Creating profit maps across Craige Mackenzie‘s 200ha of intensive arable production in Ashburton, Canterbury, New Zealand has been a key step in understanding where production can be intensified – and where inputs need to be cut back as the soil is just not capable of yielding higher.

Like most growers, Mr Mackenzie started with yield mapping. “It was a huge wake-up call – we knew we had variation, but we didn’t know where it was and how big it was.”

The yield mapping illustrated how seed rates were too high. “You were sitting on the combine watching the yield go down in the thick areas, and go up in the thinner areas – it was like a light going on.”

But his interest in precision farming really took off after a Nuffield Scholarship study tour.

“Once you understand how many kilograms you grow for every millimetre of water, and how many kilograms of nitrogen it takes to grow a tonne of wheat, and you’ve measured them and quantified them with a yield map, then you can start to do something.”

The next step was electromagnetic soil mapping and in-crop sensing with Trimble Agriculture‘s Greenseeker, which are the key to moving to a more proactive system, he says.

“Yield maps are great, but are only historical. Electromagnetic soil maps give you ideas about what you are going to do based on soil type and water holding capacity, but once you have got them, are really one-off too. It is the Greenseeker that allows you to make some in-season decisions based on what’s going on.”

The Greenseeker works on a similar principle to Yara’s N Sensor, using normalised differences in vegetative index sensing to map fields, which can be used for variably applying various inputs, including fertiliser.

“It is only a decision-support tool. There is no substitute to standing out in your fields and then using the information to help you make a decision.”

Early passes with the Greenseeker pinpointed huge variability, which soil testing confirmed as zinc and manganese deficiency. “I was able to build a map to only apply manganese sulphate only in those zones, and fixed my problem in about three days without applying much product.

“But it also meant we had zones identified, that we could continue to monitor and test. For example, when we got to a pH of 6.4 in some areas, we were able to variably apply lime.”

For the first time he had been able to locate where the man-made issues were and treat them. “To fix man-made issues is easy, and all of those tools allowed us to do that.”

Other applications include growth regulator use in ryegrass seed crops, which had increased yields significantly while decreasing costs.

That’s where profit mapping comes in, he says, whereby he overlays his yield maps with how much he has spent in different zones to come up with a profit map.

“All farmers will have zones that are costing them money and areas that are very profitable. You have to accept that some zones will never reach the profitability of the very high ones, but at least you can turn them into profitable ones.”

That involves accepting that some zones will only do 8t/ha of wheat because of some kind of physical limitation, where the target for the farm is much higher, and using reduced inputs accordingly, he says.

“If you’re going to do profit maps you have to measure everything you put on, otherwise you don’t really know what you’re doing.”

Irrigation techniques

Irrigation systems that allow each nozzle to apply variable rates of water according to soil type and moisture deficit are helping arable farmer use limited water more efficiently.

Irrigation is central to the success of both dairy and cropping systems on the Canterbury Plain, and elsewhere in New Zealand. Rainfall during the growing period is typically only 250mm in Canterbury, with the potential for north-westerly winds to remove 5-8mm of moisture a day.

But applying water, particularly with newer lateral and centre-pivot irrigation systems, has enabled farmers to take unreliable low moisture-holding soils into useful production in both sectors.

However, water use is restricted, which means farmers need to be efficient in using it.

That requirement, plus the realisation that soil types could be extremely variable under large irrigation systems, suggested that variable rate irrigation could help, says Carolyn Hedley, a soil scientist from Landcare Research.

She uses electromagnetic mapping to analyse soil type under irrigation systems, creating zones in which soil moisture probes are placed that wirelessly transmit moisture deficit information from two depths back to a central base station.

“It gives you real-time moisture monitoring accessible through a website.”

That information can then be used to produce irrigation maps for the amount of water needs to be applied in each zone.

The water required is then applied using a variable rate irrigation system developed by Precision Irrigation, which individually controls each sprinkler head to apply the correct amount of water.

In studies, farms using the system have saved up to 20% water, with no loss in yield. “With a water saving, and the same yield, there is obviously an increase in water use efficiency,” Dr Hedley says.

Potentially the whole system could be automated to apply water at a designated soil moisture deficit, for example, she adds.

More than 20 variable rate irrigation systems are now used commercially, 18 in New Zealand, including those at Eric Watson’s all arable cropping Rangitata Holdings, Wakanui in Canterbury.

He is already seeing the benefits, he says. “We had around 5.5ha of overlaps in the system, which variable rate technology has allowed us to cut out.”

Measurements on two booms using water meters suggested he was saving around 12 litres/second across both systems. “The payback is going to be quicker than I originally thought. Instead of being seven years, it might only be five.

“We’ve also found it gives the ability to use another irrigator elsewhere on the farm if we’re only using half of one boom, for example, which helps apply water when crops actually need it.”