yield that can be achieved with no added N
fertilization (YP0), where YP_{N} = (YP_{0})/ (1/RI_{NDVI}).
