"We looked at lower irrigation trigger points to see if they provided any benefits," said Bright. "This meant the soil was allowed to dry out more than usual. We also looked at different irrigation targets – varying the soil moisture content we aim to achieve through irrigation."
"We looked specifically at targets that left quite a bit of capacity in the soil to store rainfall should it occur shortly after the irrigation finished. We found that filling it up to 80 per cent of the plant available water capacity and leaving 20 per cent for rainfall was probably the best target level from the point of view of reducing the nitrate leaching substantially while avoiding pasture production losses."
Bright says he was "pleasantly surprised" by the results.
"Before we started the project we didn't know what the impact would be on pasture production, but this was not compromised. We were even more surprised by the consequences of changing the trigger level. We found we could use a much lower soil moisture trigger value in spring and in autumn without having any significant effects on pasture production. This was critical as it allowed the soil to dry out more by delaying irrigation and increased its capacity to store rainfall."
The research 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 per cent 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. These could most easily be operated using the irrigation rules developed through the research.
"About 72 per cent 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 rainfall when it occurs.
"You do have to put more effort into monitoring soil moisture and using that information on a daily basis to figure out whether to irrigate or not, and exactly how much to put on. But at the end of the day, many Canterbury farmers are required to make significant reductions in nitrogen losses to water in the next few years. For some farmers, it may be possible to achieve some or all of these reductions simply by changing the way they manage their irrigation system."
