Top dressing fertilizer usually occurs after emergence, often as a response to increased yield potential or as a remedy for noticeable deficiency symptoms.
However, growers are looking at top dressing this year as a logistical aid in getting the crop seeded faster. Here are some agronomy messages to help in your decision making.
1. If canola is emerged, expect to see some damage from top dressing liquid nitrogen (N). Canola plants are particularly sensitive to stress at cotyledon and 1-2 leaf stages. Concentrated N fertilizer bands that connect with these young plants can burn off a lot of the leaf area. Because these plants are small, a concentrated band of liquid N is not likely to contact very many plants. And those that are contacted will likely recover, but will have delayed maturity. A tip: Running against the rows instead of parallel to rows with a dribble band could knock back fewer plants.
2. Broadcasting or top dressing N in a separate pass while seeding may be one way to keep the seeder going longer between fills and, if the field is soft due to moisture, can take some of the weight off the drill and reduce the odds of getting stuck. The economic risk with this practice is that fertilizer not banded and not placed near the seed row is available to weeds and crop equally. Weed control will have to be very good to prevent weeds from taking up the applied N. (Note: Phosphate should be banded with or very near the seed to be effective as a starter fertilizer.) Using Agrotain on surface-applied N can minimize volatilization losses while waiting for a rain to move the fertilizer into the soil.
3. Mixing fertilizer with herbicide is not an option for a number of reasons. (1) Flat fan nozzles used to provide good coverage for herbicide will also provide good leaf area coverage for fertilizer, which will cause severe leaf burn in both crops and weeds and could thereby reduce herbicide uptake and efficacy in the weeds. (2) Nitrogen applied at sufficient rates will cause serious injury to crop plants and inhibit proper uptake of systemic herbicides such as glyphosate or Group 2 herbicides. (3) Fertilizer rates that are safe to the crop will be insufficient to make any noticeable difference in yield. (4) Micronutrients, can be antagonistic to herbicides. For example, calcium, magnesium and iron as well as many other multi-valent metal ions (aluminum, zinc, manganese) are antagonistic to glyphosate. Check with the herbicide manufacturer. (5) Small amounts of ammonium sulphate may be recommended as an adjuvant for some herbicides, but this is to enhance herbicide performance or overcome antagonism of the herbicide from hard or bicarbonate ions present in water. It provides no fertilizer benefit. Convenience of multiple mixes often not good for the crop and can reduce efficacy.
4. The most common and practical top dress nutrients applied to canola are nitrogen and sulphur. Nitrogen top dress should go on before the crop needs it. Uptake starts to increase rapidly around the five-leaf stage. Sulphur uptake peaks a little later than N uptake, but the ideal is to have both in place at target amounts before the crop needs them. Timing application immediately after seeding and before emergence can work well if that’s the best logistical solution. A rain shortly after application will move the fertilizer into the root zone, which is where plant uptake occurs most efficiently. Uptake through leaves is minimal.
5. Moisture that delays seeding can also increase losses for fall-applied N. Growers who are in doubt about soil N levels at this stage of the season may want to take a few soil tests at the time of seeding and then prepare to top dress after emergence if levels are lower than expected. However, keep in mind the risks with a spring soil test: Soil testing after banded nitrogen may hit a hot part of the band and get a very high reading or fall between the rows and provide a low nitrogen reading.