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When can you get the best results from late nitrogen?

Applying nitrogen late in the crop’s development (after GS60) can boost the protein level in the grain, but when is it likely to be economic to do so?

  • When leaf symptoms or comparison with an N-rich strip in the paddock indicate the crop really needs more N.
  • When N can be supplied to the crop with moderate efficiency. Efficiency of N uptake declines as the crop matures. In an N response trial in the Wimmera the proportion of applied N recovered in the crop, when applied at growth stage GS31 was 75%, at GS42 was 45%, at GS65 was 33% and at GS72 was -4%.
  • When enough N can be supplied to sufficiently increase protein. To raise the protein content of 5 t/ha wheat crop 1% with a 50% efficiency of N use requires 20 kg N/ha. GRDC supported research from the high rainfall zone in 2013 showed little difference between fluid and granular N sources, even though some N – as a urea solution especially – can be taken up by the crop canopy.
  • When the current protein estimate near enough to a price grade change gets a benefit from a 1% protein increase. Going from 8.5% to 9.5% without achieving a grade change is not likely to be worthwhile.

 

In a season with a long and cool finish applying late N can lead to a yield benefit instead of, or as well as, a protein boost. When N is applied late the N to goes to improving grain quality, not supporting the development of extra grain. In a long and cool finish, carbohydrate synthesis continues for longer than usual, long past the finish of uptake and remobilization of N, giving larger grain size. In this case, the yield increase is from heavier, not extra grain.

A query from southern NSW

Situation: a very wet season in southern NSW with expected losses of Nitrogen related to waterlogging, expected yield potential for wheat crops is to average out around 5t/ha. The crops in question have received between 250 to 300 kg/ha of Urea and are growing on Canola and wheat stubbles. Crops are now approaching head emergence.

Question: Is there a reliable plant tissue test for Nitrogen content with a method to extrapolate results to estimate a yield and protein potential?

Our answer: No, there has been research into estimating grain protein as early as pre-anthesis, but the predictive power is poor. This is basically because protein and starch deposition are not necessarily linked to each other – starch amount (grain size) depends on the conditions during grain fill.

More information

How to review Nitrogen nutrition at 3 timely points in the season

Three reasons why topdressing Nitrogen can fail

Acknowledgements

Rob Norton, ANZ IPNI

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