Why Did the Wheat Fall Over? Too Much Nitrogen and One Bad Storm

The plant goes over in one of two ways and the distinction matters for prevention. In one the stem itself buckles, usually near the base where it is weakest, which happens when the stem is long, thin-walled or weakened by disease. In the other the stem stays intact and the whole plant tips over as the root plate rotates in the soil, which happens when roots are shallow, the soil is wet and loose, or the head is heavy. Wind and rain together are the usual trigger, since rain adds weight to the head and softens the ground while wind supplies the force.

The losses come from several directions at once:

• •Grain filling stops, since the transport system is damaged
• •Flattened plants shade each other and cannot photosynthesise
• •Grain in contact with wet ground sprouts or moulds
• •A harvester cannot lift a flattened crop cleanly, so much is left
• •Harvesting is far slower, raising cost and risking weather
• •Losses of a third or more are recorded in bad cases

The problem became acute precisely because farming improved. Applying nitrogen fertiliser makes cereals grow taller and produce heavier heads, which is exactly what raises yield and also exactly what makes a plant top-heavy on a long weak stem. Through the mid twentieth century that created a ceiling, since adding more nitrogen produced more grain only up to the point where the crop fell over, after which it produced less. Traditional varieties were over a metre tall and could not carry the extra weight. The limit was not the plant's ability to make grain but its ability to stand up while doing so.

Variety choice does most of the work and the remaining measures are all about restraint. Sowing at a lower seed rate produces fewer, sturdier plants rather than a thick stand of thin ones competing for light. Splitting nitrogen into several applications, and holding back the later ones, avoids the surge of soft growth that weakens stems. Growth regulators, which are chemicals that shorten and thicken the stem directly, are applied widely in high-input systems and are effectively a chemical version of the dwarfing genes. Rolling the soil after sowing improves root anchorage. And drilling in rows across the prevailing wind rather than along it makes a measurable difference.

The answer was to make the plants shorter, and the genetics behind that changed world agriculture. Japanese wheat varieties carried genes reducing the plant's response to a growth hormone, producing short stiff straw, and Norman Borlaug's programme in Mexico crossed those into high-yielding wheats through the 1950s and 1960s. The resulting semi-dwarf varieties could absorb heavy fertiliser, put the extra growth into grain rather than straw, and stand up. Equivalent work on rice at the institute in the Philippines produced the same result. Adoption across Asia and Latin America from the 1960s raised cereal production enormously, and Borlaug received the Nobel Peace Prize in 1970.

A cereal goes down either by the stem buckling or by the root plate rotating in wet soil, and the flattened crop stops filling grain, sprouts on the ground and cannot be harvested cleanly. Nitrogen fertiliser raises yield and makes plants taller and top-heavy, which capped how much could be applied. Genes reducing response to a growth hormone produced short stiff varieties that lifted that cap.