Conclusions and Summary

Discussion

Rates are more important than timing

The results reinforce the importance of nitrogen for canola. It also reinforces previous recommendations that canola rate is more important than timing, provided the crop does not become nitrogen limited in its early growth stages.

While the trial found that ‘rule of thumb’ high rates (80kg N/ha/t) was the best rate, growers can achieve equal yields with lower (moderate) rates (60kg N/ha/t) so long as the fertiliser is supplied early enough during crop growth, including a topdress by early stem elongation.
A high yielding irrigated trial conducted by VICC under irrigation found yields of canola no different between the 80 kg/ha/t nitrogen ‘rule of thumb’ rate and a lower 60 kg/ha/t rate. It is possible that lateral movement of nitrogen between plots may have confounded that trial.

VICC canola variety trials under irrigation at Kerang have also yielded above their theoretical nitrogen-limited potential when based on the rule of thumb discussed in the introduction. One explanation may be higher growing season mineralisation rates under irrigation in the warm Mallee region than experienced in the dryland Wimmera. The calculation used to estimate in-crop mineralisation is very rough and needs to be researched further to be more useful to growers. (that is, in-crop mineralised N in kg/ha = organic carbon % x 0.15 x GSR).

Alternatively, or in combination with differences in mineralisation, the rooting depth of canola is assumed to be between 60 cm and 100 cm, with mineral N supply taken to the former and water supply to the latter depth. On these vertosols, roots of canola have been measured down to 1.8 m (Norton and Wachsmann, 2006) so that additional N and water could have been accessed during growth.

Topdressing nitrogen fertiliser during early flowering can still provide a yield response. In 2011, the efficiency of late topdressed fertiliser was lower than earlier applications. The late topdressing occurred before dry conditions. Research at wet sites by Central West Farming Systems in the 1990s showed that canola respond to nitrogen topdressed during flowering as long as the spring was wet.

Crop demand during the season

The site had about 40 kg nitrogen at sowing. The soil organic matter content suggests that around 40 kg nitrogen would be mineralised during the season, based on the rough calculation for estimating in-crop mineralisation.

By the four to six leaf stage, the crop would have extracted between 50 and 70 kg N/ha (about 1 t/ha biomass with 6% nitrogen content). At that stage the control would have been nitrogen stressed, and possibly also the low nitrogen rate at sowing.

By stem elongation, the canola would have around 2 t/ha biomass, with a lower nitrogen content of around 5%. This means the crop would have required around 100 kg N/ha. None of the low nitrogen treatments would have been able to meet this nitrogen demand and so would be nitrogen stressed.

The application of early ‘splits’ helped the moderate rate catch up a little, while the nitrogen applied at the higher rate would have kept the crop growing without nitrogen stress.

By first flower, the better growing crop is around 4 t/ha of biomass with around 4% nitrogen, so that it has demanded 160 kg nitrogen by now. If there was no additional nitrogen supplied, this would meet a 2 t/ha yield potential.

It seems that the predrilled treatment is using the N more efficiently – a placement effect largely, but all the high N treatments – even the late top dressing – still look pretty good. The late topdress recovered significantly compared with the control.

Some Rules of Thumb
Please treat these as guides only, the biggest variable is the efficiency of N use which is a culmination of many complex interactions between soil, plant and climate. The values below are largely taken from the PhD work of the co-author, Rob Norton, which was conducted in the Wimmera in the late 1980’s:

· Read your crop –even at early flowering if the lower leaves are yellow and dropping – N supply is likely to be limiting.

· Each 25 mm of rainfall has the potential to add an extra 250 kg canola per hectare. To achieve this extra yield an additional 10-20 kg N would be required.

· Each added tonne of grain yield will reduce seed oil content by around 2%

· Response

o With 50 kg N at seeding, and an extra 50 kg N will increase biomass at flowering by 50%.

o With 50 kg N at seeding, and an extra 100 kg N will increase biomass at flowering by 100%.

o With more than 120 kg N at seeding, then there will be little biomass at flowering response to extra N.

· Canola does manage to regulate its growth and yield so that harvest index (proportion of grain yield to total biomass) does not fall under moderate to high nitrogen supply. Frost and high temperatures can cause harvest index to fall though.

· Although N is important, other nutrients – especially S - should be considered.

Conclusion
· The original rule of thumb for nitrogen rates in canola of a total 80 kg N/ha/tonne of grain has stood up as the best treatment over a range of timings. However, results from this trial suggest a lower rate (60 kg) can provide similar yields as long as it is applied early.
· Canola can respond to late applied nitrogen (early flowering) but less efficiently than earlier applied nitrogen in a season with a dry spring.
Acknowledgments

· Ian and Graham Schmidt, trial co-operators;
· Peter Howie, University of Melbourne/Incitec Pivot, for sowing, spraying between plots and harvesting the trial and assisting with windrowing;
· Ashley Purdue, Victorian DPI, for windrowing;
· Mark Schumann, NuSeed for providing use of aspirator and NIR spectrophotometer.
Reference
Norton RM and Wachsmann N, (2006). Nitrogen use and crop type affect the water use of annual crops in south-eastern Australia. Australian Journal of Agricultural Research, 57, 257-267.