Tracked combines produce significantly less soil compaction than wheeled machines, according to recent research.

The study, conducted at Cranfield University, Silsoe, has shown that tracks can reduce subsoil cultivation requirements and lead to savings of about £4/ha (£1.60/acre).

Aside from the reduced ground pressure exerted by tracks, they also bring an improved climbing ability and reduced transport width.

Conducted under test conditions in a soil bin and out in the field, German PhD student Dirk Ansorge’s trials also investigated how important rear axle rubber choice was.

It was found that fitting wide, 700mm rear tyres in combination with tracks had little significant influence on soil compaction.

To explain this, he describes the tracks as leaving a shallow, flat-section compacted “roadway” on the top layers of soil.

As the rear tyres follow on this, it mimics the effect of the tracks thus spreading the pressure across the soil profile.

Mr Ansorage advises combine buyers to choose a 500/85 rear tyre with tracks.

“A taller tyre can be more beneficial than a wider tyre,” he says.

“Compaction is similar, but rolling resistance and transport width is reduced.”

But it’s not all roses.

A Lexion on tracks suffers the same problems as any twin-track machine – in headland turns it has a tendency to scuff and create a zone of horizontal smearing.

Although this is pulled out by following cultivation passes, it is no good for direct-drilling, at least on the headlands.

Costs and savings

So tracks can limit compaction, but how much do they cost?

The straight answer to that is between £16,000 and £20,000 for Claas’ TerraTracs and about £30,000 for John Deere’s equivalent.

Claas pitches this as £2.50/ha (£1/acre) based on a combine covering 800ha a year (2000 acres a year), with the cost written off over five years and allowing for a resale premium of at least £6000.

Because compaction created by a tracked combine reaches far less deeply into the soil profile than a wheeled machine, Mr Ansorge has calculated that subsequent subsoiler passes can be conducted shallower and require less draft force.

Working 450mm (18in) deep behind a wheeled combine required 241hp at a cost of £6/ha (£2.40/acre) – just to remove the combine wheelings and not tramlines.

Running at 350mm (14in) in the less compacted soil behind the tracked machine, 88hp was required at a cost of £2/ha (80p/acre).

Best tyre option

For those unconvinced by the benefits of tracks the study throws up some more interesting findings about tyre choice.

A 900mm wide front tyre was best able limit ground pressure by spreading the load over a greater area.

However, 800/65 tyres were not significantly better than 680/85s – their narrower width is offset by their increased height, which provides a greater contact area.

The best combination in limiting compaction is 900mm wide tyres up front with 700s on the tail.

Where width is an issue, 680s should be used with tall, wide rubber on the rear.

A full report is available from www.claas.com

fwmachinery@rbi.co.uk