22 March 1996


Narrow tyres may have sufficient traction for most tasks on the dairy farm but what damage are they causing to soil structure and grass productivity? Peter Hill suggests it is time for livestock farmers to pay more attention to this neglected area

THE trouble with soil compaction is that it is invisible without the aid of a spade. The symptoms are also subtle – ground that remains wet long after the rain has stopped because of restricted drainage; grass reluctant to green-up in the spring because of poor root development.

The consequences are poorer forage yields, poorer sward quality and increased poaching. All of which hits the productivity of valuable grass leys.

Even without these symptoms, compacted soil can be limiting forage production and the only way to find out is to dig down into the soil profile and inspect the structure and depth of root growth. On susceptible soils, compaction will be found deeper than expected since the effect of a heavy slurry tanker, for example, goes far beyond the depth of any wheelings or ruts left on the surface – certainly well beyond typical ploughing depths.

Compaction can be put right to some extent by subsoiling to loosen soil at depth and slitting to improve matters nearer the surface. These are useful remedies but they tackle only the consequences, not the causes. Taking steps to minimise compaction in the first place is a more positive approach.

This requires a two-part strategy:

&#8226 Keep off the land when soils are susceptible to compaction, that is, when wet.

&#8226 Use larger tyres on tractors and heavy field machinery.

Arable farmers have increasingly taken both elements of this strategy to heart in recent years and grassland farms could do the same, not necessarily by going to the extreme of fitting huge balloon tyres, which are both expensive and often impractical, but by moving to modern tyre designs (preferably radials) and fitting larger sizes to tractors and any implements that carry a hefty load.

The principle role of any tyre is to support the weight of the vehicle on which it is fitted. Its volume and the pressure of air inside the tyre, together with differences in construction, determine the weight a particular tyre can carry and these factors are detailed in the performance charts compiled by tyre manufacturers. In general, the larger the volume of air, the lower is the pressure needed to carry a given weight.

Inflation pressure is important because it broadly equates to the load or pressure imposed on the soil. There is some logic in this given that, as a tyre is pumped up harder, it stiffens and the contact patch, width and length are reduced. As inflation pressure comes down, the tyre carcass can flex, spread its tread across the ground and reduce the "ground bearing pressure" of the vehicle.

That reduces the extent and depth of compaction, surface rut depth is less and, because the tyre does not sink into the soil as much, rolling resistance is reduced and the tractor has an easier time of it, saving fuel, speeding up work rates and reducing surface-damaging wheel slip.

The aim, then should be to fit tyres that carry the weight of the vehicle at the lowest possible inflation pressure. The one compromising factor (apart from the practicality of fitting big tyres to trailers and similar equipment) is road travel. Road travel at speed imposes its own stresses and strains on tyres, notably a build-up of heat as the carcass deforms with each rotation.

Either road speed must be restricted to maintain maximum field performance, or field performance compromised by increasing the inflation pressure.

The contribution that larger implement tyres can make is well illustrated by Dutch tyre maker Vredestein. To carry a 2000kg load, a typical 11.5/80 x 15.3 tyre needs to be inflated to 3.5 bar (51 psi); but a 19.0/45 x 17 tyre which is wider but of much the same overall diameter – needs only 1.65 bar (24 psi) to carry the same load.

In assessing different tyres, the following characteristics need to be considered:

&#8226 Size: width and diameter contribute to reduced ground pressure and larger tyres should also offer less rolling resistance.

&#8226 Strength: can the tyre support the required load at a relatively low inflations pressure?

&#8226 Flexibility: at low pressures, can the tyre flex sufficiently to maximise the width and length of the tyre/soil contact patch?

Worst offenders are the old crossply truck tyres often fitted to home made trailers; these have a very rigid structure and tend to require very high inflation pressures. Front tyres on two-wheel-drive tractors also tend to skimp on size and specification, and while trailer, slurry tanker and manure spreader manufacturers have improved things in respect of tyre fitments, standard equipment is often marginal in order to remain price-competitive.

The thing to do is look for larger tyres in the options list, seek advice from a tyre specialist, or look for equipment built specifically to run on bigger tyres. &#42

Wide tyres designed to run at lower inflation pressures for a given load can significantly reduce surface damage to pasture and soil compaction – as illustrated by these 20.0/70 x 508 Dutch-made Vredestein Flotation+ tyres.

The larger the tyres the greater is the volume of air able to support the load. Inflation pressure, which broadly equates to ground pressure, can therefore be lower. Large diameter tyres, preferably supple radials, present a larger contact footprint and also offer less rolling resistance.