Managing Dairy Effluent Application and phosphate runoff.

About 60% of dairying in New Zealand is on poorly drained soils that need artificial drainage, such as mole drains and pipe systems. Drainage and surface runoff waters from dairy farms can cause significant nutrient enrichment of streams and rivers.

A research site to investigate the impacts of intensive dairying on the quality of drainage and runoff water was established on a naturally poorly drained Pallic soil, the Tokomaru silt loam, at No. 4 Dairy, and the effects of grazing, fertiliser application and the irrigation of farm dairy effluent (FDE) were measured. Dairy InSight funded the projects and Dr Mike Hedley, (director), James Hanly, (research officer), and Mike Bretherton, (research technician) from the Fertilizer & Lime Research Centre at Massey were all involved.

Grazing - During winter, the water draining from areas grazed but not irrigated averaged 14.5mgN/l and 0.29mgP/l. These concentrations will cause the nutrient enrichment of streams and lakes.

Most of the nitrogen came from early season leaching of nitrate that had built up in the soil profile over the previous grazing season. However, winter grazing immediately followed by rainfall led to significant concentrations of N and P in drainage. The total quantities of nitrogen (27 30 kgN/ha/y) and phosphate (0.3 - 0.8 kgP/ha/y) in drainage were typical of intensive dairy farms.

Urea - Applying 40kgN/ha as urea to grazed pastures in early June, when grass growth rates were slow, resulted in increases in the concentration of nitrate-N in drainage that persisted over at least seven subsequent drainage events. However, a similar application of N in spring, towards the end of the drainage season when grass growth rates were faster, had minimal effect on nitrate-N concentrations in drainage water.

Effluent - When 25mm FDE was deliberately irrigated onto wet soil, approximately a third of it left the soil profile as mole and pipe drainage and surface runoff. The total losses from that one event were 12kgN/ha and 2kgP/ ha equivalent to one third of the annual drainage N loss and twice the drainage P loss from grazed areas.

To avoid this, effluent was applied to pasture only when the soil was dry enough to absorb it without drainage loss. Consequently FDE had to be stored in the farms enlarged effluent ponds from August to October, and much of the deferred FDE was irrigated in summer and autumn.

Three years of applications of farm FDE using the deferred irrigation system did not increase N loss (compared with grazed-only pasture) in subsequent winter drainage, but did increase P loss by 1.1 kg total-P/ha. However, this increase was small less than 3.5% of the P applied as FDE.

In other words, deferred irrigation proved to be very successful for minimising nutrient losses from effluent irrigated areas.

Guidelines to farmers:

Where nutrient losses in drainage and runoff from grazed dairy pastures and paddocks used for effluent irrigation are a problem:

Store farm dairy effluent, and spray irrigate to land only when soils have the capacity to hold absorb effluent applied without generating drainage.

Limit the amount of nitrogen fertiliser applied once soils have begun their winter drainage season and grass growth rates are low. Grass growth rate models can be used to calculate how much N fertiliser it is safe to use in this cool wet period.

Nutrient losses increase when grazing is immediately followed by heavy rain and drainage. Little can be done about this unless it is possible to move the herd to stand-off pads when soils are wet.

Removing P from Drainage Water

Irrigation protocols like those above should reduce the amount of phosphate getting into drainage water, but it is not possible to stop all of it. Rain after irrigation or grazing, and lapses in management will still result in problems.

So the research team is now looking at innovative ways of cleaning it up by stripping the nutrients out. It is well known that volcanic ash absorbs phosphate readily, so he is filling mole drains with a readily available ash material to if it will strip the P from drainage water flowing through.

James Hanly has already simulated this in the laboratory using Perspex pipe as a mole channel, and it was highly successful. Early this year they created new mole drains on paddocks at No.4 Dairy and filled them with a granular ash product using a modified mole plough to inject volcanic ash into the mole channel it created. The material used is basically a volcanic gravel that comes from a quarry.

This winter the drainage water will be monitored to see how much phosphate comes out of the end of the pipes. Although the prospects are good there are many unknowns drainage water contains organics and other things and they may clog the drains or interfere in some way. In theory the ash in the mole drain should be good for 15 years or more.

If it is successful the project could point the way to reducing phosphate loads on sensitive areas, such as the Lake Taupo catchment. Putting in mole drains is relatively cheap and the ash is not particularly expensive. The estimated cost is around $900 per hectare, and the field experiment will determine how many years the cost can be written off over.