Depending on your crop rotation, most of your crop ground allocated to winter production could be either under winter pasture (wheat, rye, oats and triticale) or winter canola. Nitrogen (N) top-dress application is typically carried out in February. You can calculate top-dress application either by applying nutrients based on soil test results or by using new technology, like GreenSeeker® to make fertilizer recommendations.
If fall nitrogen was applied, some or most of that nitrogen might still be available for spring growth. Soil-available nitrogen can be determined by taking soil samples from two different depths (0 to 6 inches and 6 to 12 inches) and sending it to a laboratory for soil testing. Be sure you are collecting a representative sample of the field rather than sampling a high yielding or low yielding part of your field.
Graze-out and dual-purpose winter pastures require more nitrogen compared to grain-only production. About 140 pounds is required if grazing is involved, whereas 2 pounds of nitrogen is required for every bushel of yield goal in a grain-only system. For example, if your yield goal is 50 bushels, then you need 100 pounds of nitrogen total, including soil residual for a grain-only system.
For winter canola, about 50 pounds of nitrogen is required for every 1,000-pound yield goal. One-third should have been applied in fall and the remaining two-thirds should be applied this spring as top-dress. If you have not corrected phosphorus (P) and potassium (K) needs at planting, it is advisable to look at the soil test report to see if your field requires any of those nutrients and correct them. Top-dress nitrogen will not be efficient if P and K are deficient.
The other way to calculate top-dress applications is using technology like GreenSeeker®, which is an optical measuring device that detects plant nitrogen requirements. For this technology to work better, you should have your strip applied near planting time in fall. A reference strip could be either a nitrogen-rich strip or a ramp calibration strip. The nitrogen-rich strip is the part of the field where nitrogen is not deficient regardless of the growing conditions. If the winter pasture is used for dual purpose, pull the cattle off at least two weeks prior to sensing so there is sufficient plant growth to make an accurate recommendation.
To get an accurate nitrogen rate recommendation from the reference strip, you need to use a GreenSeeker® sensor. Based on the reflected light from the crop canopy, the GreenSeeker® sensor measures the normalized difference vegetative index. This index will then be used to predict yield potential for this year on that reference strip for that particular field. A nitrogen recommendation will then be made based on the difference from the reference strip to the rest of the field. You can get the recommendation if you have a GreenSeeker® handheld unit and input the required data on the website. For winter canola recommendations using GreenSeeker®, contact Brian Arnall at Oklahoma State University.
You also need to have your fertilizer spreaders calibrated so the intended application rate is applied instead of over- or under-applying, both of which are detrimental to your pocket.