A dry spell in one hemisphere can change bread and feed costs on another continent. Grain is a globally traded commodity supplied by a small number of exporting regions.
Exports concentrate in few places
Most countries grow grain, but only a handful produce a large surplus for export. Traded volume therefore depends on conditions in those regions rather than on world production overall.
A poor harvest in a large producing country that consumes its own output barely affects the traded price. A poor harvest in an exporting region affects it immediately.
This is why market attention concentrates on specific growing areas, and why weather reports for those areas move prices in markets far away.
Timing matters as much as quantity
Crops are vulnerable at particular growth stages, and heat or drought during flowering and grain filling reduces yield far more than the same conditions earlier or later.
Traders therefore track the crop calendar closely, and identical weather produces very different price responses depending on the point in the season.
Harvest timing also differs between hemispheres, so the market receives new supply information at several points in the year rather than once.
Stocks absorb shocks until they cannot
Carryover stocks held between harvests buffer a shortfall, and when they are ample a poor season passes with modest price movement.
When stocks are already low, the same shortfall has nowhere to draw from and prices respond sharply. The ratio of stocks to use is the figure most closely watched.
Because stocks rebuild slowly across several good seasons, markets can remain sensitive for years after a single disrupted one.
Substitution links different grains
Much grain is used for animal feed, where wheat, maize and barley substitute for one another according to relative price and nutritional content.
A shortage in one therefore raises demand for the others, transmitting a localised problem across the whole complex within weeks.
Vegetable oils and oilseeds are linked similarly through crushing and feed markets, which is why disruptions rarely stay confined to a single crop.
Policy amplifies price moves
Exporting countries facing domestic price rises sometimes restrict exports to protect local supply, which removes volume from the traded market exactly when it is scarce.
Importing countries respond by buying earlier and in larger quantities, which adds demand at the same moment. Both reactions are rational individually and destabilising together.
Freight and currency movements complete the picture, since the delivered cost in an importing country depends on shipping rates and exchange rates as much as on the grain price itself.