Every paddock on the Johnsons’ 500ha dairy farm is an intricate, swirling jigsaw of soil types.
The 1600-cow farm, run as two adjoining units, sits at the seaward end of Rangitata Island and its topography is that of a big, braided riverbed overlaid with a cover of soil that ranges in depth as well as type.
In places the water table sits just below the surface, occasionally emerging above-ground, depending on rainfall and river hydrology.
Ribbons of ancient gravels snake their way across the property while pockets of deep, heavy Wakanui and Temuka soils can also be found in both low and high spots.
Springs and wet areas appear at the slightest over-watering while the lighter, stonier areas will dry out and burn off in the hot dry Canterbury summer.
Owners of Rangitata Island Dairy Simon and Jenny say variable rate irrigation (VRI) technology is going a long way towards addressing the immense challenges the farm’s soils present.
The technology links highly detailed maps of the soil types – produced using electro-magnetic (EM) mapping – to GPS sensors on the pivots through computer software provided by Precision Irrigation.
‘If you don’t go right into paddocks on a farm this size it’s easy for something to get missed.’
In simple terms the computer programme picks up where the pivot is in relation to the EM soil map and turns individual sprinklers along the pivot on and off in a pulse-like fashion so each soil type receives a pre-determined amount of water.
The speed of the pivot is also moderated to help achieve the correct rate and depth of application.
The system allows Simon to set specific application depths for each soil type so that across any one paddock and any one pass of the pivot as many as five different application depths are being applied at any one time.
The very light soil can be getting up to 10mm while the heavy soil might be getting just 2mm and the very heavy soil no water at all, he says.
It’s the third season with VRI fitted to two of the eight pivot irrigators that water the property and compared with the next option for them of buying more water, payback was almost immediate.
That’s because when he first considered the technology he was also weighing up the option to purchase shares in the $115m Rangitata South Irrigation scheme, developed and built by Rooney Earthmoving.
“I knew we could do with 5% more water than what we have consented through our wells but there’s an ongoing annual charge for the scheme water as well as the initial capital cost to buy shares.”
By comparison the estimated savings by using VRI and not over-watering the heavy and wetter areas amounted to 30 litres/second – about 5%.
“So we achieved the same outcome for a one-off capital cost and no annual fee and it also ticked the boxes when it came to irrigation efficiencies – putting the right amount of water where we needed it, when we needed it so we could grow the most grass.
“If we’d taken that additional 5% via the scheme and then irrigated the whole farm evenly so we kept up with the dry, light soils we’d be left with permanently wet areas on the heavier areas and places where we get ground water coming up.”
The two pivots fitted with VRI cover almost half the farm and Simon is now considering putting it on two more pivots where soils are most varied.
He’s also looking at adjusting programme settings to refine the watering accuracy even further.
Variable rate irrigation for water and effluent – payback period was rapid.
A portion of the farm is Department of Conservation lease land and getting consent to put effluent over that area looked like it was going to be a challenge given it’s essentially the mid-branch of the Rangitata River.
Simon says when the family took on the lease and got consent to convert it in 1996 the area was completely covered in dense, 3.5m high gorse.
It has only fully flooded once in the 20 years the family has farmed it but it does contain some of the most complex soil type mixes.
He consulted with the local iwi, Ngai Tahu, about his plan for VRI over the DOC land, as well as with the council.
“They were happy with it. They could see we were trying to go beyond good practice and be ahead of any rules rather than chasing them.”
He chose not to inject the effluent into the water line and spread it mixed with irrigation water because that would have required safety backflow valves on each of the seven pumps on the farm’s ringmain and at $35,000 for each backflow valve he wasn’t keen.
He also wanted to be able to empty his storage ponds completely in six hours.
If the effluent was being injected to give a ratio of 10% effluent to water, emptying the 30-day storage pond was going to take much longer.
With the prevailing north-easterly wind and frequent nor’westerly events, having effluent spraying out for a long period on a big area of the farm could also pose some drift issues.
“We wanted to be able to get it out in what can be short windows of opportunity at the right rates for each soil type and have the ability to avoid some areas altogether – areas like creeks and the 20m strips along them.”
Instead of injecting it they have gone for a system where the effluent is piped to the pivot tower in a separate line. A non-return chemical valve, a backflow valve and another chemical valve on the irrigation line all have to be set before the irrigation water is closed off and the effluent valve can be opened, sending the effluent into the pivot line.
‘They could see we were trying to go beyond good practice and be ahead of any rules rather than chasing them.’
It’s been through a two-pond system and is effectively green water when it hits the pivot, and runs through the same lines and nozzles as the irrigation water would.
Exactly the same VRI technology is applied to the effluent as is applied to irrigation water.
Simon can set a programme for the effluent just as he can for the irrigation, assigning different application rates to the various soil types to ensure the nutrient-rich effluent stays in the root zone, doesn’t run down through the soil profile on the light soils, doesn’t pond on the heavier areas or isn’t applied to some areas at all.
When it’s time to put the effluent out either he or one of the two unit managers on the farm select the effluent programme at the “brains” control panel at the pivot tower when they shut down and open the appropriate valves.
Once they’ve put out what they need to they close off the effluent flow, open up the irrigation water and over-ride the variable rate irrigation programme for a few minutes to flush every line and nozzle with clean water.
That should help preserve the brass fittings on the gooseneck above the spray nozzles where the solenoid valves are fitted that create the pulsing water flow effect.
“The jury’s still out on how long the brass fittings will last with effluent going through them but we’ve been pretty careful with flushing out the system straight after the effluent’s been through and we work pretty hard at keeping fine sediment out.”
They go to the extent of sweeping out the second pond once a year when it’s emptied at the end of the season, getting rid of a 5-10cm layer of fine dust that settles out on the bottom.
Another advantage of the VRI system for the effluent has been the ability to run it through the pivot using a smaller pump.
It would have taken a pump capable of pumping 80 litres/second to maintain enough pressure to send the effluent right out to the ends of the larger pivot.
That would have meant that for one day a month on average the transformer at the farm dairy would have been too small to handle the electricity load from the pump.
VRI allows Simon to cap the effluent application depth to 2mm with some nozzles shut off altogether. That effectively boosts the pressure in the pipes enough to allow a 40l/second pump to do the job, cutting back on capital pump costs and electricity running costs, and eliminating the need for a new transformer.
They still have a few unexplained problems with a couple of valves that create the pulsing and he’s not convinced they’ve fully nailed the system yet although it’s definitely a much more precise way of getting the nutrient on.
Key points
Location: Rangitata Island, Canterbury
Owners: Simon and Jenny Johnson
Effective area: 450ha
Cows: 1600 crossbreed
Production: 430kg milksolids/cow
Farm Dairies: 46-aside herringbone and 36-aside herringbone
Supplement: 400kg drymatter/cow palm kernel, grass and maize silage
Irrigation: Eight pivot irrigators, two RotoRainers
Staff: Eight across both units.
