Pruning

Topping consists of cutting the vegetation above the crop. This prophylactic mowing aims to reduce the seed bank of the weed flora.

Objective

Operating similarly to a corn detasseler, its objective is to cut the inflorescence of unwanted plants to stop seed formation and dispersal before they become viable.

Description

The topping machine is used once the crops are well developed and control methods such as the harrow, the crust breaker/the rotary hoe, the hoe and certain herbicides on hoed crops are no longer possible. Like many weed control methods, timing is crucial but the topping machine allows thinning of grasses, wild oats, amaranth and thistle.

Nevertheless, even if the weeding is not definitive, competition is limited. This type of equipment should allow topping of weeds with thicker and tougher stems, letting thinner stems or grass leaves pass through.

Application context

The topping machine should be used in addition to other mechanical weeding tools, as a catch-up tool.

Crops: All types of crops.
Weeds: wild oats, rat's-tail fescue, ryegrass, Rumex, thistle.
Ideal weed stage: Before seed formation, above the crop.
The intervention period is potentially short:
- The time when weeds sufficiently exceed the crop can be short.
- The time lapse between heading and seed viability (= species multiplication) can be short (wild oats).

To address this issue, multiple passes may be made.

Ease of implementation varies depending on the weeds: Rat's-tail fescue does not exceed wheat much; the solution can be provided by wheat with a short straw phenotype or a late variety allowing weeds to exceed it early in the season.
Difficulties in case of varietal mixture, as crop height is less uniform.
Risk of crop damage (partial or total cutting of ears), ideally having an adjustable cutting height (2 or more sections) depending on the terrain.
Significant export in case of severe field infestation, up to 15/18 t/ha of fresh matter.
Foliar herbicide treatments can cause growth delay and/or lodging of weeds and thus a decrease in topping effectiveness.
Strong dependence on weather for intervention periods.
Points of caution:
- Adjusting the knife angle requires experience.
- The rotation speed of the deflectors must be quite high. Despite this, if vegetation is very dense, clogging on the frame can occur.
- When the crop is very weedy, a double pass is necessary; its weak point remains wild oats, which are more elusive.

Advantages and disadvantages

Advantages

The topping machine is an asset in the range of mechanical weeding tools.
Allows catch-up but tends to be used more systematically.
Replaces some herbicides (Axial) that have become ineffective.
Exported material can be valorized by methanization.
Significant reduction in weed pressure.

Disadvantages

Sensitive to clogging and wind.
Machine fragility.
Highly variable effectiveness depending on species, conditions, and context.
Crushing/flattening of part of the wheat during topping. The affected area depends on the track spacing (thus the farm's equipment) but is around 1% to 2%, i.e., a loss of about 36 euros/ha for a yield of 80 q sold at 230 euros/t.

The method's effectiveness is variable:

For ryegrass in wheat, an experiment showed that the average effectiveness of this method is 50% of plants topped. The effectiveness varies greatly depending on conditions, from 40% when ryegrass is lodged to 80% in patches or when the height difference between weeds and crop is significant.
For rat's-tail fescue in wheat, effectiveness is relatively similar with an average around 55% +/- 10%. However, an improvement in topping effectiveness is observed on rat's-tail fescue in einkorn wheat, 75% +/- 10%. This is explained because the latter reaches the stem elongation stage late, allowing weeds to exceed it more clearly and systematically.
Case of wild oats. It has a tall inflorescence and often greatly exceeds crops, very good aptitude for topping. It is observed that the earlier the topping is done, the lower the seed maturity level. Indeed, for a topping at the end of May, about 10-15% of seeds are viable and germinate, while for an intervention at the end of June, around 80% germinate. Overall, a factor of 2 to 3 is observed on seed germination depending on maturity stage. Even immature, some wild oat seeds are viable.
Effectiveness depends on the topping date (general case/rat's-tail fescue). The trend is an increase in seeds topped during later interventions. However, the later the date, the higher the viability rate of weed seeds. The optimal intervention seems to be early June with an effectiveness of 60% and seed viability below 10% (the date may vary depending on years and pedoclimatic context).

Ideally, multiple passes (2 or 3) should be made to increase the chances of hitting the maximum number of ears before their viability.

Note: When topping machines export seeds like harvesters (see 1st photo), the seed viability issue is no longer relevant. Intervention is still possible and useful until harvest if seeds have not already fallen to the ground.

Effectiveness is measured by the number of weed ears exported over the total number of weed ears.

Focus on the tool

The topping machine generally consists of several panels, each between 1 and 2 m wide, fixed on a frame with hydraulic or mechanical folding. Usually mounted frontally for better visibility of working height, it fits on front linkage (or weight carrier) or front loader.

For rotary blade tools, each panel has 3 to 5 blades of 30 to 50 cm each. They are connected at the top to a belt (sometimes a chain or gears) driven hydraulically (one hydraulic motor per panel), making the tool demanding in oil flow: 7 to 10 l/min per meter of width.

The forward speed depends directly on the density of stems to be cut and is linked to the height difference between unwanted seeds and the planted crop (the greater the difference, the greater the allowable driving imprecision, the higher the forward speed).

The hourly output ranges between 1 and 3 ha per hour depending on models, with work often done "by tasks". Some models can be equipped with hydraulic tilt correction, very useful when the field surface is not perfectly flat.


Type	Fixed reach, self-built	Foldable reach, tilt and hydraulic height
Attachment	On loader	On weight carrier or front linkage
Working width	3 m	6 m
Required power	50 HP	90 HP
Purchase price	About €3000	€11800
Hourly output	0.6 to 1.5 ha/h	1.7 to 3 ha/h
Assumption at 6 km/h	3.1 ha/h	4.7 ha/h

Example of cost price:


Suitable area	15 ha	75 ha
Maintenance and repair	€1.80/ha (blade, bearing, etc...)
Includes tool depreciation, traction, fuel, ancillary costs, and labor (€15/ha)
Cost per ha	€39.00	€24.00

For more information

You can consult the following links:

Sources

La version initiale de cet article a été rédigée par Jasmin Razongles.

Topping - Chamber of Agriculture of Gers