Grain yield trends observed for Site Year III were
similar to grain yield trends observed for Site Year II.
Mean grain yield increased with the productivity potential
of the management zones in two out of three
treatments. The low zone had the lowest grain yields
within treatments (9.3–14.7 Mg ha21), the medium zone
had slightly higher yields (11.1–14.7 Mg ha21), while the
high zone had the highest yields (10.0–15.3 Mg ha21).
In two out of three treatments, however, statistically
significant differences in grain yield were found only
for the low and high management zones. These results
are similar to those of Inman et al. (2005), who found
that the soil-color-based management zone technique
consistently differentiated N uptake levels between low
and high productivity zones. In their study, they attributed
the lack of consistent differentiation of the
medium zone from either the low or high zones to
isolated and small inclusions of both high and low
management zones within the medium management
zones. These inclusions are relicts of the intentional
“smoothing” that is performed on the management zone
maps. The smoothing makes it practical for fertilizer
application equipment to use the management zone
maps for variable-rate application.