Rates are based on yield potential and not on soil analysis (Table 4). For the corresponding rate, part of it should be applied in the fall and the rest after greenup. Generally, 20 to 30 pounds of fall-applied nitrogen should be adequate for early growth and initial growth the following spring. Spring applications may be applied between early March and early April. The spring application should be the total nitrogen recommendation less the amount applied in the fall. No credits are given for previous crops. For example, a wheat crop with a 90 bushels per acre yield goal would require 110 pounds of nitrogen (Table 4). If the grower applied 30 pounds in the fall, the remaining 80 pounds should be applied in the spring.
| Table 4. Nitrogen Recommendations for Wheat | |
|
Yield Potential (bu/A)
|
Nitrogen Rate (lb/A)
|
|
60
|
60
|
|
70
|
75
|
|
80
|
90
|
|
90
|
110
|
|
100
|
130
|
A split spring application program may be a benefit in poorly drained fields that are prone to nitrogen loss or in fields prone to lodging. For these programs, it is important that the first application occurs soon after initial green-up and the second application at initial jointing (Feekes Growth Stage 6). The time of application is not as critical in a single spring topdress (providing some nitrogen was applied in the fall), but applications should be made after initial green-up and before the second visible node on the stem (Feekes Growth Stage 7). Up to half the total intended amount of nitrogen can be applied in the second application.
Most sources of nitrogen are satisfactory for wheat. Urea and 28% solutions (urea-ammonium nitrate) are often the most common. Urea has the least potential to cause damage to the crop. Damage is generally insignificant from broadcast applications of 28% solution applied early, but the potential for damage increases as the crop matures. Dribble band applications will minimize damage from 28% solutions. Urea-ammonium nitrate solutions will have some nitrogen available immediately at application time; urea will have a short lag as it converts to ammonium and nitrate forms of nitrogen. The nitrate fraction of 28% solutions will be susceptible to loss after application until plant uptake. Urea may have volatilization losses if temperatures are exceedingly warm.
Only varieties with excellent standability should be used when high amounts of nitrogen are applied in split applications. High levels of nitrogen favor the development of a lush plant canopy that favors spread of some wheat diseases. Increased disease pressure from high levels of nitrogen should not be ignored. Select varieties resistant to powdery mildew and Stagonospora blotch and scout fields from flag-leaf emergence through flowering for disease levels so timely fungicide applications can be made if disease thresholds occur.