Keeping good traction while running at the lower pressures needed to protect soil structure isn’t a simple task, according to Kirk Walker, technical manager of Mitas’s off-road and agricultural tyre division.
“Climate change will mean that the windows for fieldwork at critical times could be smaller, so people will need to achieve greater workrates in shorter times.
“This inevitably means equipment getting bigger in the future. More horsepower also means less passes, but footprints have to be managed to look after soil structure.”
Modern machinery is heavy, too. A 400hp tractor can easily weigh 18t and a Claas Lexion 780 with tracks and a full grain tank tips the scales at 32t. That compares with a 1970s Ford 5000 at 2.7t.
But there’s already a lot growers can do to help, he says. “Much of it is about balancing inflation pressure with slippage rates to get the best draft force without damaging the soil.
“Most growers will operate at 15 – 25% slippage to maximise traction, but there’s room to reduce this without losing performance if we can reduce tyre pressures further.”
One of the biggest problems is the number of wheelings over the land, adds soil specialist Professor Dick Godwin from Harper Adams University.
“Around 85-95% of land undergoes at least one wheeling in a typical growing season with traditional tillage methods. With minimum tillage this is reduced to around 65% and direct drilling reduces it to 45%.”
Larger equipment reduces the number of passes but this adds extra weight of equipment and that’s where tyre design and ground pressure come into the equation, Prof Godwin says.
“Managing tyres, soil and water is critical. A good soil needs to be 50% void and every time a wheel passes over the land it potentially damages movement in the soil down to 60cm deep.
“The greater the traffic and the heavier the footprint the greater the compaction effect. This reduces the fissures in which the roots can grow and limits water flow into and through the soil.”
Bare soil with no tractor passes will allow 25mm/hour of water to enter the soil. After just one pass this reduces to 5mm/hour and after three passes it can be as low as just 1mm/hour, he explains.
“In practical terms, establishing fully controlled traffic systems – where all wheelmarks are confined to given tracks and only 15-25% of the soil is trafficked – has a number of technical challenges for the average farmer and full tillage is too energy demanding.
“Every time depth of cultivation is doubled, the energy requirement is quadrupled – so reducing ground pressure is the only feasible option for the future.
A lot of recent emphasis has been about developing tyres capable of carrying high load at high speed, but reducing sidewall deflection to allow lower operating pressures is now more important, Kirk Walker says.
“Modern agricultural tyres have to achieve good field and road performance, but many operators place road handling and ride above soil protection. Maybe not on purpose, but not all operators will physically reduce pressures in the field to match conditions.”
“In the future, we have to look at ways of reducing the low end of operating pressures from the current 10psi down to 6psi without compromising sidewall integrity.”
Mitas’ new PneuTrac development tyre combines elements of rubber tracks with conventional pneumatic tyres to give a lateral stability up to 170% greater than a conventional tyre so it can be safely run down to 6psi.