A desktop study led by Aqualinc Research’s research and development director Dr John Bright and funded by the Fertiliser Association has shown it is possible to achieve an average of 27% reduction in nitrogen loss.
The research examined data from 12 case study dairy farms in Canterbury.
Using computer models, including Overseer, the researchers looked at the effects of different irrigation management rules on pasture production and nitrogen leaching.
The current practice is to irrigate when soil moisture content drops below 50% of plant-available water but Bright found letting the level drop further has no effect on pasture growth in autumn and spring.
He also studied whether leaving capacity in the soil for future rain has any effect.
“We found that filling it up to 80% of the plant-available water capacity and leaving 20% for rainfall was probably the best target level from the point of view of reducing the nitrate leaching substantially while avoiding pasture production losses.”
The team also deliberately tested target levels that did cause a reduction in pasture production to gauge the limits for irrigation triggers and targets.
Applying the principles of adjusting trigger levels during the season and using an 80% irrigation target requires the appropriate irrigation system.
It is essential to have a system that can be adjusted to relatively small application levels, with a short return period.
Centre pivots and solid set sprinkler systems were found to be the most suitable irrigation methods.
They could most easily be operated using the irrigation rules developed through the research.
“About 72% of the irrigated area in Canterbury uses methods that could easily implement these irrigation rules.
“The balance of the area would require a range of capital investments to modify them or to replace them to be able to implement these irrigation rules.”
Other benefits besides reducing nitrogen loss to water include reducing irrigation water use through improved efficiency and making more effective use of rain when it occurs.
