GMP2: Nutrient and Cost Analysis

GMP2: Nutrient and Cost Analysis

Effluent Management with Tony Cairns

Ballance Agri-Nutrients and Landconnect assessed Tony and Margot Cairns’ experience with upgrading his effluent treatment system. The analysis summarises the nutrient savings, impacts on profit and the wider farm system, risks and additional on-farm skills required to implement this nutrient practice. Landcconnect logo Ballance

Nutrient losses
OVERSEER 6.2.3 was used to model the current effluent system with a 78.5 ha effluent application block, a 90-day storage pond and low-rate irrigation. This is compared with the original farm system (early 2000’s) which had a 10 ha effluent block that received effluent direct from a small pond adjacent to the dairy shed.
Tony maintains pasture production on the large effluent block at similar levels to the non-effluent blocks, despite lower nutrient inputs per hectare (even after allowing for effluent nutrient content). There are no changes in total milk solids production.


The OVERSEER comparison showed the new system decreased N losses by 3 kg/ha/yr, based on a whole farm effective area basis (218 ha). The total annual decrease in N loss was 682 kg N. Conversely, modelled P losses increased by 89 kg P/yr on the farm overall, despite the effluent block P loss risk dropping from ‘extreme’ in the old system to ‘high’ in the new system. This appears to result from a combination of (i) the much larger effluent area not ‘cancelling out’ the lower per hectare loss, and (ii) the podzol soil type.
Overall, the reduction in N losses are likely to have occurred from a combination of the improved effluent practice and the lower total N input on the enlarged effluent block.

Economic modelling
Given the practical difficulties of assessing the profit and loss impact of the effluent upgrade, a relatively simple approach was taken, using an assumed cost of capital at 6% per annum. The estimated effluent system capital costs of $225,000 were provided by the farmer, comprising:

  • 90-day rubber-lined storage pond (37m x 37m x 6m)
  • Effluent pond stirrer
  • 20,000 litre sump
  • 1 additional travelling irrigator, 8 pods
  • New pump
  • Additional pipe-work and hydrants

Running and maintenance costs are considered similar for both systems.
The capital cost of $225k equates to an annual cost $13,500 at 6%, or $19.7 per kg N of ‘avoided’ N leaching.

Key risks
From the farmer’s perspective, a major driver for the effluent system upgrade was risk reduction, particularly from (i) the much greater storage volume allowing deferral of irrigation to avoid application in the frequent wet soil conditions, and (ii) the much greater effluent block size, allowing flexibility and low per hectare nutrient loadings. More generally, risks around excessive and uncontrolled effluent discharges were reduced through the purchase of a new pump, modern pipes/hydrants, a new low-rate travelling irrigator and eight low-rate pods. The net effect is to reduce risk of non-compliance with the farm effluent consent conditions.

There are risks of lower pasture production on the effluent block due to the nil N and K fertiliser application, although this has not been observed by the farmer. There appears to be adequate N inputs from effluent N (58 kg N/ha/yr) and supplements fed on the block (54 kg N/ha/yr), plus clover N fixation. The effluent block soil Quick Test K (QTK) value is 5, below the optimum levels of 7-10, which may be limiting clover growth. Local experience is that it is difficult to achieve optimum K levels on high rainfall, free-draining soils.
Additional skills required

The relative sophistication of the upgraded effluent system does rely on a skilled operator to get the best results. This includes taking advantage of BOPRC’s on-line soil moisture monitoring data at a nearby Oturoa Road dairy farm (similar podzol soils and rainfall).

Implications for the wider farm system
The main implication of the ‘new’ effluent system is confidence that it significantly reduces effluent non-compliance risk. This helps the farmer focus on getting the best out of the wider farm system.

However, all farms are different and not all systems will work for every farm. Therefore, we strongly recommend consulting an effluent management consultant to determine if changing your system will help you reduce nutrient losses from your farm, what it will cost and what management changes are required before undertaking an expensive upgrade.