Fertiliser nitrogen effects on hill pasture and on grazing by ewes

Abstract

A study was conducted on seasonally dry hill pastures near Waipawa, central Hawkes Bay, over a two year period to investigate how the application of nitrogen (N) fertiliser changed the pasture chemical and species composition and if animals subsequently preferentially grazed that pasture. In Year 1 the short term effects of N fertiliser (30 kg N/ha) was evaluated on both north and south facing slopes after the pasture was spelled for about 12 to 25 days only after N fertiliser application. This period did not allow sufficient time for pasture dry matter (DM) responses to develop. In Year 2, using the same trial sites, the pasture was allowed to develop DM response differences of 500 kg/ha or more following N fertiliser application before animals commenced grazing. On each occasion, grazing observations were made during a 5 hour period following stock introduction to spelled pasture. Herbage mass per plot was measured prior to, and at the end of the grazing period, and the amounts of pasture utilised per plot calculated. In Year 1 there were three separate measurements and grazing observations. The first was in mid winter on pasture with a high dead matter and near zero clover content and overall low quality. N fertiliser had no effect on chemical composition. In the second and third observations in late July and September respectively, when more clover and less dead matter was present, N and crude protein (CP) concentrations were both enhanced by earlier N fertiliser application. Similarly in 2003 when herbage mass responses were allowed to accumulate following N fertiliser application, the pasture N, phosphorus, potassium and CP levels all increased and soluble sugars and starch levels decreased. These effects were more pronounced in the pasture from the south than from the north aspect. In Year 1 there were no differences between Nfertilised and non-fertilised plots in mean grazing intensity (ewes/100m2), or in the amounts of pasture DM utilised during grazing. In Year 2, grazing intensities and the amounts of pasture utilised, were higher where pasture mass was significantly higher as a result of N fertiliser application. The results suggested that where significantly greater pasture DM responses are allowed to occur, the subsequent grazing intensities will also be greater compared with grazing intensities on associatedpasture. It is likely that the differences in pasture chemical composition induced by application of N fertiliser at 30 kg /ha were insufficient to cause any difference in grazing behaviour by ewes. Higher N fertiliser rates may be required to induce such chemical changes and grazing preferences as observed in other studies. Keywords: grazing intensity, nitrogen fertiliser, pasture quality, pasture species