The effect of vehicle weight on the distribution of compaction with depth and the yield of Lolium/Trifolium grassland

Although research has demonstrated that traffic by vehicles on soils with high moisture contents causes subsoil compaction and a negative impact on grassland productivity, knowledge is lacking regarding the interactive effects of using a light tractor with a high number of passes or a heavier one reducing wheeling on the field. Research was conducted in 1994-95 in Buenos Aires State, Argentina to establish the effect of the tractor axle load on the distribution of subsoil compaction independently of the influence of ground contact pressure and to evaluate the effect of densification upon grassland production. The main species present were annual ryegrass (Lolium multiflorum Lam.) (60%) and white clover (Trifolium repens L.) (38%). The grassland was trafficked with two tractors of the same design and similar pressures on the soil contact area of the drive wheels, but with one of them having 1.8 times the mass of the other, 4.2 Mg and 2.3 Mg, respectively. The work was performed on a fine clayey, illitic, thermic Typic Argiudoll (Centeno series), at a high moisture content using three different numbers of passes for each tractor: one, five and ten. Cone index and bulk density to 600 mm in the profile were measured to identify changes in soil compactness. Dry matter yield per hectare was used to indicate the response of grassland production. Independent of pass numbers, the heavier tractor always produced greater increases in bulk density for the 300-600 mm depth range. Grass responses to compaction were significant for all treatments, either in the lane of the tractor, where the average decrease in dry matter production was 74%, or adjacent to tyre imprints, where the grass yield was reduced by 18%. For the same number of traffic passes, the heavier tractor increased bulk density at the 300-600 mm depth compared with the lighter tractor. However, the lighter tractor, with a larger number of passes, produced as much or even greater compaction than the heavier tractor with fewer passes.