Nitrogen - what is appropriate use?

Dairy farmers in the South Island have traditionally used relatively little N fertiliser with the main source of N in pasture coming from N fixation by white clover. However, because grass responds readily to N fertiliser, the amount of use has increased considerably in recent years, with a 150% increase in the last 3 years alone (Statistics New Zealand). Application rates of 150-200 kg N/ha per year are not uncommon today