The objective of this work was a complete examination of the mechanical efficiency of the tillage operations required for rice production in Cambodia, with particular emphasis on potentially beneficial changes in tractor/machinery options. Tractor performance was assessed largely in terms of the effect of tyre options, and tillage draft examined in terms of tractor wheel compaction effects for two common groundtools. In both cases, most of the work was carried out on a sandy loam soil commonly used for rice production in Cambodia, and data was collected over a large range of soil conditions from dry to very wet conditions, and untilled to tilled surfaces.
The initial focus of work was the effect of tyre options and operating parameters on traction performance and mobility. The study looked at the performance differences between two pneumatic tyre types (bias and radial ply) and two tyre sizes under constant static weight across range of field conditions, including the effects of inflation pressure. Wide add-on steel cage wheels were also tested in wet conditions, and performance evaluated with the tractor operated in 2WD and FWA mode.
Traction coefficient was used as the major criterion of comparison between tyres, because it gives a direct estimate of traction, which is useful for tractor/implement matching. Common traction prediction models were assessed and the empirical traction equations developed by Gee-Clough et al. (1978) found to simulate the traction information obtained under Cambodian conditions better than others. This has been modified for all tyre types and conditions encountered in these tests, but it has not been possible to complete a proper validation.
A study of the effects of tractor wheel compaction on tillage implement draft was carried out in both Cambodia and Australia, and included measurements of soil parameters such as bulk density, cone index and tillage draft in the wheeltracks of a bias ply tyre at three inflation pressures. Draft of chisel and sweep ground tools was measured when tilling wheeled and non-wheeled soil behind the tractor with different moisture contents and surface conditions at tillage depths between 50 and 200 mm. Work on a vertisol in Australia was restricted to a single moisture content and surface condition.
Results from this work confirm that radial ply tyres should be used instead of bias ply tyres, and that larger diameter bias ply tyres are also an option to increase traction performance. Tyre pressure should be maintained at the lowest possible value to increase traction, and reduce the impact of soil compaction on wheeltrack tillage draft, and tillage system energy consumption. Tyre recommendations for tractors used in rice production are framed in terms of these results, and the practical considerations, which apply to tractor operation in Cambodia.
Tillage draft measurements demonstrated that the draft of ground tools was significantly greater when tilling soil compacted by tractor wheels in all soil conditions other than wet, tilled sandy loam soil. The effect was much greater in the clay soils of Australia, than in the sandy loams of Cambodia, and it appears likely that this phenomenon is related to the greater plasticity of the clay soils. This is a function of soil moisture content at tillage, and the inherent properties of the soil. Sweep ground tools were more sensitive to wheeltrack compaction effects than chisel tools in dry sandy loam, but this effect was reversed in heavy clay.
These results demonstrate that substantial tillage energy inefficiency is created by wheeltrack compaction in Australia, but the effect is smaller in Cambodia. It can be avoided in dryland production by controlled traffic farming, where tillage efficiency is increased by tilling only non-wheeled soil, and tractive efficiency is increased by restricting wheels to permanent, compacted traffic lanes. This process is unlikely to be relevant to the very wet soil conditions used in the traditional rice production systems of Cambodia, which require "puddling" of the paddy fields. In the puddling situation the importance of tractive performance might also be questioned, because soil moisture content is normally greater than the upper plastic limit, and the major function of the operation is to mix and homogenise the tilled layer, while sealing the surface of the underlying hard layer. The energy dissipated in the soil by inefficient traction will probably be a positive contribution to these functions.