16 Feb 2012

Wheat nutrition following canola

Wheat nutrition following canola

Wheat grown after broadleafed crops such as canola generally yield more than wheat grown after other cereals. This has been termed a “break-crop” effect, the most recent analysis of 180 experiments by Dr John Angus from the CSIRO suggests this benefit is around 0.8 t/ha (Angus et al. 2011).

The break crop effect is most likely a result of reduced root disease inoculum levels. There are also other mechanisms acting but overall, the effect is to make the response to fertilizer nitrogen more reliable in the wheat crop.

Wheat crops with healthy root systems will extract more water and demand more mineral N from the soil. Even though there may be a small increase in mineralisation, due care to balanced nutrition is important.

In experiments on 32 wheat crops across the southern New South Wale, Dr Angus applied 40 kg N/ha at the start of stem elongation. The response to that application varied from +900 kg grain ha-1 to -640 kg grain ha-1. The nine crops that did not follow break crops had only small responses.

Shoot density prior to topdressing was a good predictor of response, but only after an effective break crop.

The amount of nutrients removed in a 3 t/ha canola crop are considerable – from the 2010 Dahlen long-term experiment, a 3 t/ha canola crop had a measured export of 110 kg N, 21 kg P and 11 kg S in the grain. The applied fertilizers for this treatment of 40N and 18P nearly balanced the P removal but N was in large deficit.

So this seasons wheat crops could be in a situation where the demand for nutrients potentially increases, while the supply could have been depleted from the heavy canola crop. Soil testing is recommended to check the supply side.

Angus J, J Kirkegaard, M Peoples, M Ryan, L Ohlander, L Hufton (2011) A review of break-crop benefits of brassicas. In: D Luckett, D McCaffery, H Raman, R Raman (eds), 17th Australian Research Assembly on Brassicas, Wagga Wagga, 2011. NSW Department of Primary Industries, pp 123-127.

Angus J, AF van Herwaarden, RA Fischer (1989) Predicting the yield response of wheat to topdressed nitrogen. 5th Australian Agronomy Conference, ASA. (http://www.regional.org.au/au/asa/1989/contributed/plant-nutrition/p-18.htm)

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