Many claims about water savings in maize and wheat are insufficiently substantiated
Much of the research into water savings in maize and wheat provides insufficient evidence for the savings attributed to it.
Much of the research into water savings in maize and wheat provides insufficient evidence for the savings attributed to it. This is evident from a new meta-study by Wageningen University & Research. Of the 208 field experiments examined, only 10 per cent contained sufficient data to properly assess the water productivity of the crops. The researchers therefore call for stricter and more uniform research methods.
Maize and wheat are among the most widely grown cereals worldwide. Their production requires a great amount of water. The question of how to grow these crops using less water has therefore been the subject of research for decades.
Around the turn of the century, the concept of ‘water productivity’ began to receive increasing attention: the amount of yield a crop produces per unit of water evaporated. Based on differences in measured water productivity, it was claimed that water savings of 20 to 40 per cent would be possible in agriculture.
A new analysis by researchers at Wageningen casts doubt on the evidence underpinning these claims for maize and wheat.
Only 21 studies meet the criteria
The researchers analysed 208 field experiments in maize and wheat, the oldest dating back to 1983. They examined whether studies reported data on biomass, whether the relationship between biomass and crop transpiration could be verified, and whether the water balance had been determined with sufficient accuracy.
This analysis revealed that 160 of the 208 studies (77 per cent) did not report data on biomass. Of the remaining studies, 27 used a simplified water balance, in which net precipitation or irrigation had not been properly accounted for. Ultimately, only 21 studies – or 10 per cent – remained with sufficient data to reliably assess water productivity: eleven for maize and ten for wheat.
Furthermore, within these studies, the variation in water productivity proved to be much smaller than previously assumed. According to the researchers, a significant proportion of the previously reported variation is likely due to inaccuracies in the experiments, rather than actual differences in the crops’ water productivity.
A long-known relationship
It is no new insight that crop growth and water use are closely linked. As early as 1958, the Wageningen researcher C.T. de Wit demonstrated that there is a linear relationship between a crop’s growth and the amount of water it evaporates. In principle, therefore, producing more also requires more water.
However, this relationship can be influenced. For example, good nutrient availability can make photosynthesis more efficient, whilst better ground cover can limit evaporation from the soil. The way in which water stress is managed also plays a role.
“The reliable data show that the relationship between water use and crop growth is remarkably consistent, across different years and varieties of the same crop. The scope for producing more with less water is limited, and such improvements are often difficult for farmers to put into practice. Producing more food will therefore also require more water. This is not a particularly encouraging message, but it is an important insight when assessing the implications for our water resources,” says lead author Gerardo van Halsema.
Better standards for new research
According to the researchers, new field experiments are therefore needed that meet stricter and more uniform requirements. In particular, the measurement of biomass and crop transpiration must become more accurate and comparable, in both rain-fed and irrigated agriculture.
In the publication, the researchers propose new standards for field experiments and scientific publications. This should make future studies more verifiable and comparable with one another. Ultimately, this will enable a more accurate assessment of the actual scope for water savings in the cultivation of maize and wheat.
Students urge lecturers to publish
Master’s students at Wageningen University also played a key role in the development of the research. As part of the course Water Delivery, they spent years analysing scientific publications and checking, amongst other things, water balances and calculations of water productivity.
In doing so, the students discovered time and again that many published results could not be properly verified. Their findings, and their repeated urging of the lecturers to investigate this further, were among the factors that led to the eventual meta-study.
