The response of a ryegrass sward to wheel traffic and applied nitrogen

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Abstract
The dry matter and nitrogen yield and estimated metabolizable energy of perennial ryegrass grown for silage were recorded from 1988 to 1990 for three levels of wheel traffic (zero, light and severe) at four rates of nitrogen fertilizer. The traffic treatments were applied by tractor wheels in the spring and summer of 1987 and in the spring of 1988 and 1990. First‐harvest yields were reduced consistently by severe traffic: for example, at a rate of 100kg N ha‐1, dry matter (DM) produced in the severe treatment was 58, 72 and 84% of that in the zero traffic treatment in successive years. Wheel traffic effects on yield were markedly smaller at second and third cuts than at first cut. Nitrogen uptake and apparent recovery of fertilizer nitrogen were usually less after the relatively severe traffic treatment than after zero or light traffic treatments. Denitrification fluxes, measured in the second and third years, indicated that gaseous losses of nitrogen were largest when soil compaction was greatest.Reduction of herbage yield at first cut in 1989, the year in which no wheel traffic had been applied at the start of the growing season, indicated that impaired soil physical conditions were implicated, quite apart from possible damage to the grass plants. Soil structure was damaged in both light and severe traffic treatments: in the latter, the volume of air‐filled pores during the wetter periods early in each growing season was especially small (<4%, v/v). It seemed likely, therefore, that poor aeration was a key factor in limiting both grass growth and nitrogen utilization.The effect of wheel traffic on herbage production tended to decline over the 3‐year period of the experiment. However, it was not clear whether that trend was primarily a consequence of a progressive improvement in the structure of the most dense soil, or was a degree of variation caused by differences in weather patterns between years.
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