A review of literature on the land treatment of farm‐dairy effluent in New Zealand and its impact on water quality

Abstract

Dairy farming is the largest agricultural industry in New Zealand, contributing 20% of export earnings but providing a challenge for the environmentally acceptable treatment of wastes from dairy farms. Nutrient‐rich farm‐dairy effluent (FDE), which consists of cattle excreta diluted with wash‐down water, is a by‐product of dairy cattle spending time in yards, feed‐pads, and the farm dairy. Traditionally, FDE has been treated in standard two‐pond systems and then discharged into a receiving fresh water stream. Changes brought about primarily due to the Resource Management Act 1991 have meant that most regional councils now prefer dairy farms to land treat their FDE. This allows the water and nutrients applied to land in FDE to be utilised by the soil‐plant system. Research on the effects of land‐treating FDE, and its affects on water quality, has shown that between 2 and 20% of the nitrogen (N) and phosphorus (P) applied in FDE is leached through the soil profile. In all studies, the measured concentration of N and P in drainage water was higher than the ecological limits considered likely to stimulate unwanted aquatic weed growth. Gaps in the current research have been identified with respect to the application of FDE to artificially drained soils, and the lack of research that has taken place with long term application of FDE to land and at appropriate farm scale with realistic rates of application. Whilst the land treatment of FDE represents a huge improvement on the loss of nutrients discharged to fresh water compared with standard two‐pond systems, there is room for improvement in the management of FDE land‐treatment systems. In particular, it is necessary to prevent the direct discharge of partially treated FDE by taking into account soil physical properties and soil moisture status. Scheduling effluent irrigations based on soil moisture deficits results in a considerable decrease in nutrient loss and may result in a zero loss of raw or partially treated effluent due to direct drainage.