High yielding cereals and canola – with a good start to the season, what management practices still need to be considered?

Take home messages

• To develop a nitrogen topdressing strategy you need to have a target yield and get the timing right.
• Allow approximately 40kg N/ha for every tonne of wheat grain yield and 75kg N/ha for every tonne of canola grain yield.
• Stripe rust in wheat and sclerotinia in canola are significant diseases and growers will need to consider an appropriate fungicide strategy.

Background

To achieve maximum yield in any year, there needs to be a high level of management throughout the growing season. With the 2020 season underway, growers now need to consider what they still need to do to take advantage of the good start. At this stage of the season, there are still three main management areas left to consider.

Key Question 1 - What are some of the key management decisions and timings from now on for setting up wheat crops for high yields?

High yielding irrigated wheat production requires effective nitrogen management

Production of high yielding wheat crops requires split applications of nitrogenous fertiliser over the growing season at three main stages. The first application is usually at sowing, the second (or first topdressing) at mid–late tillering to stem elongation and the third (or second topdressing) at flag leaf to booting. The application of three smaller amounts of nitrogen allows for better matching of crop nitrogen demands.

The speed of biomass production from sowing until the end of tiller development (during the winter months) is relatively slow and thus nitrogen requirement is minimal. Application of large amounts of nitrogen (in excess of the plant’s requirement) during this period will promote excess vegetative growth resulting in the crop developing a large canopy. As a result, the crop will have increased risk of foliar disease and lodging.

During stem elongation, the wheat plant undergoes a period of rapid accumulation of biomass in order to build the main structure of the plant both above and below the ground. Nitrogen uptake during this period can be as high as 3kg N/ha/day (Angus 2001). During this growth phase, available nitrogen levels must meet the crop’s requirement to ensure that plant growth and leaf area are produced to maximise grain yield. The first nitrogen topdressing is generally applied at the beginning of stem elongation. If nitrogen levels are not sufficient during stem elongation, tillers will start to die off and grain yield will be reduced.

Due to the high demand for nitrogen over the stem elongation period, plant nitrogen reserves may be low when it reaches the booting stage. Irrigated wheat crops may require a second nitrogen topdressing to achieve the target yield. Low nitrogen availability during grain filling will cause low grain protein concentration. The second topdressing should be applied before booting as delaying the nitrogen application until after flowering will not increase yield potential.

Determining a target yield

Having a target yield is useful to calculate an accurate nitrogen budget for irrigated wheat. When targeting a high yield, all management factors must be adequately addressed including varietal selection, sowing time and water availability.

After sowing and throughout the growing season, the crop needs a higher level of monitoring to ensure the target yield is still achievable. Having an adequate plant population is a key consideration in achieving a high yield. If the plant population falls below the general recommendation (150–200 plants/m2; minimum 120 plants/m2) the target yield may need to be lowered.

Low and uncertain irrigation water availability is a major factor limiting irrigated wheat yields. A high yielding wheat crop requires approximately 5.5ML/ha (from rain and irrigation). Achieving a grain yield of 8 t/ha with 5.5 ML/ha of water would give a water use efficiency (WUE), or water productivity, of 14.5 kg/ha/mm. If irrigation water is unavailable or the allocation is reduced during the growing season, growers should consider lowering the target yield and reduce inputs accordingly to ensure the crop is still profitable.

Preparing a nitrogen budget

For the nitrogen budget, allow approximately 40kg N/ha for every tonne of wheat grain yield. Therefore, if the target yield is 8t/ha, the crop needs a total of 320kg N/ha. The nitrogen will come from three sources including nitrogen already in the soil at sowing (pre-sowing soil test), nitrogen mineralised during the growing season and the amount of nitrogen applied as fertiliser during the growing season.

An irrigated wheat crop requires approximately 100–120 kg N/ha at sowing which includes starting soil nitrogen and nitrogen fertiliser applied at sowing. Therefore, if the soil test indicates 90kg N/ha prior to sowing and 145kg/ha of MAP (15kg N/ha) is applied at sowing, the crop will have a total of 105kg N/ha (and 32kg P/ha). If the pre-sowing soil test indicates a lower level of nitrogen in the soil of 75kg N/ha, applying 160kg/ha of DAP will supply 29kg N/ha giving a total of 104kg N/ha (and 32kg P/ha). Where the starting nitrogen is even lower, the addition of another nitrogen fertiliser will be required at sowing to achieve the base requirements of 100–120 kg N/ha.

The first decision regarding topdressing of irrigated wheat should be made when the crop is approaching stem elongation (GS31, first node visible). If the number of tillers is 500– 800 tillers/m2 the crop is on target to yield 8t/ha and a minimum of 30–50 kg N/ha should be applied. If establishment was poor and the tiller count is below 500 tillers/m2 a yield of 8t/ha is unlikely, and therefore, a new lower yield target should be considered. If the tiller count is above 800 tillers/m2 (sown with high levels of soil N available) there is no need to topdress. The first topdressing is generally applied between the first and second node stage. In situations where the starting N levels are low, topdressing can commence a few weeks earlier during mid tillering.

The second decision regarding topdressing is made between flag leaf emergence and booting (GS41–GS47). Topdressing at this stage with 60–90 kg N/ha is usually required to achieve 8t/ha with a grain protein concentration above 11.5%. Delaying topdressing after this time will reduce the effectiveness of the application. It is especially important to time the application of this topdressing so that it occurs before irrigation or rainfall to minimise nitrogen losses through volatilisation.

Table 1. An example nitrogen budget for a high yielding (8t/ha) irrigated wheat crop.

Disease management of wheat

Wheat foliar diseases are most likely to cause yield loss when there is increased biomass, humidity and prolonged green leaf retention. Stubble-borne and soil-borne wheat diseases are likely to occur in retained stubble systems. During the second half of the season growers need to monitor for all leaf diseases, especially stripe rust. Wheat varieties also vary in their levels of stripe rust resistance. To limit its infection and spread, it is recommended to sow varieties with a resistance rating of moderately resistant (MR) – moderately susceptible (MS) or better. There are seed and fertiliser treatments available to suppress stripe rust on wheat plants at earlier growth stages until adult plant resistance (APR) activates around heading. Foliar fungicides should be applied to protect the youngest three leaves. Generally, this would occur at GS39 (flag leaf emergence). Under high disease pressure, an earlier application at GS31 (first node) and then GS39 might be warranted.

Key Question 2 - What are some of the key management decisions and timings from now on for setting up canola crops for high yields?

A higher level of nutrient management is also required for high yielding irrigated canola

High yielding irrigated canola crops with high oil content (above 42%) require adequate nitrogen levels for the target yield. To achieve a 4t/ha canola crop, approximately 14t/ha of total biomass is required with a harvest index of 0.28.

Canola dry matter production and thus plant nitrogen requirement is relatively low during the early emergence and rosette stages, prior to the commencement of stem elongation and branch initiation. Once ‘bud visible’ commences, the crop will go through a rapid growth period until the end of flowering. Nitrogen topdressing needs to provide enough nitrogen for maximum plant growth during this period. If all the nitrogen fertiliser is applied at sowing, achieving the target yield is unlikely. When targeting a maximum grain yield, it is recommended that some nitrogen needs to be delayed and applied as a top-dressing before the crop starts flowering.

Determining a target yield

The target yield is required to calculate an accurate nitrogen budget for irrigated canola. When targeting a high yield, all management factors must be taken into account, including varietal selection, sowing time and irrigation scheduling.

After sowing, the crop needs to be regularly monitored to ensure the target yield is still achievable. Establishing the desired plant population is a key factor in achieving a high yield. The general recommended plant population for maximum irrigated canola yields is about 40 plants/m2. Research has shown that crops with an evenly distributed plant population of 20 plants/m2 can still achieve very high yields. When the plant population falls below this, consider lowering the yield target.

Sowing time also needs to be considered when determining a target yield. In the irrigated regions of southern NSW and northern Victoria, sowing generally occurs during April to minimise the risk of early winter waterlogging, and high temperatures causing heat damage during flowering/early podding.

Low and uncertain irrigation water availability is a common factor limiting irrigated canola grain yields. A high yielding canola crop will need about 5.0ML/ha (from rain or irrigation). Achieving a grain yield of 4t/ha with 5ML/ha of water would give a water use efficiency (WUE), or water productivity, of 8kg/ha/mm. Growers will also need to consider lowering yield expectations if irrigation water supply becomes limited.

Preparing a nitrogen budget

Every tonne of canola grain produced requires about 75kg N/ha (depending on nitrogen uptake efficiency). Therefore, a 4t/ha canola crop requires a total of 300kg N/ha. The nitrogen will come from three sources – nitrogen already in the soil at sowing (pre-sowing soil test), nitrogen mineralised during the growing season and nitrogen applied as fertiliser.

A pre-sowing soil test (0–100 cm) should be conducted to determine the amount of nitrogen in the soil at sowing. Soil tests need to be conducted early enough to have the results back before sowing. Soil nitrogen levels can vary considerably depending on farming practices and cropping history while the mineralisation rate is influenced by the level of organic carbon and available moisture in the soil.

Nitrogen uptake is relatively low in the early growth stages of an irrigated canola crop. The general recommendation for a high yielding irrigated canola crop is to apply a portion of the nitrogen fertiliser at or before sowing and the rest by topdressing after the crop has established. Applying all of the nitrogen fertiliser at sowing can increase the risk of excess foliage, crop lodging before maturity, and can result in more leaf and stem disease.

If the crop is healthy and has a uniform plant population of greater than 20 plants/m2, maximum yields can still be achieved and high nitrogen topdressing rates should be applied. If establishment is poor and the plant population is below 15 plants/m2, a yield of 4t/ha is unlikely and nitrogen topdressing rates will need to be lowered accordingly.

The water budget developed at the beginning of the season should be reviewed before topdressing as this will help with decisions on yield targets and crop inputs. If irrigation allocations are low or uncertain, nitrogen topdressing rates will need to be reassessed. When nitrogen is topdressed, it should be applied just before an irrigation or rainfall to improve nitrogen uptake efficiency and to minimise losses through volatilisation. A rainfall of 10mm is enough to wash topdressed fertiliser into the soil.

An irrigated canola crop with a yield target of 4t/ha requires a total of approximately 310kg N/ha throughout the growing season. If 50kg N/ha is present in the soil before sowing and a further 50kg N/ha will become available from mineralisation during the season, 210kg N/ha must be applied as fertiliser to reach the target yield.

A base application of DAP provides early nitrogen for the canola seedlings and all the phosphorus requirements for the 4t/ha target. If possible, the DAP should not be sown with the seed but banded close by to avoid negative effects on establishment. The timing of the nitrogen fertiliser (urea) will vary depending on location, paddock history, climate zone and soil type. The first application of nitrogen (urea) is often applied at sowing in southern NSW. If high rates of nitrogen are applied at sowing, it is essential that the N is separated from the seed. The second nitrogen application is normally applied around the ‘bud visible’ (green bud) stage and before the crop reaches flowering.

Table 2. An example nitrogen budget for high yielding (4 t/ha) irrigated canola crop.

Disease management of canola

During the second half of the season, canola growers need to be on top of their sclerotinia management strategy. Sclerotinia is a significant disease of canola with no commercial canola cultivars in Australia with resistance to sclerotinia stem rot. Management of the disease relies on the use of cultural and chemical methods of control. Foliar fungicides should be considered in those districts which are at a high risk of disease development (e.g. where the disease frequently occurs, has a long flowering period, is fully irrigated, receives reliable spring rainfall). There are several foliar fungicides currently registered for use in Australia to manage sclerotinia stem rot.

Plants become susceptible to infection once flowering commences. Research in Australia and Canada has shown that an application of foliar fungicide around the 20–30% bloom stage (20% bloom is 14–16 flowers on the main stem, 30% bloom is approximately 20 flowers on the main stem) can be effective in reducing the level of sclerotinia stem infection. Most registered products can be applied up to the 50% bloom (full bloom) stage.

The objective of the fungicide application is to prevent early infection of petals while ensuring that fungicide also penetrates into the lower crop canopy to protect potential infection sites (such as lower leaves, leaf axils and stems). Timing of fungicide application is critical.

Key Question 3 – How to determine the timing and number of spring irrigations?

Irrigated wheat

Spring irrigation of a wheat crop is one of the major factors influencing grain yield. The decision of when and how often to irrigate is complex and depends on several factors including available soil moisture, rainfall, time of irrigation water availability in relation to plant development, potential yield benefit, risk of waterlogging or lodging, and returns from using the water on another crop or simply selling it.

Regardless of how many irrigations you intend to apply to your wheat crop during the growing season the most important factor to consider is that adequate moisture is available during the heading stage. Head emergence is the most sensitive growth stage to moisture stress in a wheat crop.

If you are planning to limit the number of spring irrigations applied to a wheat crop it is important to find a balance between irrigating before significant moisture stress to the wheat plants occurs while also ensuring adequate moisture is available during head emergence.

If only one irrigation is going to be applied to the crop, the best timing for this is around mid-stem elongation. The crop will still have time to increase biomass with some moisture remaining for the critical flowering and early grain fill stages. Any late season rainfall will also then be more beneficial.

If two irrigations are possible, the first irrigation should be applied at early stem elongation with the second applied between flag leaf emergence and flowering.

Timing of the first irrigation is very important to maximise water productivity and speed at which the water can be applied. If the first irrigation is not applied until the soil profile is very dry it will take much longer and use considerably more water for the irrigation, both of which are not desirable. One irrigation applied once the soil has dried to depth can use almost as much water as two irrigations applied at the correct times.

An experiment conducted by NSW DPI at Leeton NSW in 2015 found that two spring irrigations produced the highest wheat grain yield (7.61t/ha), but one irrigation provided the highest water productivity (1.7t/ML). Another important point from that experiment was that ponding the irrigation water for 48 hours did not reduce grain yield (due to the excellent soil structure) but it did increase water use and subsequently reduced water productivity by 25%.

Irrigated canola

Spring irrigation of a canola crop is one of the major factors influencing grain yield. The decision of when and how often to irrigate canola is complex and depends on several factors including available soil moisture, rainfall, time of irrigation water availability in relation to plant development, potential yield benefit, risk of waterlogging or lodging, and returns from using the water on another crop or selling it.

A fully irrigated canola crop will require between two and four spring irrigations to achieve maximum grain yield depending on the irrigation system being used, location, rainfall and soil water holding capacity. In a dry winter when the soil profile is dry to depth it is very important to irrigate as soon as possible in the new irrigation season (usually early–mid August) or significant grain yield reduction will occur.

When irrigation water is limited, one irrigation applied at early flowering will give the best grain yield response and economic return in a dry season. Canola is very sensitive to moisture stress at flowering, particularly early flowering, and moisture stress during this period can dramatically reduce grain yields. Supplying adequate moisture to the crop at this time will maximise the number of pods that set seed and later irrigations will maximise seed size.

Canola is less tolerant of waterlogging in the period from flowering to maturity than most cereal crops. It is therefore very important that the opportunity time with each irrigation or large rainfall event is within the guidelines for each soil type, or reduced grain yield will occur.

Irrigation scheduling

Scheduling irrigations is important to ensure that the crop is irrigated before moisture stress occurs.

There are several methods available to assist with irrigation scheduling. The methods are divided into either plant based or soil-based tools. Daily evapotranspiration (ETo) figures are a common plant-based tool while soil capacitance probes and gypsum blocks are soil-based options.

The use of ETo requires growers to access weather data from the internet and keep records for each of their fields. A crop coefficient that depends on the growth stage of the crop is also required. Once familiar with this method and the plant available water for each soil type it is easy to use to predict when future irrigations will be required.

There are several providers of services to install and monitor soil capacitance and gypsum blocks as well as an increasing number of data logging methods allowing the soil moisture data to be accessed in the field or in real-time from the internet. It is important that this equipment is installed in locations that are representative of the majority of the field or the results may be misleading.

Regardless of method used, it is important to monitor crop water use in order to avoid crop moisture stress. Do not allow soil water to deplete below 60% of plant available water capacity (PAWC) referred to as readily available water (RAW). The point of timely irrigation is commonly known as the ‘refill point’. Plant growth and yield potential will decline considerably if soils are allowed to dry down beyond the point of RAW, which is particularly important in crops fully irrigated for maximum yield potential. Readily available water will vary across soil types. Successfully growing an irrigated wheat and canola crop and achieving high levels of water productivity is dependent on matching the layout with the soil type and creating bay sizes that allow ponded water to be on the field for as short as practical, preferably less than 10 hours.

Reducing the number of irrigations can often increase water productivity in irrigated canola but it is important that timing of the irrigations is planned to maximise grain yield.

Conclusion

There has been a good start to the season so far, but it is very unlikely that there will be sufficient irrigation water during spring to fully irrigate crops. Nitrogen application rates and timing must be made in conjunction with how much water the crop is likely to get. Lower water allocations will mean lowering the target yield and lowering nitrogen topdressing rates. When high water allocations are available, growers can increase their nitrogen topdressing rates to achieve higher target yields. Growers can only target maximum yields in years of full irrigation allocations.

Crop monitoring is recommended every year for disease management. A targeted fungicide program will help avoid yield loss.

Acknowledgements

I would like to acknowledge the authors of “Irrigated wheat in southern cropping systems” and “Irrigated canola in southern cropping systems”. These two publications were produced as a part of the “Southern irrigated cereal and canola varieties achieving target yield’ project which was made possible by the significant contributions from both NSW DPI and GRDC. Many of the recommendations in this paper were taken from these two manuals.

Useful resources

GRDC Publication: Irrigated canola in southern cropping systems

GRDC Publication: Irrigated wheat in southern cropping systems

GRDC Factsheet: Irrigated wheat in the murrumbidgee and murray fact sheet

NSW DPI Publication: Eight tonnes per hectare of irrigated wheat

Contact details

Tony Napier
Yanco Agricultural Institute
02 6951 2796
tony.napier@dpi.nsw.gov.au