Rural Management Strategies consultant Chris Minehan says using Yield Prophet and plant available water capacity mapping to manage nitrogen can be more profitable than applying a blanket rate.
At about 30 percent of the variable cost of growing wheat and canola, nitrogen is one of the single largest costs in a continuous cropping program, so it is no surprise that grain growers are looking for a way to manage their nitrogen more profitably.
With that in mind, recent research conducted by Rural Management Strategies in New South Wales has found that managing nitrogen separately across varying soil types using Yield Prophet and plant available water capacity (PAWC) mapping can be more profitable than applying a blanket rate of nitrogen due to savings in fertiliser costs.
During the 2013 and 2014 winter cropping seasons, the Grains Research and Development Corporation (GRDC) funded trials at Barmedman to demonstrate the use of Yield Prophet and PAWC mapping to improve nitrogen decision-making in wheat and canola. The trial was part of GRDC’s Agribusiness Trial Extension Network.
Yield Prophet has the potential to improve nitrogen decision-making but it relies on accurate soil characterisation data which can be expensive and time-consuming to determine, according to Rural Management Strategies (RMS) consultant Chris Minehan.
“While many growers have yield mapping and variable rate technology, they are often unsure how to determine different rates for different areas,” he said. “PAWC mapping is a potential method which can be used to zone paddocks, allowing soil types to be managed separately for nitrogen using Yield Prophet predictions. However, the expertise and time required for soil characterisation is a limitation to the accuracy of Yield Prophet predictions, which has affected uptake of Yield Prophet in southern NSW.”
The project, conducted by RMS, had two parts. Part one of the project involved the development of a decision-making framework (Table 1) incorporating historical crop performance to be used in conjunction with Yield Prophet which was tested during the 2014 season on a canola crop. This resulted in a reduction of fertiliser cost and improved profitability compared to the district practice of blanket urea application.
Table 1: Decision making framework developed in part 1
Yield Prophet % probability of achieving median yield (1.6t/ha)
Likelihood of yield response to additional N/action
with existing N + PAW
| with additional N
|| Adequate N and PAW
|| LOW/wait, reassess
| PAW is likely to be the major limitation to yield
|| Adequate PAW but inadequate N
RMS consultant Chris Minehan says this was achieved by identifying a soil type within the paddock where additional nitrogen was unnecessary. However, he added that some concern remains around the level of accuracy achieved by the Yield Prophet model in canola.
“Some insight was gained as to the importance of having crop specific soil information such as crop lower limits (CLL) and subsoil constraints,” he said. “The impact of temperature extremes was also identified as a limitation of the model which must be considered when making decisions based on Yield Prophet.”
Figure 1: MG soil type zones
The trial paddock at Barmedman (MG) was split into three zones (Figure 1) based on soil type. They were red sandy loam (MG1), shallow sand and paddock edges (MG2) and sandy clay (MG3). Each zone was soil-tested to assess plant-available water and nitrogen (Table 1). The resulting data was entered into Yield Prophet along with sowing time, variety, fertiliser and seeding rates, which gave an estimated yield potential of each zone.
“Yield Prophet modelling in May 2014 indicated approximately 80 percent chance of increasing yields by more than 0.5 tonnes per hectare by applying additional nitrogen,” Mr Minehan said. “However based on our decision-making framework, the decision was made to not apply any nitrogen to MG3 which was predicted to exceed the median yield with current nitrogen and to apply a smaller amount of nitrogen as urea (60 kilograms/ha) to MG1 and MG2.”
Table 2: Average N and PAW to one metre
|| Total N
For comparison, standard urea treatments of 100kg/ha were included in strips across the paddock encompassing each of the three zones (Figure 2).
It was found that while applying nitrogen in 2014 increased yields with the 100kg/ha treatment the most at 0.74t/ha, applying variable rate (VR) nitrogen using PAWC mapping was the most profitable (Table 3).
Table 3: Nitrogen management strategies
|| Nitrogen management
| Average yield
| Canola income @ $450/t
| Less urea cost
| Less VR cost
| Difference from nil ($/ha)
“If considered in isolation, the 2014 nil treatment was the most profitable treatment, as it was in 2013,” Mr Minehan said. “However, the use of the 2013+2014 nil treatment shows that taking this approach every year is a risky strategy likely to see lower yields and less profit. The rundown in organic nitrogen and organic carbon must be considered as part of sustainable farming practices, especially in long term or continuous cropping systems.
Figure 2: Nitrogen treatments
“Yield Prophet is likely to be a useful tool in these situations however, as it would help identify situations where soil nitrogen was low and hence investment in nitrogen was most likely to be profitable.”
Another aspect of the project was to assess the potential for extrapolating soil characteristics across landscapes. Mr Minehan says this is possible, provided the additional paddocks and farms are in similar positions within the landscape.
“Results from the ‘AA’ site – a paddock with a similar soil type to zone MG3 north east of the original trial site selected to compare soil characterisations - suggested that soil characterisations can be extrapolated across paddocks, farms and landscapes and used effectively in Yield Prophet,” he said. “A sufficient understanding of key soil characteristics and their implications both for the model and for crop yield potential is required in order to make use of this potential.”
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