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Maximising yield – by the numbers

Crop ecologist Professor Victor Sadras spoke at a recent GRDC Hyper Yielding Crops field day about the critical period.
Photo: Melissa Marino

In Australia’s complex grain production systems marked by varying environmental and economic conditions, some key concepts are worth revisiting, according to crop ecophysiologist Professor Victor Sadras.

These crop physiology principles are so strong that they apply to all grain production systems regardless of soil type, climate or management, Professor Sadras says.

Chief among them is that in the range from crop failure to yield potential, grain number is typically responsible for 80 per cent of the variation in yield.

“Grain number is king,” he told growers at the Field Applied Research (FAR) Hyper Yielding Crops Tasmanian field day in November.

“And don’t get me wrong – you don’t want screenings; you want good-quality grain – but the point is not to become distracted chasing grain weight when the main game is getting the numbers.”

Professor Sadras says that yield is a function of grain number, and that grain number is defined in a species-specific critical window.

In wheat, this critical period has been determined through extensive trials dating back to the 1980s led by former International Maize and Wheat Improvement Center (CIMMYT) and CSIRO crop physiologist Dr Tony Fischer.

Using controlled shading as a stressor, Dr Fischer showed the critical period for wheat grain yield spans from late stem elongation to 10 days after flowering.

Further trials following a similar method found the critical period for barley and oats was similar to that of wheat.

For pulses and canola, the critical period shifted to later in the growth cycle, with the most-sensitive stage at podding.

“In canola, grain yield does not correlate much with biomass at flowering, but correlates with biomass at podding,” Professor Sadras says.

Maximising yield

He says yield depends on three factors in the critical period when grain number is set:

  • how fast the crop grows;
  • how much biomass is allocated to the head and grain; and
  • the duration of the critical period.

Growers can influence these factors through a range of management tools.

For example, to maximise growth and grain set in the critical period, the strategic application of nitrogen at sowing and top-dressing aids the development of a healthy and full canopy that promotes photosynthesis, he says.

“Plots at the Tasmanian HYC Research Centre at Hagley showed the importance of crop protection in the critical period through an appropriate fungicide program,” he says.

“We saw at the field day that some late-sown crops were very green and healthy, whilst other varieties clearly showed disease issues, so variety choice (for more-resistant germplasm) and disease control are critical.”

Matching variety to the environment to ensure the critical period aligns with favourable environmental conditions is also important for biomass partitioning to the plant heads, Professor Sadras says.

Plant growth regulators can be a helpful tool for reducing plant height and controlling lodging, particularly in high rainfall zones. They can also reduce excessive plant growth, which can alter the overall biomass partitioning to the grain and heads in the critical period.

Another element affecting growth in the critical period is water, but while this could be managed in irrigated systems, growers have less control in rain-fed systems and is often not the limiting factor in the HRZ.

Temperature driver

When it comes to the duration of the critical period, management strategies should aim to maximise its length to favour grain production, Professor Sadras says.

Ambient temperature is the main driver of the duration of the critical period. Professor Sadras explains that because plants cannot regulate their temperature, high temperatures accelerate the speed of development, resulting in crops reaching the end of the critical period too soon.

If the temperature is higher, that window will be shorter, and this can affect late-sown crops, for example, or those at inland locations that experience higher temperatures.

By contrast, in the cool conditions experienced at the Tasmanian GRDC HYC site, crops routinely return yields of 10 tonnes per hectare. “Mild temperatures in the critical period extend the window for grain production,” he says.

Professor Sadras says research by FAR senior research manager Dr Ben Jones is furthering knowledge around the impact of temperature on yield in the critical period and why yield potential is higher in cooler climates.

Using the photothermal quotient (PTQ) during the critical period – the sum of light received per day divided by average temperature – Dr Jones is exploring how a high PTQ in Tasmania (high light intensity and low average temperatures) fosters a longer critical period, leading to higher yields.

Variety and sowing date

Regardless of location, there are several ways to increase the length of the critical period, Professor Sadras says. These include selecting varieties and adjusting sowing time to reduce frost and heat risk based on knowledge of local conditions.

“There will always be risk because the environment is a moving target and no two seasons are ever the same,” he says. “But if crops reach their critical period too early and the location is frost prone, then you have a high chance of a frost that damages the crops in that sensitive stage. If they reach that period too late, they may avoid frost, but risk heat stress.

“So, growers have to manage that risk by selecting varieties and sowing dates to reach that critical period in environmental conditions where the two risks are accounted for.”

A range of GRDC investments looking into sowing dates and varieties in several locations is helping to mitigate against these risks by quantifying the impact of frost and heat, he says. These studies are complemented with modelling incorporating long-term climate data, building a clearer picture of risk of environmental factors.

Critical period the key

Contrary to popular belief, long-seasons such as those in the UK are not required to produce record-breaking yields, Professor Sadras says. Rather, the key is a long critical period when the crop is growing strongly and directing its resources to grain production.

In southern Chile, for example, spring wheat crops sown in September and harvested in late January or February have yielded 12t/ha. These high yields were achieved with a shorter season than in the UK.

“So, it’s not the season length that matters, but the critical period – how long it is, how healthy the crop is and whether there are available nutrients.”

Professor Sadras says the FAR Australia GRDC HYC program has shown that with appropriate fertiliser and fungicide management, a healthy canopy in the critical period can be reliably achieved. In mild conditions, this translates to significant yields.

“If you told a grower 20 years ago they could regularly grow 10t/ha or more it would have been hard to convince them,” he says. “But what the project is showing is that you can grow crops yielding 10t/ha year-in, year-out.”

In Tasmania, growers have traditionally prioritised higher-value vegetable crops over cereals, but the HYC program has demonstrated cereals can provide a significant income stream, he says.

“The driver is profit and if you can show that you can grow 10t/ha reliably, that might help cereals to gain traction in these areas.”

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