By Matt Cawood
Since the Green Revolution of the 1960s, the world has produced a lot more grain–but there may be a lot less in it, a unique experiment in the United Kingdom has revealed.
Recent analysis of 160 years of crop samples from Rothamsted Research Station near London discovered that levels of essential micronutrients remained consistent in wheat grain from 1844 to the late 1960s, but then began a decline that continues to this day.
The nutrient decline began when traditional long-straw wheat varieties where phased out in favour of higher-yielding semi-dwarf varieties.
As wheat plants have grown smaller since the 1960s, grain nutrient density has continued to decrease.
Compared to the old long-straw varieties, Rothamsted’s modern dwarf wheat grain carries on average 20-30 per cent less zinc, iron, copper and magnesium.
For zinc, a critical human nutrient, the decline is even more pronounced if the most recent five years of data are compared, with average nutrient levels in wheat harvested from 1844-1967.
The Rothamsted work supports a United States Department of Agriculture study, published in 2006, that compared nutrient levels in hard red winter wheat varieties grown from 1873 to 2000.
The US Department of Agriculture (USDA) researchers found that compared to 130 years ago, modern varieties deliver 36 per cent less selenium, 34 per cent less zinc and 28 per cent less iron in their grain.
Nutrient decline in food is a driving force behind the organic farming sector, on the assumption that high-tech agricultural methods have depleted the mineral levels in soils and thus made less available for plants to take up.
But at Rothamsted, it seems that soil is not the issue.
The research project used the soil and grain archives of the research station’s 166-year-old Broadbalk experiment, the longest-running agricultural experiment in the world.
The Broadbalk wheat plots were established in 1843, and were laid out to compare the relative performance of wheat fertilised with inorganic fertilisers, farmyard manure, and wheat given no treatment at all.
Nutrient declines in grain have occurred across all these treatments to a similar degree–but mineral levels in the soil have either remained stable or increased.
Concentrations of zinc in the treated Broadbalk soils have increased 40-60 per cent since 1860, and yet zinc densities in grain have declined more than any other measured nutrient.
“We can’t put it down to soil impoverishment, so my guess is that the nutrient decline is related to plant physiology,” said Rothamsted researcher Professor Steve McGrath.
That presents three possible areas for further research.
One is that in selecting for dwarf plant genes, breeders have inadvertently dwarfed root mass and made the plant less capable of foraging for nutrients.
Another possibility arises if micronutrients are relocated from vegetative material into growing grain. “If the vegetative mass is smaller, it may mean there is less ability to translocate micronutrients into the grain,” Prof. McGrath said.
“The third possibility is dilution. The grain is bigger, the grain yield is larger than old varieties; is it just that less mobile micronutrients are just not catching up with grain production so you get a dilution effect?”
That answer isn’t likely to emerge anytime soon.
Prof. McGrath said there has been little apparent interest in the finding, and making a case for further research in a restricted funding environment is a long and complicated process with no guarantee of success.
However, he understands that micronutrient deficiency, zinc and iron in particular, are implicated in health problems across the developed and developing worlds alike.
“People are suffering growth, health and effects on mental development from lack of zinc and iron,” he said.
Dr Carole Hungerford, author of the Australian medical nutrition textbook “Good Health in the 21st Century”, wrote that zinc is an essential nutrient for fertility, bone and joint health and immunity.
It is also essential for the structural integrity of the DNA molecule, which has led some researchers to speculate that zinc deficiency may contribute to cancer.
* Matthew Cawood visited Rothamsted Research on a travel scholarship provided by the World Conference of Science Journalists.