Using
a "Partial Delta Yield" Approach to Fine Tuning N Rates
Keith Reid, Soil Fertility Specialist
Any trial involves making some assumptions. I will try to list the assumptions underlying this approach. If these assumptions are not valid on your farm, the results won't necessarily give you the best answers.
1. Yield response to N follows a quadratic
or quadratic-plateau pattern (ie. The law of diminishing returns).
2. The optimum N rate is fairly consistent over the whole field, even though
the yield may vary.
3. Optimum N rates are fairly consistent from year to year.
4. A farmer's crop rotation and pattern of manure use is fairly consistent
from year to year.
5. Most farmers are already applying optimum or above-optimum rates for their
crop (although the same approach can be modified for investigating increasing
N rates).
6. Farmers are able to vary the rate of nitrogen applied to part of the field,
and can measure the yields at the end of the season.
Laying out the strips:
To provide good information, the partial N
strips need to be repeated, and need to be located so that they are representative
of the field. Each field that is being checked should include a minimum of
three partial N strips. If resources are limited, do the comparisons in one
field, rather than doing a smaller number of comparisons spread over several
fields. The strips should go across any major variations on topography or
soil type. If this is not possible, widely contrasting areas in the field
should be treated as separate comparisons. Uneven manure applications across
the field will make interpretation of the results difficult or impossible.
The partial N rate should be two-thirds of the normal rate applied to the
rest of the field. This should provide a large enough difference that the
difference in N response is larger than the random variation between adjacent
strips in the field, but not so large that yield losses are extreme.
Each partial N strip needs to be wider than the width of the combine header,
but as narrow as possible beyond that. They must be laid out so that the strips
can be harvested as a full header width with the minimum disruption of the
harvest. Location of the strips must be carefully marked with flags or some
other identification system.
Physically applying the reduced rate of nitrogen in strips will be easiest
with side-dress applicators. Spray application of UAN or airflow application
of granular fertilizer can be varied relatively easily, but the strips will
be quite wide unless you can convince your applicator to apply a half boom
width at the partial rate, and then double back and apply the other half boom
width at the full rate. The most difficult system to apply partial rates in
will be pull type spinner spreaders - these will require some very careful
planning and implementation on the part of the farmer.
Soil nitrate levels can vary from year to year, depending on the amount of
nitrate loss over winter, and the amount of N mineralized from organic sources
in the spring. We are undertaking a project to determine if a benchmark site,
which is sampled each year, can track the relative availability of soil nitrate
from year to year, without having to sample the entire field. If this approach
is valid, it will be useful to have a history of soil nitrate levels from
a benchmark within your field. Choose a site typical of the field, away from
headlands or other sources of variation, and collect 6-8 cores, 12 inches
(30 cm) deep, for analysis for nitrate. This will provide a baseline for that
site.
Measurements at harvest:
a) Weigh wagons or scales
At harvest time, combine each strip with the reduced N rate (or a full header
width within that strip), and weigh. Determine the moisture content from a
sample. Repeat for an adjacent strip with the full N rate. Measure the length
and width of each strip, so that yields per acre can be calculated. Average
the yields from the reduced N strips, and compare this to the average of the
yields from the full N strips. This method can be used to assess field scale
N rates, but cannot assess smaller areas within the field.
b) Combine Yield Monitors
As with any comparison using a combine yield monitor, make sure that the monitor
is properly calibrated before attempting to harvest the trial. When harvesting,
save each reduced N strip, and the adjacent full rate strip, as a separate
load. This will allow much more accurate comparisons of the average values
for each strip.
Interpreting the Results:
Compare the average of the yields from the
reduced N strips with the average yields from the adjacent full rate strips.
DO NOT compare the average of the reduced N strips with the average yield
of the rest of field - there will be too much random variation introduced
for the comparison to mean anything.
If the yields from the reduced rate strips are the same as the yields from
the full rate strips (within two or three percent), or higher than, you can
have a fair degree of confidence that your normal rate of nitrogen fertilizer
was too high for that field and that year. If you cannot identify any reason
why the response to nitrogen would be lower than normal, you can consider
reducing N rates next year.
If there is a moderately higher yield in the strips receiving the full rate
of nitrogen fertilizer, the N rate used was probably close to the optimum
for that field. If the yield was much higher in the full N strips, it may
mean that the rate used was below the optimum for that field. Further work
is needed to identify how large a yield discrepancy is needed to indicate
that fertilizer rates should be increased.