| Summary: | We investigated the impact of tractor traffic on grassland yield, soil porosity and penetration resistance in three field trials on contrasting soils in different regions of Norway (medium sand at Tjøtta, Nordland, silty medium sand at Fureneset, Sogn og Fjordane, silt at Løken, Oppland). The trials had a split-plot design with three levels of two wheel-by-wheel tractor passes after each cut (no traffic, light tractor 3-4 Mg, heavy tractor 6-7 Mg), three different seed mixtures and two fertilization levels. At three other field trials (Balsfjord in Troms, Fjærland in Sogn og Fjordane and Hokksund in Buskerud) impacts of both traffic and the inclusion of clovers in the seed mixture on grassland yield and penetration resistance were studied. These trials had a split-plot design with three levels of one wheel-by-wheel tractor and harvesting machinery pass and three different seed mixtures. During the first three ley years the yield reduction by tractor traffic was 26% at Løken, 4% at Fureneset and zero at Tjøtta. There was a positive correlation between yield reduction and the soil moisture content at wheeling. Tractor traffic reduced pore volume and air capacity and increased bulk density, degree of compactness and penetration resistance, with the largest effect on the silty soil at Løken and the smallest on the sandy soil at Tjøtta. Prediction of soil compaction by using the Terranimo® model gave a good picture of the situation at Fureneset and Tjøtta, but at Løken the soil was far more compacted than predicted by the model. The depth of tracks after wheeling was not found to give a reliable measure of soil compaction effects, as the yield reduction on the silty soil at Løken was far larger than at Tjøtta and Fureneset at the same level of track depth. In the field trials at Balsfjord, Fjærland and Hokksund the yield reduction after wheeling varied between 8 and 27% and penetration resistance increased. Inclusion of clovers (red and white) in the seed mixture increased yields at second cuts, but 30% clover compared to 15% did not affect yields. During these first three years of the experimental period there were no significant differences in either yield, soil physical properties or penetration resistance between wheeling with light or heavy tractors at any of the sites. At Løken and Fureneset, the experimental period was extended for two years. At Løken the experimental setup was kept the same and the yield reduction from traffic was about 15%. At Fureneset the number of cuts and wheelings was increased from two to three times per year, and both the fertilization level and tyre pressure were also increased. This gave significant yield reductions in the third cuts. In the last year of the experimental period (2015) there was also a significant beneficial effect on yield of using a light tractor rather than a heavy tractor. Overall soil texture and soil moisture content are major factors explaining traffic effects on soil physical properties and grassland yield. It seems that these factors explain more of the yield reduction than do the weight of tractors and machinery, at least in the short run when using low tyre pressures.
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