Wheat-yields-at-a-standstill

Audio: Dr Zvi Hochman
Senior Principal Research Scientist
Agriculture & Food
CSIRO

In this study we asked why Australia’s wheat yields had not increased since 1990, in contrast with the previous 90 years when wheat yields more than trebled.

We found that 1990 was a watershed year, after which rainfall declines and increases in maximum daily temperatures started affecting wheat yield potential. What we found was that that the yield potential, as determined mainly by the environment but also by the genetics of wheat and by the soil types in which it’s grown and by current best management technologies, has declined. And it’s declined 27 percent over a 26 year period.

This means that whereas in 1990 potential wheat yields were 4.4 tonnes per hectare, by 2015 potential wheat yields went down to 3.2 tonnes per hectare – quite a significant drop.

And so this begs the question, why has the actual yield stayed constant while the potential yield has gone down. To answer that question I need to introduce the concept of the yield gap. There’s a gap between yield potential and the average yield that farmers are actually achieving every year across Australia. When we’ve looked at some of the farmers who are achieving the very highest yields we found they were achieving around 80 percent of the yield potential. We consider that to be the ceiling. It’s uneconomic to try to go above that.

Over the last 26 years, that yield gap has narrowed. So whilst in 1990 farmers were achieving around 38 percent of their yield potential, by 2015 they were achieving 55 percent, and it’s by closing that yield gap that they have been able to keep yields constant whilst the potential has been going down.

The closing of the gap is driven by both technological advances including better wheat varieties and better practices. So to find out what has happened to yield potential we were keen to work with high quality weather stations, and so we chose 50 weather stations that had the most complete record. They had to be located in an area which was a significant wheat growing area. They also had to be at least 20 kilometres away from each other so that each one was telling us something different. And we then found that they were also representative of 10 of the agroecological zones, and that meant that they represented more than 95 percent of the area and that they also did a good job of representing the different soil types that are relevant to wheat production.

We found that there was a 28 percent reduction over the 26 years in the amount of rainfall that fell during the growing period and there was a 1.05C increase in maximum temperatures during that same period over the 26 year period.

This work is also important because it will obviously affect the welfare of Australian farmers, and it is also important to the Australian economy because wheat production on average is currently worth about AUD$5 billion a year. So wheat is Australia’s most important grain crop, but other important grain crops like barely, canola, various pulse crops that we grow, all are likely to be similarly affected, because they will respond to the same climatic variables in a similar way.

Farmers have done really well in adapting technology in an unprecedented rate to keep up with current yields. And what we can say is that if the next 26 years look like the past 26 years in terms of the trend in reduced rainfall and increasing temperatures, then even if farmers can reach that 80 percent yield potential what we are calculating is that in 26 years’ time the yield potential will have dropped from 1.74 tonnes per hectare to 1.55 tonnes per hectare. So that means that by 2041 farmers, especially those in areas in which the decline has been higher, there will be quite a few farmers who will find it too marginal to keep growing wheat.

So in terms of attribution to climate change, there’s universal agreement that increased C02, or increased greenhouse gases overall, are responsible for the rising temperature. There is less certainty, but an increasing amount of evidence, and recent papers back this for the area we’re interested in Australia, that the decline we’ve seen in rainfall is also a consequence largely of climate change.

When you look at the observed reduction in yield potential, there’s only a less than a one in hundred billion chance that trend could be observed by chance alone or by random variability in climate.

The World Food Organisation have calculated that to meet the demand of growing populations and of a more affluent world in countries like India and China having the means to purchase high quality foods and more food, that means that by 2050 the world is going to have to produce 60 percent more food than it currently produces.