Aim: To test and calibrate the extension of nitrogen-rich strips (N Gauges), Normalised Difference Vegetation Index (NDVI) measurement and the In-Season Nitrogen Calculator N-CALC) to assist in mid-season nitrogen decisions and maximise nitrogen efficiency and returns.

Key Messages:

  • In a season with close to average rainfall, the application of nitrogen at seeding resulted in the best wheat yield and gross margin.
  • Early biomass from the nitrogen applied at seeding set up a higher yield potential. Later top up applications determined by Summit’s In-Season Nitrogen Calculator did not bridge the gap created by lower nitrogen rates at seeding.
  • Greatest returns were generated with the higher rate of nitrogen at seeding topped up with the N-calculator rate seven weeks after sowing.

Table 1. Soil Test Results

Depth pH Al OC NO3- NH4+ P PBI K S Cu Zn
0-10cm 6.0 0.1 0.6 13 3 20 8 65 15 0.6 0.8
10-20cm 4.5 0.4   6 1 21 10 35 9    
20-30cm 4.3 0.5   1 0 19 19 51 7    


Summit’s In-Season Nitrogen Calculator aims to calculate a precise N rate to maximize yield potential of a paddock where N may be limiting, using NDVI as on-the-spot growth measurement. Previous trials at Binnu and around the state have shown its value in recommending N rates that were not in excess or deficient to crop requirements and therefore maximised returns, meaning N applied was efficiently used. In 2015 at Binnu, value of the Calculator was demonstrated in determining best timing for N application, after low early N inputs, by repeated measurement of NDVI and running the model. Later applications were possible, but higher N rates were required to optimise yield and returns, increasing grower risk somewhat. In 2016, we looked to determine how the Calculator performs following different rates of early N input to a Mace wheat crop.


Figure 1. Biomass assessment of plots on 16/06/16 byNDVI using hand-held Greenseeker®. NDVI readings were utilised as inputs for the In-Season N calculator.Figure 2. Average yield response to K application rate at the trial site. Quadratic function fitted.

At 7 weeks after sowing, using NDVI assessment comparisons (Figure 2), for rates of [a] 6, [b] 20 and [c] 40 kg/ha N up-front, the In-Season Nitrogen Calculator:

               i)     projected a maximum potential yield of 4.239 t/ha and

              ii)     calculated an optimized N application rate of [a] 34, [b] 23 and [c] 19 kg/ha;

             iii)     predicted this regime would out-yield a crop with no N top-up by [a] 933, [b] 646 and [c] 529  kg/ha (Figure 3). 




Figure 2. In-Season N Calculator input/output example for the N-gauge plot (D, 90 kg/ha N) versus treatment plots (2-4, 20 kg/ha N up front) NDVI comparison on 16/6/2016 – 45 days after sowing.


Figure 3.  Yield response to N rate applied in split treatments determined by In-Season N calculator compared with standard N response curve from up-front N rates only


The N rates recommended by the N-calculator optimised yield and gross margin with yield differences being significant when the recommended rate was altered up or down by 50% (Figure 3). This has consistently been shown in Summit field trials for wheat and barley in different conditions over recent seasons and has even shown reasonable applicability to oat grain crops.


  • 40N upfront generated higher yields than 20N upfront which in turn yielded greater than 6N upfront (P<0.05), highlighting the importance of plant available N during early growth and vigour.
  • 40N upfront topped up with the N-calculator rate of 19 kg/ha N produced the highest returns for the N-calc treatments.
  • An above average finish to the season meant the trial yielded higher than the 4.2 t/ha potential predicted by the N-calculator. Figure 3 plots the N-calculator rates on to the N rate curve and suggests that further N upfront would have continued to increase yield above our N-calculator upfront rates. 
  • The N-calculator predicted a yield penalty of 646 kg/ha from not applying N on the 20N upfront treatments which was very close to the 610 kg/ha actual penalty. Similarly, predictions for 40N upfront were very close with a yield penalty of 529 kg/ha compared to the actual 590 kg/ha.
  • Grain quality data saw some protein response to increasing N but minimal, high hectolitre weights, no difference in screenings except for low N.


Figure 4.  Gross margin, net of all fertiliser costs, from plots that received top-up N in various proportions of the In-Season N Calculator recommendations.



  • Yield potential was set up by applying nitrogen at seeding.
  • Top up nitrogen, even at higher rates did not lift yields to levels achieved when nitrogen was applied at seeding.
  • For this trial, the N-calculator prediction maximised or near maximised yield and gross margin in Mace wheat for the N-calculator treatments.
  • Getting mid-season N decisions right can have a significant impact on profitability since optimal top up N rates indicatively increased yields by 610 kg/ha and net returns by 94 $/ha for 20N upfront treatments, and 590 kg/ha and 99 $/ha for 40N upfront treatments.
  • This trial provides more weight to the building confidence that the N calculator can provide real and robust N recommendations that are not in excess of crop requirement and produce maximum or near maximum margin returns from mid-season N applications.