When it comes to fuel efficiency of the family car or farm ute, most growers could give a reasonably accurate number. Yet this is not usually the case for nitrogen use efficiency, says the Back Paddock Company’s Dr Chris Dowling.
Speaking at the GRDC Adviser Update at Temora, NSW, earlier this year, he said that in most farm situations nitrogen use efficiency was not being measured or compared with industry benchmarks or management targets.
Dr Dowling said the nitrogen budget at the start of the season should not be the finishing place for determining nitrogen use because seasonal conditions change, as do crop nitrogen requirements.
“Crop nitrogen needs to be monitored during the season to ensure nitrogen use efficiency is optimised. At the end of the season nitrogen use efficiency can be calculated, benchmarked and then possible sources of inefficiency identified,” he said.
While there are several ways to measure nitrogen use efficiency, the most common calculation is nitrogen transfer efficiency – or the kilograms of nitrogen in grain per kilogram of total crop available N.
This measure is the factor that translates grain N demand to required N supply.
Potential causes of inefficiency
- N is oversupplied. The amount of N applied is greater than the crop or grain’s demand, either because yield potential was overestimated; soil N mineralisation potential was not considered; or residual soil mineral N was underestimated.
- There is inefficient N uptake. This can be a result of volatilisation, denitrification, leaching or immobilisation of nutrients in soil. Applying N from GS31 to GS37 (stem elongation) reduces the time it is exposed to potential losses and increases the chance of uptake by roots because plant growth is rapid.
- N is applied but temporarily unavailable. Up to 18kg N/tonne of cereal stubble can be unavailable to the crop if N is immobilised in soil.
- Applied or mineralised N is lost from the soil. Mineral N is at risk of leaching below the root zone, particularly in sandy soils. N that volatilises can be lost at the rate of 10 to 20 per cent in the four days after application, while denitrification occurs mostly in conditions of high soil moisture. Recent research has shown that fallow N can be reduced by 35 per cent if summer weeds are not controlled.
- N is available in soil but not taken up. N is not taken up in soil because the mineral N is not near the active root mass for most of the crop’s growth; root depth or density is restricted by chemical, physical or biological factors such as phosphorus, compaction or root disease; or because the products applied may not release N when the crop wants it.
- N is taken up to biomass but not transferred to grain. N is taken into grain in two parts – uptake from the soil to vegetative parts of the plant and then from the plant’s vegetation into grain proteins. Some newly released varieties seem to have higher grain protein but whether this relates to the plant’s ability to take up N from soil or incorporate it into grain protein is not clear.
Dr Chris Dowling, Back Paddock Company
0407 692 251
A fact sheet on Plant Available Nitrogen is available at: www.grdc.com.au/GRDC-FS-PlantAvailableNitrogen
End of Ground Cover #110 (Southern region edition):
Read the accompanying Ground Cover Supplement: Ground Cover Supplement issue 110: Cereal foliar fungal diseases
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