Take home messages

• While cereals are often deprioritised for irrigation in favour of higher-value crops, targeted irrigation during critical growth stages – especially grain fill – can substantially improve yield.
• Different cereal growth stages respond uniquely to moisture stress. Managing moisture at these stages can prevent yield loss due to pinched or underdeveloped grains.
• Shallow, frequent irrigations can support root development into deeper layers while still maintaining cereal yield, offering a win-win for both current and future crops.

Introduction

For most cropping growers in Tasmania, cereals fill a different role in the rotation compared to many other areas in the country. With a wide variety of options, including several crops with higher potential returns, cereals are often considered a low value crop. Nevertheless, they still play an important role in the cropping rotation, particularly on more marginal soils in the Tasmanian Midlands.

Irrigated cropping growers will tend to focus on the higher value crops during the peak irrigation periods. Cereal yields can be improved using strategic irrigation timings, however, these irrigation timings are not prioritised, leading to yield potentials not being met.

While cereals could be irrigated with a target of maximising yield, the most profitable cropping systems come from generating the most profit across an entire rotation. This doesn’t always mean maximising yield in each individual crop within that rotation, and there are times when one crop will impact the subsequent crops in the rotation. Subsoil moisture conditions and structure are an example of this, and a more strategic approach to cereal irrigation management may be justified on some soil types.

Irrigating for maximum yield

Compared to many of the other crops commonly found in Tasmanian rotations, cereals are one of the least complex crops with regards to irrigation scheduling because the impact of soil moisture stress on plant physiology and the sink-source relationship is well understood. The impact of soil moisture availability on productivity at key growth stages has received much research, to the extent that it can be quantified in models and applied in practice.

Maximising optimal soil moisture throughout the entire season is the best way to maximise cereal yields, however, that is not always possible due to irrigation demands from other crops. This demand doesn’t match the capacity of many Tasmanian cropping farms, particularly in dry seasons when higher value crops are also needing water. Nevertheless, an understanding of the impact of soil moisture stress on different growth stages can highlight the possible value to improved cereal yield by strategically timed irrigations.

Yield in cereals is a function of grain weight and grains per hectare. In turn, grains per hectare is a function of the number and size of the flower spikes (Figure 1). Moisture availability impacts yield in different ways at different times in the growing cycle (Table 1).

Table 1: The impact of insufficient moisture on cereal growth stages.

There is other feedback within the season, for instance, excess vegetative growth allowed by an absence of moisture stress or high fertiliser application will lead to greater evapotranspiration. Without follow-up rain or irrigation, this can cause healthy crops to hit moisture stress before unhealthy crops. This can lead to high biomass crops with pinched grain, giving less yield than lower biomass crops that have less grains per hectare but a greater grain weight.

The main irrigation season occurs during December and January when evapotranspiration is high, and many crops have active canopies. However, skipping irrigation at that time of year can contribute to a reduced grain size. Work by Southern Farming Systems has indicated that, in some seasons, a mid-December irrigation can delay plants reaching a critical stress level during grain fill, with the example of a single 20mm irrigation increasing yield by 1.4 tonne/ha.

Figure 1. Yield in cereals is a function of grain number and grain weight. Grain number is a function of the number of spikes and the number of grains on each spike. All these factors are influenced by soil moisture status at different parts of the growing season (Slafer et al. 2014).

Cereals as team players – irrigating to improve the overall rotation

Many irrigated cropping areas in Tasmania include vertic duplex soils, which provide unique challenges. The B horizons of these soils are often poorly structured, which create risks such as perched waterlogging that can negatively impact crops. This can lead to drainage constraints, which may lead to saturated soil layers in the profile and, if these are within key parts of the root zone, can create waterlogging stress on the plants. Root growth can be impeded either indirectly, by waterlogged zones such as this, or because of structural issues creating physical barriers to development. The impact of either of these constraints increases as A horizons become shallower.

One way to improve duplex soils starts with encouraging the formation of cracks. Cracks form when soils dry out, with the size of those cracks varying depending on soil type and structure. When cracks form, this allows soil from higher horizons to fall deeper into the clays. Due to the higher level of organic matter, these A horizon soils are generally more fertile than the deeper clays. This, along with their ability to open up and maintain a physical pathway through the clay, can make root growth into the clay more effective. A larger root zone can provide plants with a larger pool of water to utilise for growth, meaning water deficit stress is delayed. It also provides more capacity to take in water, which can mean greater ability to deal with excess moisture, for instance, from rain events during the season.

The catch is that the drying required to create cracking relies on plant root activity. Creating the conditions that allow plant roots to dry out can lead to some moisture stress on the plants growing at the time, which can cause a yield penalty. However, the subsequent crop can have one or two boosts to its productivity, depending on the conditions over winter. If the growing season dry down of the B horizon is substantial enough and the winter is relatively dry, then winter recharge may not fully fill the B horizon to saturation prior to spring. These conditions can allow accelerated root growth and much less risk of waterlogging when combined with shallow irrigations to prevent stress from a water deficit.

There is still a benefit when winter rainfall is enough to recharge the B horizon fully. Root growth will be easier into the clays through the soil pathways left where the cracks used to be, so the rootzone can still increase faster as the water content drops.

Cereals are able to send roots to substantial depths, meaning they have the physical characteristics needed to facilitate B horizon cracking. They also have a reduced opportunity cost, as one of the lower value crops within a rotation.

As a tool in the system, it can benefit a rotation if a cereal crop is grown with some moisture stress and potentially a lower yield, but is followed by a higher value crop, such as poppies or potatoes. The gain in productivity of the higher value crop has the potential to more than offset the loss in cereal yield.

Having your cereal cake and eating it too

Careful management of irrigation can minimise the yield loss by creating conditions that allow roots to explore deeper horizons but still provide moisture for plant growth. The key is to apply irrigations that are so small, they only refill the shallow soil layers.

Figure 2 shows a soil moisture profile from a wheat crop in Hagley. The summed soil moisture graph clearly shows the continual dropping of the total moisture in the profile that commenced after flowering. At the same time, the stacked soil moisture graph (showing the moisture level at eight points down the profile, 10cm apart) shows that this coincides with moisture uptake commencing at 50cm and below. Importantly, irrigation is still being applied during grain fill, but this is being held in the top 20cm. This will be enough to minimise the impact of moisture stress, but not enough to stop the plant adding to water use with the moisture extracted from the clays.

Figure 2. Soil moisture data from an irrigated cereal crop in Hagley.

Conclusion

Irrigation at key periods, such as grain fill, can lead to improvements in cereal yield. Strategic late season irrigations can prevent issues with small and pinched grain by optimising grain fill. Cereals may have a larger role to play in the cropping rotation, by having the ability to tolerate moisture stress but still provide a profitable yield. While moisture stress may drop the cereal crop yield, this could be offset by improved yield in the subsequent crop due to soil improvements in duplex soils. If the subsequent crop has a higher potential value than the cereal crop, this yield increase can offset the yield hit in the cereal, improving profit across the crop rotation.

Acknowledgements

This paper was assisted by knowledge contributed by Christian Bannan, soil scientist at South East Soil and Water, and by Brett Davey from Southern Farming Systems.

References

Slafer GA, Savin R, Sadras VO (2014) Coarse and fine regulation of wheat yield components in response to genotype and environment. Field Crops Research 157, 71–83.

Contact details

Mark Matuszek
Ag Logic
0419 000 267