Performance of five pasture-based dairy systems with increasing levels of nitrogen fertiliser and associated stocking rates

A farm systems experiment comparing five pasture-based dairy systems was conducted in south-western Australia over 4 years on an annual ryegrass:subterranean clover sward. Fertiliser N inputs (0, 0.5, 1, 1.5 and 2 kg N ha(-1) day(-1)) and stocking rate (SR; 1.25, 1.50, 1.75, 2.0 and 2.25 milking cows ha(-1)) increased concurrently for each treatment (F1-F5, respectively). By design, milk yield per cow did not differ between treatments; therefore, the response was measured in terms of changes in herbage dry matter (DM) utilisation and nutritive characteristics, as well as differences in feed input and imported forage required. Total annual pasture utilisation (t DM ha(-1)) increased from treatment F1-F3 (5.1, 5.9 and 7.3 t DM-1 ha(-1), respectively), but responses beyond treatment F3 were not significant. Mean total N response efficiency was 9, 13, 8 and 7 kg DM per kg applied N for F2-F5, respectively, while mean marginal N response efficiency was 9, 17, -2 and 3 kg DM per kg applied N for F2-F5, respectively. The most efficient use of N fertiliser was achieved on treatment F3, where annual pasture utilisation levels in excess of 7 t DM ha(-1) yr(-1) were consistently demonstrated under the management strategies outlined, substantially greater than the local industry average.