Pastoral agriculture in New Zealand produces more than a third of all internationally traded dairy products, and approximately 20% and 10% of all internationally traded sheep and beef, respectively. For a country smaller than the size of Nebraska, with a population of only 4 million, these are impressive statistics.
The country’s natural advantage is an equable climate, and the ability to grow grass all year round. However, increased productivity over recent years largely to fulfill dairy export demands has led to increased dependence on irrigation systems to extend dairying into new regions and maintain productivity levels through the dry summer months.
This has put pressure on freshwater allocations with resultant restrictions, so that farmers are looking to optimize the water-use efficiency of their irrigated pastoral systems, aiming for best conversion of each millimeter of applied water to plant growth. Consequently, there is a current trend to replace traditional flood irrigation systems with more efficient center pivot and lateral sprinkler systems, with an estimated 40% of all irrigated land now under these sprinkler systems. This opens up opportunities for variable rate irrigation scheduling, with individual sprinkler control.
The New Zealand Centre for Precision Agriculture and Landcare Research (an environmental research Crown Research Institute) are developing a soil-based decision support tool for variable rate irrigation scheduling, so that a sprinkler system can be directed to deliver different depths of water to different zones based on soil differences. This gives better use of stored soil water, especially in the variable, young alluvial soils, which typify many of New Zealand’s soils.
For example, some of our research in the Canterbury Plains of the South Island has found that a 600-meter (approximately 1,968 feet) center pivot, covering an area of 113 hectares (about 280 acres), irrigates soils with available water holding capacities ranging between 40 and 100 mm available water (approx. 1.5 to 4 inches), so that some zones ideally require irrigation earlier that others to maintain potential yields.
EM mapping is being used to delineate soil spatial variability on a basis of differences in soil apparent electrical conductivity (EC). Soil EC differences are largely due to differences in soil texture and moisture in these non-saline soils, and our research has found good relationships between soil available water holding capacity (AWC) and soil EC, so that soil AWC maps can be produced. These maps indicate the maximum amount of available water that a soil can supply to plants, and are used, with daily soil moisture predictions or measurements of wetting and drying within each zone, to produce soil water status maps. Soil water status maps predict the day on which each zone reaches it’s irrigation trigger point. The maps are available for upload to an automated variable rate irrigation system.
The variable rate irrigation system is being developed by WMC Technology Ltd. (NZ) and is at present in the testing stage. Solenoids are fitted to sprinklers and controlled in banks of four, with each controller contributing to a wireless network. The system is controlled by software that determines the application depth at any point under the irrigator.
An example of the need for water management is Manawatu Sand Country, where corn is being grown as a grain and fodder crop. Sandy knolls reside in an undulating sand plain topography, which tend to dry out very quickly in early summer and become hydrophobic (water repellent). Ideally they require very frequent irrigation events to maintain soil moisture for potential crop growth.
In comparison, other low lying zones that are just meters away stay wet by receiving additional water as a result of capillary rise from a high water table, and therefore require less irrigation to maintain the optimum soil moisture for potential crop growth. This is a situation where variable-rate irrigation is desirable as it (1) meets the needs of high water-use soils and (2) decreases over-watering of low-lying areas that might otherwise become flooded, which in turn stunts plant growth and increases the likelihood of nutrient leaching and plant disease, as well as wasting water.
Variable rate irrigation (VRI) is an enabling technology with multiple benefits. It improves water-use efficiency and allows crop flexibility under one irrigator where crop type can be matched to soil type. It also increases options for chemigation and fertigation. Saved water can be redirected elsewhere allowing better strategic use when it is limited or restrictions are imposed mid-season when crop demand is highest. Our research shows water savings of 10% to 20% under VRI systems, based on hypothetical irrigation scheduling using a water balance model where soil zones are only irrigated when they reach their specific irrigation trigger point (i.e. a known AWC depletion factor). This research will continue to (1) develop the variable rate irrigation soil decision tool, and to (2) investigate the potential advantages and uptake of these systems by New Zealand agriculture.
The New Zealand Centre for Precision Agriculture is undertaking a wide range of research to meet the demands of current agricultural systems in the country. The Centre has developed a pasture growth meter to map pasture yields. The maps are a grazing management tool aimed at improved pasture utilization. The rapid pasture meter is now produced and marketed by a local manufacturing company CDAX Systems Ltd., Palmerston North, New Zealand.
Carolyn Hedley is a soil scientist at Landcare Research, based in Palmerstown North, New Zealand. Ian Yule leads the New Zealand Precision Agriculture Centre, also based in Palmerston North. More insight into precision agriculture practices in New Zealand will be highlighted in the Spring edition of PrecisionAg Special Reports.