Marena Lin and Peter Huybers from Harvard University believe that there are many reasons for this trend, including losses from regional warming, a reduction in some countries in the amount of fertilizer used and some wheat yields possibly being near their yield potential.

"Many studies suggest that global wheat yields are levelling off," Lin told environmentalresearchweb. "But modelling wheat yields is a complex issue and many models do not account for intrinsic interannual variations, their autocorrelation and the differences in the models compared."

Lin and Huybers developed a new statistical test for whether a yield time series has levelled off. They applied it to wheat yield data from 50 different regions to show that nearly half of the production within their sample has transitioned to level trajectories.

"With the major exception of India, the majority of levelling in wheat yields occurs within developed nations, including the United Kingdom, France and Germany," said Lin. "This levelling coincides with policies in these countries, which appear to have disincentivized yield increases relative to other objectives. It is interesting to note that in these cases, the levelling wheat yield is probably due to management decisions, rather than the reaching of a yield limit."

In India and Bangladesh, however, there seem to be different factors at play. In these two countries, which have greater food insecurity and have been using increasing amounts of fertilizers, wheat yields are also levelling off. "This could be due to the effects of climate change, which may be expected to decrease yield potential," said Lin. "Also, wheat yields in India and Bangladesh are near their estimated yield potential." She also pointed out that developing countries were more likely to be excluded from this analysis due to poor data quality and may show different trends, perhaps corresponding more with management factors.

Of the 50 regions that Lin and Huybers tested for yield stagnation, 27 show yields that have levelled off when performing the test at the 80% confidence level. Using 2007 numbers, wheat accounts for 19% of the total calories of food produced, and the 47 countries sampled in their analysis account for 75% of total global wheat production.

The 27 regions with confirmed plateaus at the 80% confidence level account for 35% of global wheat production. "We prefer to report values at 80% confidence to reduce the probability of false negatives, but note that 18 regions have levelled off with at least 95% confidence and that they still account for 28% of global wheat production," said Lin.

The researchers also caution that the current observed trends are not predictive of future wheat yields. "There is wide scope for adaptation in terms of policy revision, technological innovations and increased economic incentives that cannot be captured in such a forecast," said Lin.

The scientists published their study in Environmental Research Letters (ERL).