Yield ceiling becomes the breakthrough must

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PHOTO: Evan Collis

Pressure continues to mount on cereal pre-breeders to find ways to quickly increase the yield potential of staple crops – for global food security and for the viability of commercial growers

Key points

  • The increases in cereal yields needed to meet food security demands to 2050 pose complex new challenges for researchers at the pre-breeding stage
  • A concerted global R&D effort is underway to probe for unexplored opportunities to accelerate gains in wheat yield potential
  • Many researchers are not confident the necessary genetic gain is achievable within the timetable set by the rising world population

No sooner had pre-breeders made progress towards solving cereals’ sensitivity to environmental stresses such as salinity in wheat or flooding in rice, than a familiar challenge resurfaced with renewed urgency in 2012. The issue is yield, and new projections show demand growing much faster than advances so far in yield potential.

Adding urgency, from a global perspective, is a decline in grain stocks and a continuing decline in water resources, all compounded by jumps in fertiliser prices and climate-related crop losses this year through much of the Northern Hemisphere.

As a result, yield is back on the agenda – the need to get more grain from each cereal plant grown. Helping to frame the nature of the challenge are projections that the planet’s population is expected to level out at nine billion people in about 2050, marking the likely demand peak for agricultural production.

Australian scientists have been quick to respond to the challenge, which has both a global humanitarian imperative and, for Australian grain growers, a crucial farm-viability objective.

The scale and ambition of projects being initiated also necessitates more collaborative partnerships among both public and private sector researchers. In Australia, for example, this includes projects that bring together bodies such as the GRDC, CSIRO’s Plant Industry Division, Bayer CropScience and the International Maize and Wheat Improvement Center (CIMMYT) in Mexico.

An unprecedented challenge

Dr Antonio Hall from the University of Buenos Aires in Argentina and CSIRO Plant Industry researcher Dr Richard Richards recently analysed the scale of the projected yield shortfall. 

In a review published in 2012, they quantified the rate at which yields of wheat, maize and rice need to increase to prevent food insecurity and the tragedies and social strife that inevitably flow from widespread hunger.

Image of a close-up of man standing in greenhouse

Dr Richard Richards: existing rates of yield
improvement are falling well short of what is needed.

PHOTO: Brad Collis

The researchers weighed up yield needs against the background of pressure being put on farmland from biofuel production. “We found that to meet expected demand for food and animal feed by 2050 requires yield gains to increase at annual compound rates of 1.16 per cent with a low biofuel requirement and 1.31 per cent with high biofuel requirements,” Dr Richards says.

“However, we found that existing rates of yield improvement are falling well short of these benchmarks.”

He says the evidence for wheat shows rates of yield gain of between 0.3 and 0.76 per cent per year. “And in several countries and regions, there is strong evidence of yield having plateaued.”

The most comprehensive data sets analysed for wheat come from the UK. There, the results from rain-fed trials are good approximations of a variety’s true yield potential given generally adequate rainfall, mild seasons, and complete pest and disease protection built into the annual trials. 

“Data from such trials show consistent upward trends in wheat yields, and since 1982 at least 88 per cent of improvements in yields could be attributed to genetic improvement,” Dr Richards says. “From 1948 to 2007 the rate of yield increase was linear and amounted to about 69 kilograms per hectare. But that translates into a relative rate of just 0.76 per cent annually.”

Other studies of yield potential follow a similar pattern, including gains of:

  • 0.3 per cent annually in irrigated spring wheat based on experiments in the Yaqui Valley in Mexico in 2005 (although the rate halves in Mexico’s warmer Tlaltizapán and has shown little tendency to increase in cultivars released after 1966); and
  • 0.68 and 0.75 per cent annually in irrigated winter wheats based on historic set studies from different periods in the main winter wheat regions in China.

“The hard truth emerging is that the vast majority of data point to relative rates of yield progress that fall below the necessary exponential rate required to meet projected demand to 2050,” Dr Richards says.

The world faced a similar situation in the 1960s, with famine averted by the agricultural innovations of the Green Revolution. However, today’s researchers note that key factors in past gains – dwarf varieties, nitrogen fertilisers, irrigation especially from groundwater, and expanding the land area being cropped – can today contribute only a small proportion of the required increases in food production.

That places serious limits on how extra gains can be made, especially regarding diminishing water resources. 

The water link

According to a Stockholm International Water Institute report, Feeding a Thirsty World, the dependence of the world’s large breadbasket regions on irrigation is becoming a problem. The rate of depletion of groundwater in the past 50 years has doubled, to about 300 cubic kilometres per year, and that has raised concerns for three large groundwater aquifers: the Ogallala aquifer in the US, the North China Plain and Gujarat in north-west India. 

The report states: “The conditions that challenge agriculture today are very different from those of the 1960s. From a water perspective, rivers are drying up, groundwater is being depleted, and ‘water crisis’ is now a commonly used term. Agriculture now consumes 70–80 per cent of all human water withdrawals, with severe consequences for many ecosystems and the related services on which we all depend. We now know that we can no longer view water as an inexhaustible and free input to a global food production system.”

For Dr Richards, such issues serve to inextricably link the issue of yield with issues of water use efficiency, in particular traits that lift yields in rain-fed wheat under water-limited conditions. 

Rates of progress in water-limited dryland environments for wheat were estimated at close to 0.54 per cent per year based on side-by-side trial data from Australia published in 2009, and 0.87 per cent per year in France.

The important message emerging is that opportunities to increase yields under water-limited conditions exist, but studies of past gains indicate the rates of advance are not likely to be high enough to meet the challenge pressing down on growers.

Australian approach

Dr Richards anticipates that conventional breeding will remain the mainstay of genetic advances since it is the most effective way to select a genetic background with appropriate quality, resilience, adaptability, and disease and pest-resistance traits.

However, the yield crisis coincides with the emergence of new pre-breeding biotechnologies. The technical arsenal now available to accelerate gains in yield potential in both unstressed environments and water-limited dryland environments includes:

  • improved, and faster, phenotypic selection for complex traits;
  • molecular markers for simply inherited traits to hasten delivery of new varieties; 
  • improved and more widespread hybrid technologies to exploit hybrid vigour, such as has occurred in rice;
  • genetic engineering technologies;
  • crop simulation models combined with long-term climatic records and soil information to facilitate the design of breeding programs and the selection process; and
  • more efficient experimental designs and statistical data processing.

Among already identified research targets is a re-engineering of photosynthesis to improve a plant’s radiation use efficiency. This project is being undertaken through the Wheat Yield Potential Consortium coordinated by CIMMYT and involving Dr Richards. Other projects are targeting improved root capacity for water uptake as part of an Indian–Australian wheat breeding initiative funded by the Australian Centre for International Agricultural Research and the Indian Council of Agricultural Research.

There are also efforts to deliver to breeding companies above-ground traits that influence canopy architecture to improve both radiation and water use efficiency.

The important link between new pre-breeding technology and global food security, says Dr Richards, are the timeframes for delivery of grower-ready varieties. He and Dr Hall have estimated that going from initial idea to the release of new cultivars takes about 20 years for complex traits and 10 years for simple traits.

“The time required is a function of trait complexity, inheritance mode, screen simplicity, linkage drag, money and effort invested,” Dr Richards says. “Serendipity has also been known to come into the equation. 

“However, in the challenge to progress yield enough to meet projected demand for affordable cereals by 2050, the timescales that have emerged are not a good basis for optimism. While this is obviously speculative, it needs to be noted that so too is the optimistic alternative.”

More information:

Dr Richard Richards
richard.richards@csiro.au

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