Spinner cuts are shallow trenches cut into flood irrigation bays, approximately 10m apart, or 4 per standard bay. They run the length of the bay, following the slope of the bay, and aim to move water away more quickly, reducing waterlogging after irrigation/rainfall.
Spinner cuts can be installed at the rate of 4 ha/hour with a tractor and rotary drainer, at a rate of approximately $40/ha. A GPS equipped tractor keeps cuts straight and prevents management issues. The cuts are simple, with the first cut within 5 m of the checkbank and the remaining cuts an equal distance of approximately 10 m apart. This means every part of the bay is within 5m of a drainage point. The best spinner cuts are taken top to bottom of the bay. When forming the cuts, if it is irrigation season, it is best to leave small uncut areas along the length so that water does not rush down when the cuts are freshly installed. As the cuts become colonised with plants and sediment, the uncut areas can be removed to allow water to flow more freely.
Deeper cuts (50mm) last a few years, shallow cuts last a season, and can be rescraped and neatened up easily with a rotary drainer and tractor on-farm.
WHAT DIFFERENCE DO SPINNER CUTS MAKE?
Spinner cuts can reduce
Pugging
Transient waterlogging
Pasture weeds
Nitrogen losses
Waterlogging causes low trafficability and pugging in winter and reduced pasture growth and weed infiltration in summer. It also means nitrogen and water is lost through deep drainage or emissions and evaporation into the atmosphere and not collected on-farm. These wasted inputs are expensive.
The effects of waterlogging in summer are very apparent and can cost the farm a lot of margin. After flood irrigation, surface water heats up in the sun if it does not absorb or run off effectively. Hot water loses oxygen and when this hot water soaks into the root zone, it means plants have no oxygen and their roots are surrounded by hot water- effectively boiling the plant. Ryegrass is far more susceptible to this effect than summer weeds such as distichum and wild millet. In severe cases, when temperatures are very hot and pasture has just been grazed, the thin film of water in bare patches absorbs heat and scalds the ryegrass plants, causing die off. This encourages weed growth and can change pasture composition to weedy, low quality fodder.
Spinner cuts move water away more quickly than would ordinarily be possible, improving the irrigation flow and reducing the time the soil is waterlogged, which increases growing time, makes pasture more N efficient and maintain pasture quality.
HOW DO THEY WORK?
The slope of the irrigation bay is designed to carry the water off the bay, as is the natural flow pressure of the water. Sometimes the slope isn’t effective enough on its own, depending on soil type, laser grading frequency, pugging and other factors. As waterlogging is one of the key issues on-farm, we aim to reduce it in any way possible, and spinner cuts are one way.
Spinner cuts improve poorly draining irrigation bays by creating localised areas of slope across the bay, so that water moves preferentially into the cuts, and can move more quickly off the bay.
Once most water has moved across the bay, or infiltrated down into the soil, the root zone soaks with water, and becomes anaerobic. Usually a thin film of water (1cm) is left on the surface of the bay, held by surface tension. This water prevents air re-entering the soil, is slow to evaporate and can hold up pasture production as ryegrass can’t begin growing again until it has oxygen in the root zone. Any delay in moisture movement means lost production from the bay.
Spinner cuts reduce the layer of water on the soil surface, so that it absorbs more quickly by creating regular areas of the paddock where the gradient, due to the shallow trench, is slightly lower than the rest of the paddock, creating localised slope to allow water to travel. Not only does this mean that the water reaches the mid and back areas of the bay faster, creating greater efficiency, it also means that the shallow remnants of the irrigation water can flow into a drain, and allow air to re-enter the soil more rapidly to promote optimum pasture growth.