High nitrogen (N) fertiliser rates can have long-lasting yield effects in canola-wheat rotations. Since 2010, Grain Orana Alliance has looked at yield and oil responses in canola to increased nitrogen (N) rates. Yield increases often continued well beyond N rates that had been thought reasonable.
There’s a risk of applying excess N, and a risk of under supplying a canola crop’s demand for N. Recent Central West NSW trials looked at pairing high N applications in canola with response to residual N in a following wheat crop. The wheat crop ideally uses any leftover soil N from the high N canola crop. Sites at Tullamore and Geurie were planted with canola in 2014 and wheat in 2015.
How high nitrogen?
Up to 200 kg N per hectare was applied to canola crops in 2014. This is well above typical canola rates of 50 – 120 kg N/ha. The 2015 wheat crop did not receive any N fertiliser.
How effective was the high nitrogen rate?
Yields in both canola and wheat increased with higher N rates. This means that:
- optimal N rates for canola may be higher than current practice, and
- the wheat crop used residual N from the canola crop.
Compared to 0 kg N/ha, the 200 kg N/ha plots had:
- more residual N: 64 kg N/ha at Geurie, and 25 kgN/ha at Tullamore
- more canola: extra 1.34 t/ha at Geurie, and 0.84 t/Ha at Tullamore.
- less oil: canola oil percentage dropped by 3.8 – 3.9%.
- more wheat: extra 0.88t/ha at Geurie and 0.75 t/Ha at Tullamore.
- more protein: wheat grain protein increased 1.2% at Geurie, and 2.5% at Tullamore – but not enough for a jump up in wheat grade.
Can you apply too much nitrogen?
Too much N can reduce yield in grain crops. ‘Haying off’ is when high N rates drive crops to develop thick, vegetative growth. A ‘bulky’ crop uses too much soil water leaving little to fill the grain. Canola does not appear prone to haying off: 30+ trials in different seasons and locations have not shown canola yields declining from too much N.
Was high nitrogen upfront profitable?
Over the two years, and with the increased fertiliser costs deducted, profits still increased by $546/ha at Geurie and $280/ha at Tullamore with 200 kg/ha of N applied for the canola of 2014.
At Geurie, canola yield and profit increased up to the highest rate of N. This suggests, at this site, that at rates of up to 200 kg/ha N, N was more limiting than soil moisture. Wheat yields also increased from higher N applied for canola the previous year.
At Tullamore, 50 kg of N per hectare maximised profit in canola. With 100+ mm less rainfall than Geurie, water-limited yield more than N. Wheat yields increased up to the highest N rate indicating the wheat crop was using residual fertiliser N from the canola crop.
What does this all mean?
Higher rates of N fertiliser may be needed to maximise canola yield and profit. However, applying high amounts of N early means you lose the opportunity to adapt N rates to the season. As canola appears to resist ‘haying off’, growers can be confident that over-applying N will not decrease yields if the season turns bad. There is also strong evidence that some excess N can be used by a following wheat crop.
Fertiliser programs ideally refer to the 4R best practice. Matching fertiliser applications to crop demand reduces risks of N losses through leaching or denitrification in wet periods. Both of these N losses are usually driven by large rainfall events, which are difficult to predict at the start of the season. Weeds can also consume some N during the fallow, but at least this risk can be controlled.
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