Second top dressing of wheat and foliar feeding: when and how

By Nikola Mojsa, MSc in Agricultural Engineering

When it is time for the second top dressing of wheat

The second top dressing should go on at the very end of tillering or the start of stem elongation, most often 3 to 4 weeks after the first (Agrosava); advisory services such as PSS Zrenjanin give the optimal window as “up to three weeks”. Still, the calendar only gives a general frame. In practice (following the advice of Professor Vukadinović), the ideal moment is set by the state of the field alone: exactly when the developing ear has visibly lifted (about 2 cm) above the tillering node.

If you spread the first top dressing in the first half of February, this window falls in mid-March, just before wheat enters stem elongation by the calendar (late March or early April).

For this second pass, nitrate or ammonium nitrate fertilisers (such as CAN or AN) are recommended, and urea should be avoided at this stage as far as possible (Prof. Vukadinović). When the total nitrogen for the field exceeds 60 kg/ha, the rule is to split it 60:40 or 70:30; for example, out of 100 kg of nitrogen in total, the second top dressing carries 30 to 40 kg (Agrosava). Ideally the dose is set by testing the deeper soil layers with the Nmin method (60 to 90 cm deep).

Don't make the mistake of thinking more fertiliser is always better. Pushing nitrogen makes the crop too dense (unproductive tillers) and sharply raises the plant's susceptibility to disease. Because wheat takes up most of its nitrogen between stem elongation and heading (as much as 50%), the nutrient has to go on in time (Agroklub).

Sources (in Serbian): Agroklub, Prof. Vukadinović, Agrosava.

What foliar feeding is and why it helps

Foliar feeding works like an injection for the plant when the roots can't keep up with nutrient uptake: in extreme drought, after late frost or when the root system is underdeveloped (AgriNova). Its big advantage is that the nutrients are taken up whether or not there is moisture in the soil. On the other hand, you can only give the plant small, limited amounts of the major nutrients (nitrogen, phosphorus, potassium) through the leaf; more than that causes serious leaf scorch (Agroklub).

Foliar feeding is therefore best used to deliver key micronutrients: iron, magnesium, copper, boron and zinc (Agroklub). Specialists (AgriNova) suggest 1 to 2 targeted treatments per season: at tillering or stem elongation, and then just before heading.

Avoid treating in the midday heat: above 24 °C some species close their stomata (the pores in the leaf) to stop water loss, so they take up the product less well (Agroklub). Late afternoon or early morning works best, and the drone stops as soon as the wind goes above 3 to 5 m/s.

Foliar feeding of wheat mixed with a fungicide

Two jobs in one pass are possible, but only if the labels of both products allow it. Combining a foliar fertiliser with a crop protection product saves money by cutting out one pass (which costs €20 to €25 per hectare).

There is always a risk of chemical incompatibility, though. The rule is: jar test first! If the mixture separates into layers, turns cloudy or curdles, it must not go into the tank. The recommended mixing order is: water first, then powder formulations, then liquid suspensions and then emulsions, the foliar fertiliser second to last and the adjuvant (wetting agent) at the very end (AgriNova). If you have doubts about compatibility, always do the protection first and the feeding 48 hours later. Our agronomist does the mixing at the field.

Of all the treatments in wheat, the most critical is the protective fungicide at flowering. We cover the treatments and timings in detail here: wheat protection and feeding.

How strong is the solution a drone sprays

An agricultural drone uses only 15 to 20 litres of water per hectare, compared with the 150 to 400 litres a tractor needs. Because the amount of product or fertiliser is fixed (by the label), the drone applies a highly concentrated solution.

For example, if you apply urea, the recommended solution is 0.5 to 2% (AgriNova): 1% in 250 litres of water is 2.5 kg of urea per hectare. Put the same 2.5 kg of urea into 20 litres of water and you get a concentration of 12.5%, far above the recommendation for tractor application. Because of this difference, before every flight we have to check the label and get confirmation from the advisory service that the chosen product is safe to apply in a very low water volume (ULV, ultra-low volume).

The drone's strength is good physical coverage: hovering about 3 metres above the crop, its rotor downdraught pushes easily through the wheat and spreads the nutrient over the whole leaf.

Why spread the second top dressing by drone

Flexibility in the field is the clear advantage. After rain the drone goes to work as soon as the access road dries, and the same day if there is a tarmac road. Since the window for the second top dressing is fairly short (the very start of stem elongation), a muddy field no longer has to hold up your crop programme.

The job is done with a dedicated spreader module (100-litre capacity), which carries up to 70 kg of granular mineral fertiliser per flight and spreads it over a width of 3 to 10 metres (DJI manufacturer data).

What foliar feeding and top dressing cost

Foliar feeding costs exactly the same as standard spraying: €25, €22 or €20 per hectare (depending on whether the job is up to 20 ha, up to 100 ha or over 100 ha). Spreading granular fertiliser by drone costs a flat €40 per hectare for any area. You bring the fertiliser and the water to the field. Travel to fields within 50 km of Opovo is free.

If you agree a package of three treatments with us (for example an early spring top dressing, then foliar feeding and the fungicide at flowering), the total price drops by 5%. You can see your own figures in the price calculator, and read how foliar feeding is done in foliar feeding by drone.