Redefining the production potential of hill pastures using fertiliser nitrogen

Abstract

Farmer perceptions of the productive potential of land influence their attitudes to productivity improvement. Pasture production information has generally been gathered within management systems that were considered feasible at the time. A sheep grazing trial was run over 2 years in southern Hawkes Bay hill country in which a Fer tiliser treatment involving a very high rate of nitrogen (N) fertiliser (400 kg N/ha annually as 8 split dressings of 50 kg N/ha) was applied to blocks with historically low, medium or high fertility. Additional fertiliser P and S was also applied as part of the Fertiliser treatment, however the responses were judged to have been mainly due to the N fertiliser Annual pasture production increased from an average 9.2 t DM/ha in Control areas to 17.1 t DM/ha in Fertiliser treatment areas. Similar responses of 18 to 22 kg DM/kg N fertiliser occurred across the 3 fer tility blocks. Average annual pasture production for the historically high fertility-Fertiliser treatment combination was 19.4 t DM/ha, higher than on most dairy farms. Pasture content of high-fertilityresponsive grasses such as ryegrass increased, at the expense of low-fertility-tolerant grasses such as browntop, legumes such as white clover, and herbaceous weeds. Nitrate leaching measurements suggested annual losses were increased by 25 kg N/ ha or 6% of the N applied. Given present-day cost structures and returns, these results suggest use of higher rates of N fertiliser than is current practice is an economic option for hill farmers. However longterm sustainability of systems receiving such high dressings of fertiliser N r equires further investig ation before such an approach should be adopted as part of commercial practice. Keywords: botanical composition, hill country, New Zealand, nitrate leaching, nitrogen fertiliser, pasture production potential