Post-emergence herbicides for maize: up to 8 or 10 leaves
Every post-emergence maize herbicide has a latest permitted stage stated on the label, most often 8 or 10 leaves. Basic weed control should be finished by the 4 to 5-leaf stage at the latest (Syngenta), and Johnson grass is controlled while it is 10 to 20 cm tall. That is a job for the tractor; the drone steps in when rain or crop height keeps the tractor out of the field.
By Nikola Mojsa, MSc in Agricultural Engineering
How maize leaves are counted
According to Purdue University, you count only the leaves with a visible collar: the pale band where the leaf blade meets the stalk. A leaf still rolled in the whorl, with no collar, does not count (Purdue Extension). As the plant grows, the lower leaves drop off; when the count on an older plant doesn't add up, dig the plant up and split the stalk down to the swollen node that carries the fifth leaf.
Galenika Fitofarmacija, on the other hand, advises counting all the leaves (Galenika Fitofarmacija). The two methods give different numbers, so before spraying check how the product's instructions count leaves, or ask your agronomist: one leaf's difference can make you late.
Up to which leaf stage maize can be sprayed with herbicide
The latest permitted stage is stated on the label and applies only to that product. The sources we reviewed give different limits, because the products themselves differ:
| Source (in Serbian) | What it says |
|---|---|
| Syngenta | Basic weed control by the 4 to 5-leaf stage at the latest; relying on a single post-emergence treatment is the riskiest approach. |
| Agrosava | Grass-weed herbicides up to 7 to 8 leaves; Johnson grass control can start as early as the second leaf. |
| Agrolekar, Gordana Forgić MSc | Up to 8 leaves; after the 7th leaf some herbicides can be phytotoxic to maize. |
Why you shouldn't be late: after 8 leaves the reproductive organs start to develop, so spraying later, up to 10 leaves, can affect yield quality (Agrolekar).
Johnson grass and couch grass in maize: when to control them
Johnson grass from rhizomes is controlled while it is up to 15 to 20 cm tall: Agrolekar gives this as the optimum, while Agrosava advises 10 to 15 cm, starting as early as the second leaf of the maize (Agrolekar, Agrosava). On most maize ground, Johnson grass is controlled in two rounds, in May and June; where weed pressure is heavy, Agrosava suggests a second treatment 10 to 14 days after the first.
According to Agrolekar, an afternoon treatment raises efficacy by 20 to 30% and improves selectivity towards maize, because the lower afternoon temperature helps the weed take up more of the product and the maize less.
Maize past 10 leaves: what if you are late
A drone does not move the label deadline. There are two answers to whether a drone can spray after 10 leaves: for a herbicide, the label decides, while the drone applies fungicides and insecticides even in maize taller than 1.5 m (European corn borer: when to spray).
In a field that has fallen behind, where the weeds have already outgrown the product's stage, ask your agronomist what is still possible. Syngenta warns that heavier weed pressure in the early stages shortens the time you have to apply a herbicide.
When a tractor tramples and a drone doesn't
Herbicides come last in our order of treatments, after fungicides, insecticides, foliar feeding and desiccation. The reason is drift: herbicide carried off by the wind can destroy a neighbouring crop. So we apply herbicide to maize by drone only when both conditions are met:
- The tractor cannot get into the field, after heavy rain or because the crop has grown. In maize 1 to 1.5 m tall, a tractor tramples and destroys 5 to 6% of the crop.
- The fields around are the same crop, which that herbicide does not harm. If the maize borders sunflower, soybean, vegetables or an orchard, we do not apply herbicide by drone (how wind and flight height affect drift).
Up to eight leaves maize is not tall, so in that window rain is usually the only reason a tractor has to wait. When the field is waterlogged, the drone needs only the access road to dry, and next to a tarmac road we work the same day, even after heavy rain. We work in wind up to 3 to 5 m/s, but apply herbicide, following the drone manufacturer's advice, only when the wind is below 3 m/s; we don't work in the worst of the heat. On field crops we use 15 to 20 litres of water per hectare, and for a herbicide the exact volume is set by the label and the agronomist. Nikola mixes the product at the field; you supply the product and the water.
In the early stages, while the weeds are small and the product needs to land close to them, a tractor is the better choice.
Price of herbicide spraying in maize by drone
Drone spraying costs €25, €22 or €20 per hectare, for areas up to 20 ha, from 20 to 100 ha and above 100 ha. Contract spraying by tractor in Banat costs about €30 per hectare; when a tractor can get into the field, compare both prices. Travel within 50 km of Opovo is free, and new clients ordering at least 10 ha get the first hectare free on their first order. Three treatments on the same field in a season earn 5% off, and five or more 10% off. Check the price for your field in the calculator.
Frequently asked questions
Up to the latest stage on the product label, most often 8 or 10 leaves. Basic weed control should be finished by 4 to 5 leaves (Syngenta), and grass-weed products are applied up to 7 to 8 leaves (Agrosava).
While it is 10 to 20 cm tall, with a herbicide for grass weeds. Where weed pressure is heavy, Agrosava suggests a second treatment 10 to 14 days later. The agronomist chooses the product and rate from the label.
Not with a herbicide: the label sets the latest stage, and a drone does not change it. We apply fungicides and insecticides by drone even in maize taller than 1.5 m.
- First hectare free
- Free travel up to 50 km
When rain keeps the tractor out of maize in the herbicide window
Tell us how many hectares you have, where, what stage the maize is at and what grows around the field. We will tell you whether a drone is an option or a tractor is the better choice.