Application of the nutrient budgeting model OVERSEERâ„¢ to assess management options and Regional Council consent requirements on a Hawke's Bay dairy farm

Abstract

OVERSEER™ is a nutrient budgeting model which estimates the inputs, outputs and balances of N, P, K and S for different farming systems. Its main role is as an index of the sustainability of farm systems and to provide information on the potential environmental impacts through estimates of nutrient leaching. OVERSEER was used to assess the N flows and balances associated with different management practices on the farm of Hawke's Bay Dairies Ltd. The 210 ha farm includes a feed-pad for feeding cows with fruit and vegetable processing wastes and pasture which is cut and carried from a 41 ha effluent area (the latter is a requirement of the Regional Council consent). Outputs from OVERSEER for the effluent area revealed that N removal in cut pasture was 1.4 × total N inputs in effluent and clover N2 fixation and the total N balance was -117 kg N/ha/year. This could lead to significant pasture deterioration and would be unsustainable in its current state. Current N inputs for the whole farm system were estimated at 80, 147 and 87 kg N/ha/year in fertiliser, brought-in feed and clover N2 fixation. Outputs of N in milk and nitrate leaching were estimated at 154 and 66 kg N/ha/year, respectively. Hawke's Bay Dairies propose changes in management to exclude the cut-and-carry operation. The effects of these changes on N flows and balances are presented and compared with those from the long-term farmlet trial at the Dairying Research Corporation Number 2 Dairy farm. In the Hawke's Bay Dairies farm, high milk production (2200 kg milksolids/ ha) is being achieved through utilisation of locallyavailable fruit and vegetable wastes which are high in energy and low in protein. This results in very efficient conversion of total N inputs into milk (49%) and lower leaching losses than those measured in farmlet systems based on high (400 kg N/ ha/year) inputs of N fertiliser. Keywords: dairy farm, feed-pad, nitrate leaching, nitrogen model, OVERSEER™