Managing cereal crop N

Several years ago Ballance co-funded development of the Sirius Wheat Calculator (SWC) – an advanced crop model that has proven to be an extremely useful and accurate tool for forecasting the required rate and timing of nitrogen (N) fertiliser inputs. In addition, it also provides growers with information on irrigation scheduling and rate of crop maturation. 

The beauty of crop models is that farmers themselves don’t have to use them to benefit from them. These tools are extremely useful for consultants, helping them to understand regional climatic effects on soil N availability and rate of crop development. In addition, they provide excellent benchmarks for standardisation of fertiliser N recommendations, and can be used for training purposes. Many of the current N fertiliser strategies employed on farm have stemmed from work done in the development of this model. 

One of the limitations of the SWC is that its focus has been quite narrow – dealing predominantly with wheat varieties and focusing solely on grain crops. This left a large gap with regards to nutrient management in cereal crops, so Ballance has been working with Plant & Food Research to expand the capability of the SWC. The outcome of our project will be a new version of the SWC – called the Cereal Grain and Silage Calculator (CGSC). This model will cater for barley, wheat, oats and triticale, as well as crops grown for either grain or whole crop silage. 

The CGSC model will use much the same functionality as the existing SWC model – that is, using a combination of current and long-term average climate data to simulate the growth of the crop, linked with deep soil mineral N availability and irrigation (if available) inputs. In addition to forecasting rate and timing of fertiliser N and irrigation, the model gives growers the benefit of being able to forecast ahead for the likely ‘maturation window’, depending on whether the crop will be taken for silage or grain. Economics of fertiliser N inputs, as well as end use (grain or silage) can also be assessed – a useful tool in a volatile marketplace. 

As an example of the role these models can play in assisting growers with nutrient management, Ballance set up a randomised plot trial on a property in the Manawatu. The paddock was sown into autumn wheat coming out of maize, on a very low N soil (a deep soil mineral N of 3 ppm). 

The SWC was used to estimate an optimised fertiliser N strategy: it suggested 190 kg N/ha was required, on-third at the beginning of stem elongation (GS 30-32) and two-thirds at booting (GS 40-49). N treatments were designed around this forecast N rate, to test the crop’s actual N response against the predicted response. The treatments used are shown in Table 1.


GS 0-10 (kg N/ha)

GS 30-32 (kg N/ha)

GS 40-49 (kg N/ha)





50 N




100 N




200 N




200 N (early N)




300 N




Table 1: N application rate and timing in the Manawatu autumn wheat trial.

Plant & Food Research were recruited to harvest the trial with a small plot harvester, and then to statistically analyse the results (shown in Figure 1).

Figure 1: Grain yield results from the Manawatu autumn wheat trial.

As you can see, the crop was strongly N responsive. Yield appeared to plateau at the 200 kg N/ha rate, consistent with the forecast from the SWC. Increasing the rate of fertiliser N at this site to 300 kg N/ha appeared to give no further yield response.

The trial is also consistent with current fertiliser N recommendations, in that early N (applied in the autumn soon after sowing to encourage tillering over winter) did not give a yield advantage, despite the particularly low soil N status of the site. In fact, yields appear to have been somewhat reduced in this treatment, compared to the same total rate of N applied in spring only. This is not surprising - it was a very wet winter, and therefore some of the early-applied N may well have been leached before spring.

Results from trial work such as this are collated and fed into the database that is being used to build the CGSC, which will be released in winter 2011.

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