Report: TR 2008/39

Author: RD Longhurst, DC Smeaton(Agresearch)

Abstract

In 2006, Environment Waikato (EW) launched an Integrated Catchment Management (ICM) initiative to improve nutrient efficiency in two sub-catchments (Little Waipa and Waipapa) within the Upper Waikato Catchment. At the time of compiling this report, fifteen dairy farmers had been approached by EW and had agreed to participate in the ICM study. On eight farms, detailed Environmental Farm Plans had been constructed and form the basis of the case studies presented here.

AgResearch was contracted to undertake nutrient management analysis and to integrate this with agronomics and farm economics. A modelling study using the OVERSEER® nutrient budget model was undertaken on these farms to investigate nitrogen (N) and phosphorus (P) losses through on-farm mitigation strategies. Two of the eight farms were then used as case studies using the UDDER1 dairy production farm model to determine the likely cost of implementing these various mitigation strategies.

The soils in the catchment are predominantly free-draining allophanic and pumice. Average annual rainfall is 1470 mm (range 1400 to 1550 mm). On an average ICM Farm, 500 dairy cows were stocked at 3.0 cows/ha and produced 1,070 kg milk solids/ha. Average N and P losses on the ICM farms were 45 and 2.2 kg/ha/yr respectively.

Greatest contributors to N loss were stocking rate and the amount of fertiliser N input as fertiliser. Most ICM farms used land application for effluent treatment and all but one farm had some form of effluent storage. Average nutrient loading rates on effluent blocks were 120 kg N/ha and 158 kg K/ha. Potassium (K) loadings were far in excess of pasture K maintenance requirements. Surface runoff losses of P were predicted to be higher when soil Olsen P concentrations were above optimum levels and greater with pumice soils than on allophanic soils.

The most effective mitigation strategies that were investigated for reducing N losses were a reduction in fertiliser N rates, avoiding fertiliser N applications in winter and using a DCD nitrification inhibitor (over two applications) on grazed pasture in autumn/winter. Wintering the herd off the farm was also very effective but was unacceptable to the ICM catchments involved in this study because of its N exporting effects. Increasing the size of the effluent block and reducing fertiliser inputs to this block reduces the overall amount of fertiliser N required and reduces the K loading to pastures.

Following best management practices and using a combination of the above strategies could reduce N losses by approximately 10 kg N/ha, down to an average of 35 kg N/ha. Further reductions in N leaching can only be achieved if cows are removed from pastures as much as possible during the high-risk leaching period (May-July). Construction of specialised stand-off/animal shelters to achieve this requires substantial capital and operational costs and is difficult to justify although farmers report other benefits.

The UDDER model was used for in-depth case study analysis of two ICM dairy farms. Several mitigation scenarios were tested and reductions in N loss were calculated to be of the order of 6% for not applying fertiliser N in winter; 4-10% for using nitrification inhibitors, 12% for switching to a land based effluent treatment system; 8-10% for stock wintering systems. Large reductions in N leaching can, potentially, be obtained through reducing fertiliser N inputs (0-40% depending on the level of fertiliser N reduction). An organic farming scenario was predicted to greatly reduce N leaching, and deserves serious consideration, as it was also quite profitable, although further study is required to confirm this.

Some of the N reduction scenarios had a slightly positive to neutral effect on farm profitability and some reduced profitability. Gross Margins were affected to a greater or lesser degree depending on the level of milk payout ($5, $6, and $7/kg MS were tested). Generally speaking, scenarios to reduce N leaching were more likely to be profitable at high payouts. This is a major consideration in the adoption of any mitigation system.

A simplistic approach to reducing N loss would be to limit N inputs to a farm as has been the legislative approach in Europe. Another approach, however, is to allow farmers to develop management systems that limit leaching losses by building a number of mitigations into their systems (i.e., putting an emphasis on N outputs rather than on N inputs).

Overall, the ICM project has demonstrated a need for EW and AgResearch to work with farmers on how they can fit mitigations into their current dairying systems and has shown that progress can be made in the management of N leaching without com-promising farm profitability.

Improving Nutrient Efficiency Through Integrated Catchment Management in Little Waipa and Waipapa [PDF, 870 KB]