Modifying Nitrogen Rates for Delayed Seeding of Cereals and Canola

As a result of cold, wet weather, seeding of spring cereals and canola is delayed into late May and June.  Therefore, growers will be asking about reductions in N rates for the crops.  However, this type of situation is not straightforward.  There are at least 4 factors at play with delayed seeding under these conditions:

1. Reduced yield potential and hence reduced N demand
2. High rates of N relative to crop requirements may delay crop maturity
3. Mineralization of N from soil OM and crop residues may better match crop uptake patterns
4. Losses of soil N and fall-applied fertilizer N may be substantial

1. Reduced yield potential

If someone is looking for criteria for reducing N demand with lower yields, they could consider the uptake values provided by the Canadian Fertilizer Institute (modified in Table 1).

Table 1.  Nitrogen uptake and removal of cereals and canola (Canadian Fertilizer Institute, 2001)

* adapted from Canadian Fertilizer Institute – Nutrient Uptake and removal – Western Canada 2001

If a farmer feels that yield loss has occurred due to delayed seeding or other factors, they may consider deducting the N required for the yield difference from their original fertilizer recommendation.  So if one presumes that yield potential of wheat has dropped by 5 bu/ac, then N might be reduced by 7.5-10.5 lb N/ac. Of course this is a very rough guide, and assumes that one can estimate yield potential so early in the season.  To be good at this would require much experience.  One source of additional information is to consider Crop Insurance records of yield potential by planting date. 

2. High rates of N

High N status might delay crop maturity - Some farmers in areas of the province where the growing season is short may be concerned that continuing high nitrogen rates may delay maturity and cause harvest quality and yield issues.

3. Mineralization of N

Better synchronization of mineralization with crop requirements - Release of nitrogen by mineralization from soil organic N reserves requires moisture, oxygen and heat for microbial activity.  Therefore, planting later in the spring might allow for more mineralization during the period that the crop takes up substantial amounts of N.  However, this increase in mineralization during the growing season is probably more than offset by low rates of mineralization so far this spring, as well as losses of soil and fertilizer N due to excess moisture (see next point).

4. Losses of N

Substantial losses of N due to excess moisture - Currently we are so long on moisture that we have been losing nitrogen due to leaching and/or denitrification.  In fact, losses of fall soil nitrate and fall applied fertilizer will assuredly be greater than the average 20% MAFRI thumb rule, and so N fertilization rates may need to be increased, rather than decreased.  This is especially the case considering the favourable price projections for 2011 crops.

Summary

Due to the soil and fertilizer N losses experienced to date and high expected returns, growers should not be too quick to reduce fertilizer rates.  However, if seeding is delayed into June, growers may then want to consider modest reductions based on their estimates of yield potential.