A post-graduate agricultural science study by Massey University’s Will Meads looked into the effects on nutrient budgets of mapping out the soils using electromagnetic (EM) surveying.
He based the study on actual onfarm information from Three Springs Dairies, which is close to Methven in Canterbury.
The EM survey information allowed him to input more blocks into the Overseer model because it identified the areas of predicted soil types more accurately than the S-Map information Overseer uses as its default.
When Overseer 6.2 was released earlier this year Will re-ran Three Springs Dairies Overseer file twice – once with the more accurate, increased block inputs and once with the standard defaults which allowed just half the number of blocks.
He found with the increased data input the nitrogen loss to water value dropped by 7% from 42kg/ha/year to 37kg/ha/year.
“That was the only difference in inputs between the two Overseer budgets – there were no other mitigations,” Will said.
Will said for Three Springs Dairies the EM survey detailed the variability between the two main soil types in the area, Mayfield silt loam and the lighter Eyre soils, giving more precise indications of where each was on the farm and the area each covered.
That allowed him to double the number of blocks in the Overseer model based on soil type predictions and indicate the area in each.
Without the information he would only have been able to define four blocks – two of which were based on effluent categories. With the EM survey information he could break each block into two – one for each soil type – and more accurately identify eight blocks.
As well as their cropping and dairying farming enterprises Craige and Roz own Agri Optics along with daughter Jemma Mulvihill. It’s a precision agriculture company that, among other activities, does EM surveys.
They were intrigued to see how much the EM information on its own could affect the outcomes in Overseer budgets.
Three Springs Dairies has three centre pivots, one with variable rate irrigation. Water is applied based on soil moisture probes with the aim to keep soil moisture between refill point and field capacity to prevent drainage.
Craige said over the past two milking seasons there had been no drainage events where water had moved below the 600mm point in the soil profile.
“And if there’s no drainage there’s no leaching,” he said.
Through the winter months rainfall created drainage but the farm is part of a larger independent study that’s collecting the leachate and measuring the concentration on nitrogen in the drainage water.
Although the results haven’t been formally released yet he said preliminary data had shown very low levels of nitrogen were present.
“If we’re keeping soil moisture at the optimum levels during the growing season so moisture and nutrients are staying in the root zone it makes sense that the plants are using up most of those nutrients so when drainage does occur through the winter there’s very little to be lost,” he says.
At the time the initial Overseer budget was carried out the farm was peak milking 1195 cows and using just over one tonne of drymatter/cow of supplement including ryegrass straw, palm kernel, barley, pasture silage and maize silage.
Cows were producing 519kg MS/cow.
A total of 220kg N/ha was being applied on areas not receiving effluent.
Effluent is spread via a centre pivot.
The mapping process
EM surveying uses electromagnetic conductivity to identify variations in soil properties.
Sands have low conductivity, silts have medium conductivity and clays have high conductivity.
Research has found strong correlations between electromagnetic conductivity and a number of soil properties including water holding capacity, cation exchange capacity, bulk density and even salinity.
A tow-behind device, which doesn’t touch the soil surface, emits an electromagnetic field into the soil and then measures how far it travels through the soil profile.
Data is collected almost continuously as the device travels across the paddock, allowing a detailed and precise map of the soil variations to be produced. The map can be used as the basis for other technologies so irrigation or nutrient applications can be varied to match soil characteristics.
Reducing leaching losses
The greater the level of control farmers have over irrigation application depths and return periods the better their chances of reducing leaching losses.
Will Meads’ study using Three Springs Dairies Overseer input data also looked at the effect of different irrigation options on nitrate loss levels.
Table one shows the significant gains to be made by being able to vary both the application depth and return period as Three Springs Dairies does using centre pivot irrigation and soil moisture monitoring.
Irrigation decisions at Three Springs Dairies are based on defined trigger points of profile available water (PAW).
The aim is to maintain the soil moisture at about 80% of the soil’s water-holding capacity which gives a buffer to accommodate rainfall events and prevent soil moisture reaching field capacity when drainage will happen.
Overseer 6.2 allows for selection of four irrigation scheduling types:
FF = Fixed Depth Applied, Fixed Return Period
FV = Fixed Depth Applied, Variable Return Period (based on soil moisture trigger point)
VF = Variable Depth Applied (applied to achieve target), Fixed Return Period
VV = Variable Depth Applied (depth applied to achieve target), Variable Return Period (based on soil moisture trigger point)
Three Springs Dairies uses a VV irrigation strategy.
If the farm was limited to a FF strategy whereby it wasn’t able to adjust the application depth and was committed to a fixed return interval it would be almost impossible to maintain soil moisture within the correct band and drainage would occur through the season.
The farm’s Overseer budget in this case would mean nitrogen leaching losses would be 118kg N/ha/year or more than three times what they are using a VV strategy and EM mapping.
Without the EM mapping leaching levels would be even greater at 146kg N/ha/year or close to four times worse.
It paints a bleak picture for those farms using irrigation equipment that applies large volumes of water of 50-70mm at each run and because of water availability or shortage of irrigators has a set return interval.
Large gun and rotary spray irrigators can fall into the FF category.
“Being able to manage irrigation to keep all of the applied water and rainfall in the root zone makes a huge difference to those leaching numbers,” Craige said.
They have installed variable rate irrigation (VRI) on one of their centre pivots on the dairy farm and will do the same with the others.
That gives them even greater control, allowing them to vary application rates along the length of the pivot based on the soil type under each nozzle as it passes.
They’ve employed similar systems on their cropping farm and by moving to VRI using telemetered soil moisture probes to help schedule their irrigation very accurately they saved 30% of their water.
“We didn’t have enough water for the whole farm originally but with this system we do.”
The cost of the new technology is about $2100/ha and with the additional productivity and returns, thanks to being able to spread the water further, the cost benefit and rate of return looks very attractive, Craige says.
He estimates anyone shifting from an FF type system to a VV operation would find similar water savings.
When compared with a district average for water use of 600mm/ha/year Three Springs Dairies has proven significant water use efficiency gains cutting usage in half to 300mm/ha/year.
Contrary to what some groups would claim water isn’t free – pumping costs, irrigation scheme share costs and water use charges can be significant.
Better water use efficiency can therefore create substantial financial savings.
While the proposition has to add up financially the environmental drivers in Canterbury have also increased significantly with nutrient caps and restrictions becoming part of consent compliance.
Farmers across the region are facing the prospect of having to make large cuts to nitrate leaching and more precise control over irrigation looks as though it can go some way to achieving them.
