Features Agronomy Fertility and Nutrients
What’s the right rate of nitrogen for winter wheat?

Nitrogen is one of the largest input costs for wheat production. The amount
applied can impact both profitability and the environment. The nitrogen rate
on any field must be high enough to optimize yield and quality, but too much
nitrogen will result in lower profitability and potentially negative environmental
consequences. The Agronomy Guide, OMAFRA Publication 811, has recommendation
tables for nitrogen rates, but just how accurate are these recommendations?

Research/demonstration plots were established to study nitrogen response on
six winter wheat fields in Halton and Middlesex counties from 2003 to 2005.
Various N rates and timing of N application were investigated.

The maximum economic rate of nitrogen (MERN) was highly variable from field-to-field
(see Figure 1). The MERN in three of six fields was close to the rates recommended
in the Agronomy Guide. However, the MERNs for two fields were considerably
higher (>120lb/ac N, >135kg/ha N) than the current recommended rate and for
one field, the best management practice would have been no N application at
all.

Various counties leading the search
The Middlesex 2005 field with a zero MERN suffered from winterkill and drought.
The Halton 2004 field with the next-lowest MERN also had winterkill issues.
Early yield potential for both were rather low. This suggests that a critical
evaluation of yield potential should be done in the spring, and N rates adjusted
accordingly. Otherwise, N applications will result in more N being applied than
the reduced crop potential can utilize and profits will drop.

Wheat yields of zero-N and N-rich fertilized plots were highly variable from
field-to-field. Yield response to N fertilizer was also highly variable (see
Figure 2). N remaining in the soil increased with higher N rates at all locations.
The proportion of fertilizer N recovered in the grain and straw decreased with
higher rates of N application (see Figure 3). Lower recovery means more potential
for N leaching, denitrification and loss as nitrous oxide, a greenhouse gas.

Application of N in mid to late April tended to result in the highest yields,
similar to current recommendations, while more losses were associated with early
or late applications. The practice of split rate applications between early-mid
April and early-mid May was not beneficial.

Proof is in the particulars
Current N recommendations in the Agronomy Guide are, on average, accurate. However,
they have been developed over many fields and growing conditions and are extremely
generalized. Research is now underway to capture this field-to-field and yield-to-yield
variability to see if nitrogen rates can be adjusted more accurately for a field
or possibly within a field.

This project was funded by Agriculture and Agri-Food Canada
via the Greenhouse Gas Mitigation Program for Agriculture, administered by the
Soil Conservation Council of Canada. The project was a co-operative effort between
the Halton and Middlesex Soil and Crop Improvement Associations and the Ontario
Ministry of Agriculture, Food and Rural Affairs' crops staff. The program was
co-ordinated by the Ontario Soil and Crop Improvement Association in conjunction
with Innovative Farmers Association of Ontario.

*Dave Hooker is with the University of Guelph; Peter Johnson, Adam Hayes,
Ian McDonald are with OMAFRA and Greg Kitching is with OSCIA.