Sunflower desiccation: when to do it and how

By Nikola Mojsa, MSc in Agricultural Engineering

Why sunflower is desiccated at all

Sunflower stays green and full of moisture even when the seed is fully ripe: the leaves, the thick stem and the head all hold water that gets in the way of harvest. For the combine to work without clogging the sieves, the moisture of the whole plant has to come down. That is why desiccation speeds up and evens out drying and makes sure the seed reaches the buyer at the right, low moisture.

If you harvest without desiccating, green parts of the head, leaf stalks and leaves go into the combine. Because they are much wetter than the seed, they directly raise its moisture. Seed like this heats up quickly in the heap, spoils and needs expensive artificial drying at the silo. On top of that, waiting a long time for sunflower to dry naturally in the field exposes it to bird damage, lodging in strong wind, autumn rain and, with late hybrids, the first frosts (Đukić et al., 2007). An earlier harvest also frees the field in time for the next crop, and the desiccant burns down any remaining weeds as well.

Keep in mind that a desiccant is by nature a non-selective herbicide that destroys green plant tissue (Agro Info Net, PSSS Šabac). That is why following the label (and your agronomist's advice) to the letter is a must, while simply going by the calendar leads to mistakes.

When to desiccate sunflower: seed moisture, not the calendar

The treatment is timed only to physiological maturity: the stage at which the seed stops taking up nutrients from the mother plant. This happens 25 to 40 days after flowering, depending on the hybrid and the weather.

Judging maturity by eye (the colour of the back of the head, ray florets dropping, lower leaves drying) is very subjective and unreliable, especially because sunflower flowers in rings. Specialists agree: seed moisture content is the only reliable parameter (Đukić et al., 2007).

Different hybrids, however, need different approaches. The recommendations in the literature, which the 2007 paper compiles from the 1970s and 1980s, put the ideal moisture for desiccation across a wide range: from a strict 25% up to 45%, depending on genotype and weather. Hybrids can differ by as much as 10 percentage points.

So the textbook rules have to be adapted to the field: always measure moisture on the field itself, and let your agronomist set the threshold for your hybrid. Desiccating too early costs yield and significantly lowers thousand-seed weight, while desiccating too late simply defeats the purpose, because it no longer shortens the wait for the combine.

How long after desiccation harvest can start

The earliest you can go into the field is set by the pre-harvest interval on the product label: the required number of days between spraying and harvest. More on this in pre-harvest and re-entry intervals.

How fast the crop dries after spraying depends directly on the weather. If temperatures stay above 20 °C, harvest can start 7 to 14 days earlier than if you had waited for natural drying. On the other hand, if cooler weather, rain and high humidity follow straight after the treatment, the product works more slowly (Đukić et al., 2007).

Sunflower desiccation by drone: speed without trampling

The problem with a tractor sprayer in sunflower taller than 1.5 m is serious stem breakage. Every pass permanently flattens about 5 to 6% of the crop in the wheel tracks. The drone removes the problem at the root: it flies 3 metres above the sunflower heads and does not trample a single plant.

According to a calculation in the 2007 paper, a tractor sprayer with raised booms, which bends over two rows, flattens about 7% of the plants, and half of those (about 3.5%) are lost for good, while the drone causes no yield loss from trampling (Đukić et al., 2007). The drone also needs only 15 to 20 litres of water per hectare, while a tractor uses 150 to 400 litres.

After rain the drone doesn't wait days for the mud between the rows to dry; all we need is a dry road to the field.

Because non-selective products attack any green tissue, we take great care with the surrounding crops. We work in winds of up to 3 to 5 m/s, and with desiccants, following DJI's guidance for the Agras T70P, only below 3 m/s, which keeps the risk of drift to a minimum. In detail: drift in drone spraying. For an overview of the service, see desiccation by drone. The full sunflower protection programme is at sunflower protection by drone.

What sunflower desiccation by drone costs

Prices for sunflower desiccation by drone:

The price band is set by the total area of the order. Example: desiccating 40 ha of sunflower costs 40 × €22 = €880. You supply the product and the water at the field. There is no travel charge within 50 km of Opovo, and new clients get the first hectare free on a first order of at least 10 ha.

If we agree a package of three treatments on the same field in a season (for example fungicide, foliar feeding and desiccation), you get 5% off the whole package. You can run a quick estimate in our calculator.