The chart below shows that there are many production variables that affect yield and protein in a wheat crop. All these variables need to be adequately addressed to ensure that the desired outcome is achieved and one component, such as nitrogen cannot be used in isolation to make up for a deficiency in the others. The correct use of N can significantly improve the crop outcome when the other variables are satisfactory.
Matching nitrogen to crop growth stages
Most dry matter in cereal crops is produced during the reproductive phase starting when the first node can be detected. Only 10 – 20 percent of total dry matter growth occurs prior to first node formation.
While early crop growth is high in nitrogen, the small bulk of vegetation means only a small amount of total N is required to support the crop. In many cases, residual N carry-over from the previous crop and autumn mineralisation will provide adequate N to produce a crop canopy with good yield potential.
The graph below shows peak demand for N (where the curve is steepest) occurs between fully tillered and boot stage.
This is the period when cereals usually have maximum active root surface area to intercept mobile, plant available nitrate N, so applying N in this window is likely to give the highest efficiency of uptake.
Success with N application is influenced by the following factors:
- An identified need for additional N;
- Freedom from root disease, pests and weeds;
- Adequate plant available water;
- Follow up rainfall (spring);
- N applied at the right time for targeted production outcome;
- Adequate supply of other nutrients, particularly Phosphorus.
Extra N needed?
When deciding whether or not a crop requires extra N the following issues need to be included in the decision:
- Paddock history including yield limiting factors such as soil acidity, sodicity, etc;
- Variety responsiveness;
- Surface and deep soil test results;
- Experience or “gut feel”.
Disease, pests and weeds
Nitrogen will not kill weeds, rarely mitigate the effect of disease and may increase the attractiveness of a crop to insect pests.
Adequate plant available water (PAW)
During cooler months of the year, cereal crops use water at 1 – 2 mm/day. So a crop with only 10 – 20 mm of PAW will run out of water within 5 – 15 days unless it rains, but a crop with more than 50 mm of PAW will last 25 days in the cooler months. PAW is important because plant uptake of nitrate N is by mass flow to roots in soil water. If the roots cannot access water they can’t access the N. PAW in the soil profile can be estimated in the paddock with a push probe or calibrated soil moisture probes.
Rain after topdressing moves nitrogen into the root zone and ensures volatilisation losses to the atmosphere are minimised. The amount of rainfall required to move N into the root zone is dictated by soil texture (lighter soils need less rain) and the amount of PAW already present in the soil surface layers. Where good moisture is present, less rain is needed to get the N into the root zone. In warm spring conditions on soil with high clay content about 10 – 15 mm of rain is needed in the week following top dressing to minimise N losses from Urea.
Topdressing Urea onto a highly alkaline soil surface with high wind speeds, an open crop canopy and warm spring conditions can result in losses of two percent of product per day, meaning that two weeks without rain would result in about 30 percent of product loss.
However, losses are minimal when topdressing on acidic soils into a good crop canopy with cooler late winter or mild spring conditions, even when there is some time between spreading and rainfall.
Timing of N
Ideal N timing is largely driven by the desire to grow a crop that runs out of N at about the same time as it runs out of water.
Early excessive supply of nitrogen can encourage a crop to use extra water that then is unavailable later in the season when it is needed for grain filling.
Improving protein with late-season topdressing
Can nitrogen be top dressed late to increase grain protein?
Yes, but it’s not easy!
The crop has to be set up for “critical protein” by the boot stage. Critical protein is the point at which the addition of nitrogen will no longer increase yield but will increase protein. Critical protein for wheat is about 11.5 percent. The table below shows the potential yield loss that can occur when protein levels are below 11.5 percent, however the economics of the yield loss needs to be compared against current grain prices and the cost of nitrogen application.
Potential yield loss indicated by protein levels below 11.5 %
In wetter, high yielding years, topdressing at flowering can result in yield increases rather than the desired higher protein. Crops where this occurs have been destined for low protein (8.5 – 10.5 percent) before topdressing. Supplying such crops with late N increases yield by a smaller amount than if N was applied earlier and produces negligible protein increase.
Rarely will late topdressing work if there is not a significant supply of PAW under the crop. Available water is critical at flowering because crop water use increases to 4 – 7 mm/day. Topdressing in front of a rainfall event is of increased importance around flowering, not only to rapidly move N into the root zone but because volatilisation losses of N from Urea increase with temperature.
Topdressing late generally results in lower N-use efficiencies. Topdressing from 5-leaf to boot stage typically gives recovery of fertiliser N in grain of 40-45 percent but at flowering, applications more commonly give recoveries of 30-35 percent.
Given these figures, it is vital to ensure there is adequate protein spreads in the market and be confident that early season nitrogen management is on target to produce the desired result.
The main reasons for the lower N recovery from late season applications are the dying root system of the maturing crop and reduced nitrogen uptake under moisture stressed conditions. To grow a high-protein crop it needs to run out of water before it runs out of N.
Nitrogen strategy for wheat
• Set the crop up for around 70 percent of targeted yield at a reasonable protein, say 11.5 percent by sowing / pre-tillering. This provides a hedge against the risk of a poor season and leaves the option of “do nothing” should this eventuate. This approach by necessity will require regular soil testing of paddocks to understand what nutrients exist before cropping.
• Providing the crop is progressing well based on regular visual assessment, do nothing until the crop is fully tillered.
• Where N status is doubtful or there is good rainfall during autumn, take in-crop deep N samples before the crop is fully tillered. This will help account for mineralisation in the first half of the growing season and assist with answering the question of “how much N is needed?”
• As an extra check, do tiller counts when the crop is fully tillered or first node present.
• Where an N deficiency is indicated by deep N levels and sub-optimal tiller counts, topdress with an appropriate rate of N as soon as conditions are right (good soil moisture, rain coming).
• Where an N deficiency is identified but tiller counts are optimal or above optimal, adopt a “holding pattern” until additional tillers drop off. In reliable high rainfall areas and where yield increase is the primary objective, this holding pattern may continue to the boot stage.
• Where protein increase is the objective, ensure that the crop is on target for 11.5 percent protein as a base by managing N up until the boot stage. The window for protein increase is from boot stage to mid-flowering. In more marginal areas target the earlier application timing whereas in higher rainfall environments with good subsoil moisture and a strong likelihood of follow up rain, flowering application can be attempted. Remember though that N application for protein increase is a risky exercise with no guarantee of the desired outcome being achieved.
Content sourced from GRDC and Back Paddock Company publications.