Some significant global premiums are emerging for high protein wheat grades. These international prices are starting to flow through to domestic wheat prices. Prime hard wheat (standard is a minimum 13% protein) was recently quoted at a premium of $85/tonne over APW wheat (10.5% protein, 5% screenings, 12.5% moisture) in northern NSW.
Applying nitrogen (N) late in crop development (from GS39) can boost the protein level in the grain. Best results come from tactical use of late N fertiliser to shift grain protein into a higher price grade when yield is already maximised with adequate N.
How much nitrogen?
Grain protein increases in response to N fertiliser are limited. A 1.5% increase in grain protein might be the highest reasonable target to aim for. Yield increases usually tail off above 11-12% grain protein.
To raise the protein content of a 5 t/ha wheat crop by 1% would require around 9 kg N/ha – if 100% of the N was taken up by the crop and transferred to grain. But, the transfer efficiency of N declines as the crop matures (see table).
A crop is better placed for the opportunity to move up a protein grade if it is heading for somewhere in the 10.5-12% protein range already. Low protein grain means the crop probably ran out of N before it ran out of water. This should be avoidable if N budgets are informed by soil testing, and respond to changes in water limited yield potentials. A high yielding crop that is heading towards <10% protein is unlikely to see enough change in protein to go up a protein class, no matter how late N is applied.
A summary of yield and protein responses to application of 50 kg N/ha in trials in Victorian and South Australian high rainfall cropping zones.
Source: Charlie Walker, Incitec Pivot, 2004.
Late N can also boost grain size
Late N can lead to a yield benefit instead, or as well as, a protein boost. Carbohydrate synthesis is a separate process to protein deposition. In seasons with a long and cool finish carbohydrates can continue accumulating for longer than usual, long past the finish of uptake and remobilization of N. More carbohydrate production gives larger individual grains. The yield increase is from heavier, not extra grain. The protein percentage in the grain can be diluted, or offset by longer carbohydrate accumulation. It may be difficult to achieve a crop with both large grain size and high protein.
Trials comparing the efficiency of different forms of N fertiliser suggest timing is more important than type of N fertiliser.
When applying N to increase protein keep in mind that the usual topdressing rules still apply. Fertiliser response depends on:
- sufficient rain or irrigation to get N into the root zone
- a healthy crop not limited by other stresses like weeds, pests or disease etc.