Have we got the foundation of N nutrition right

Chris Dowling, Back Paddock Company

What is low protein in cereals telling us?

In essence, low protein content in cereal grain is symptomatic of the inability of the soil and/or vegetative mass of the crop to maintain an adequate nitrogen (N) content to maintain a protein content relative to grain yield. In reviewing the 2012 season it is pertinent to review what constitutes low grain protein. 

Grain protein can be an important measure of crop performance from both economic and agronomic perspectives. From an economic perspective the dollar value of some cereal crops is directly or indirectly related to it grain protein content, while from an agronomic perspective it is a reasonable barometer of seasonal adequacy of nitrogen supply in relation to crop water availability. In recent years the benefit of higher protein in wheat has been negligible from an economic perspective due to the low premiums hence selection of wheat varieties with yield advantages at the expense of protein have frequently provided the best economic return and now dominate crop areas.

For wheat grown in northern NSW and Qld achievement of grain protein of 11.5 – 12.5 %  in varieties with hard classification is thought to indicate optimisation of N availability (quantity and location) for the seasonal moisture availability (quantity and timing). This protein range also includes the 90 % Relative Yield (RY) point that is commonly associated with economic optimum use of nitrogen.  Desired protein ranges for other commonly grown cereal crops include sorghum – 8.5 to 9.5 % and barley – 9.5 to 10.5 %.

Agronomically, grain protein also provides a historical perspective of the risk associated with having added more N (Table 1), and the likely residual soil N at the commencement of the next fallow (Figure 1).

Table 1. Probability of grain yield increase from added nitrogen based on historical grain protein % (Strong 2000)

Figure 1. Post harvest soil residual nitrate-N is related to grain protein in wheat

How does grain protein relate to soil and fertiliser nitrogen?

A multitude of factors affect the pathway of N from the soil to the grain of cereal crops and therefore influence grain protein.  The rate of nitrogen fertiliser applied is an attempt to bridge the gap between the nitrogen a soil can supply (mineralisation plus residual mineral-N minus denitrification), the crop demand for the season, and the efficiency of the crop accessing and transferring soil and fertiliser N to grain (Figure 2).

Figure 2. Example of a gross seasonal nitrogen budget for winter cereal, pools and efficiencies

When the system fails and yield and/or grain protein are below optimum the causes can generally be put into a couple of major categories

1. Insufficient nitrogen available across the season from the soil (inadequate quantity)

• The combination of mineralisable N and fertiliser N does not match the yield potential as determined by climatic conditions. The decline in soil organic matter  (OC % in soil tests) has also seen a parallel and related decline in soil N supply capacity If the decline in organic N supply is not matched by a parallel increase in N from other sources (fertiliser, recycle organics, legume) with equivalent uptake efficiency then yield and /or grain protein will decline (Figure 3). For large areas of cropping with soil organic carbon around 1%  and standard fallow lengths  to produce excess of 2 t/ha yield will require at least 50 % of the crops available N to be supplied by fertiliser or legume N for grain protein to reach Hard and APH quality.
• Nitrogen loss prior to crop access – denitrification depletes the mineral N pool that is derived from both mineralisation of organic N and fertiliser N residues. Where in the past it was thought that most denitrification was limited to the surface soil, recent anecdotal evidence suggests that under some circumstances denitrification may be occurring down to 60 cm where decaying root systems are present at the time of waterlogging.

2. Sufficient nitrogen available but not taken up (low efficiency)

• Nitrogen available to the crop not accessed -  located in dry soil or hostile (salinity, sodicity, nutrient toxicity, density) soil
• Low supply of another key nutrient such as phosphorus that can affect N acquisition.

3. Nitrogen not transferred to the grain (internal nitrogen use efficiency)

• Variety – based on the NVT trials data there appears to be reasonably significant and consistent difference in cereal varieties in their apparent nitrogen partitioning (Figure 5) affecting the yield – protein balance (Figure 4). Dominance of a particular variety can bias the average protein outcomes of a farm, district and region.
• Low supply of another key nutrient such as potassium, sulphur and zinc that can affect internal use efficiency of nitrogen.

Figure 4. Relative crop nitrogen partition characteristics of selected wheat varieties, Bellata and SW Qld and NVT Trials 2009 to 20112.

What happened in 2012?

In any unexpected event, such as industry-wide low grain protein in dryland crops in 2012, the causes can generally be understood with the clarity of hindsight but prediction is frequently difficult. Prediction is clouded by complexity and uncertainty, particularly where weather events are a large component of the unknown. In the 2012 winter crop the combination of near term extremes such as the wet summer fallows and denitrification, small cumulative effects such as the depletion of subsoil mineral nitrogen reserves (built up during the mid-2000s drier seasons) by the crops in 2009 and 2010 as well as organic matter decline, and the seasonal growing conditions combined to create conditions where low protein was inevitable and probably reasonably predictable.

Low protein in cereal grains is indicative of poor nitrogen supply to the grain during the grain fill period. This can, and was, influenced by factors such as location of concentrations of mineral nitrogen in the soil, soil moisture, seasonal rainfall and temperature patterns and crop species and variety genetics.

Identified significant contributing factors in the 2012 crop included:

• Long term loss of soil nitrogen supply elasticity as a result of long-term organic matter decline.
• General lack of legumes in rotations to supplement background organic matter nitrogen.
• A record wet summer prior to the 2012 winter crop leading to significant denitrification through the soil profile.  Soil test data from autumn 2012 suggests that many paddocks had no benefit from normal 20 – 50 kg/ha summer fallow N mineralisation due to losses from waterlogging or the >20 kg/ha residual mineral N present in the post-harvest .
• High yields in crop preceding 2012 winter crop creating low mineral N availability in the subsoil.
• Fertiliser N management strategies – quantity and timing unsuited to the seasonal soil moisture use pattern. Due to the wet summer and autumn, N was applied in late autumn and into full soil profiles. As a consequence there was little possibility for movement of fertiliser N away from the application zone.
• Dry spring – depending on the sowing dates of the crop, from mid-tillering or first main-stem node onward, many crops were dependent on reserves of soil moisture and mineral N below 30 cm because the last effective rainfall was in July.  This meant that greater than 60 % of the above ground dry matter was grown with adequate moisture from subsoil reserves but limited nitrogen. During grain fill, the low N reserves in the biomass appears to have been sufficient to sustain yield but was not sufficient to maintain protein. After long fallows where there was generally more N in the subsoil, despite denitrification, grain proteins were frequently significantly higher.
• Widespread adoption of varieties that are yield dominant at the expense of grain protein content.
• Apparent low soil N transfer efficiency (% of soil and fertiliser N in the grain) in N Budgets resulting from underestimate of denitrification loss and possible stranding of fertiliser nitrogen during the period of majority of dry matter accumulation.
• Plant tissue analysis for N prior to first node does not appear to provide any information about final protein.

Not all of the above conditions were necessarily active in each case but the widespread seasonal low protein outcomes suggest a number of these conditions were present for many growers in 2012.

How do we avoid or adapt to the soil N supply and low protein challenge?

• Ignore it! The protein price premium signals are not sufficient to forgo yield or chase protein.
• Consider varietal performance in relation to N use. As for pest and diseases select a variety with it reaction to N supply and end market value in mind.
• Know your situation – don’t ignore the signs from previous crop outcomes.  Frequent occurrence of low protein indicates N supply is not well matched to water supply.
• Recognise that high protein now needs to be a planned outcome and may be less frequently a consequence of the season.
• Measure to manage. Need to consider a range of tools to establish N management strategies, tactics and monitor seasonal progress. Soil testing 3 months before the crop is sown may be a good starting point for seasonal N but it should not be the only consideration.
• Rethink “set and forget” – there is no right rate of N to achieve a particular yield and protein outcome that can be defined at sowing, but there is a range of acceptable and unacceptable yield and grain protein outcomes from N that are available (Figure 5). The outcomes can be biased in positive direction using a combination of paddock and variety selection and judicious use of the 4 Rs (Right Product, Right Rate, Right Timing and Right Placement) for N fertiliser management.
• Consider new fertiliser strategies – higher efficiency products and strategies, application timing for protein only?

Figure 5.  A range of grain yield and protein outcomes based on a range of  soil and fertiliser N availability and  standard nitrogen transfer efficiencies associated with grain protein%.  Total crop N supply 60 (No N added), 90 (Low), 120 (Expected) and 150 (High) kg/ha N.

References

Simpson RJ, Lambers H, and Dalling MJ. 1983. Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.). iv. Development of a quantitative model of the translocation of nitrogen to the grain. Plant Physiol., 71, 7-14

Strong WM 2000 pers. comm.

Contact details

Chris Dowling
Back Paddock Company
Mb: 0407692251
Ph: 07 38213544
Email: cdowling@backpaddock.com.au