Precision N application test costs 'outweigh returns'
Precision application of nitrogen can improve yields, but the costs of testing currently outweigh improved returns, according to new research from Plant and Food Research, MPI and Ravensdown.
Cropping centres around 180ha of forage maize that when harvested reverts to annual ryegrass, red clover or annual ryegrass-clover mix, with the latter established by strip tilling. Having grown maize, initially for grain, since the 1960s, this was the cultivation regime until 2017. This followed the “conventional” ploughing route, followed by a power harrow to establish a seedbed.
Since 2018, strip tillage has been the preferred method for all maize crops, although one trial paddock has been striptilled since 2013.
Having chosen the strip-tilling route, Hunter says he saw many benefits, not including reduced cultivation costs, less run-off in adverse weather events and retention of organic matter in the soil profile.
Part of the ‘Growers Leading Change Project’ established by The Foundation for Arable Research (FAR) and the Ministry for Primary Industries (MPI) in late 2020, Mulroy Farm became one of four Pathfinder farms in October 2022. The aim of the programme is testing new innovation and techniques, aided by the FAR research team.
Currently, a trial is under way to look at nitrogen-use efficiency in maize growing, with an emphasis on residual N post-harvest and usage over the growing season. Deep N post-harvest sampling showed residues that ranged from 23 kg/ ha to 117 kg/ha – with an average reading 60 kg/ha over the test area.
Looking at the establishment regime, preplanting designated maize paddocks are sprayed off, followed by the installation of the cultivated areas where maize will be planted, using an American- built, 8-row, Soil Warrior Unit. Drilling is carried out using a John Deere planter equipped with Precision Planting Technology. This allows variable rate planting with seed rates from 85,000 to 110,000 seeds per ha, based on historical data and remote sensing.
During the trial, three nitrogen application rates were undertaken, using 210kg Sustain N/ha, a variable rate application that ranged from 200 to 285 kg/ha, with the final option at 420kg Sustain N/ha.
At three key stages during the growing season, Optimum Soil Adjusted Vegetation Indexes (OSAVI) indexes were recorded via drones, although visually there were no obvious signs to differentiate the three fertiliser regimes.
However, at harvest in March 2023, analysis of starch, sugars, ADF and NDF threw up some interesting findings. On average, the area applied with 420 kgN/ha showed 3% lower dry matter and the variable rate area delivered the highest yield at 29 tonnes / ha. This compared to 26.5 and 27.8 tonnes/ha in the other areas.
The lowest starch percentage was in the variably applied area, although it also offered the highest starch yield per hectare.
Comparing fertiliser costs per tonne of Dry Matter, the 219 kg/ ha application rate saw an average cost of $10.50/ tonne DM, the variable rate area at $11.80/tonne and the 420 kg/ha area the highest at $21.80 per tonne DM (all figures were based on a Sustain N cost of $1,380/tonne - June 2022).
In summary, the initial stages of the trial suggest that growers should make greater use of Deep N testing pre-establishment. In doing so they will get a better idea of residual N and the ability to adjust total N use over the growing season.
