A scorching hot, dry summer has ended five years of plenty in many wheat-producing countries and drawn down the reserves of major exporters to their lowest levels since 2007/08, when low grain stocks contributed to food riots across Africa and Asia.

Although global stocks are expected to hit an all-time high of 273 million tons at the start of the 2018/19 grain marketing season, according to US Department of Agriculture (USDA) estimates, the problem is nearly half of it is in China, which is unlikely to release any onto global markets.

A spring drought in Black Sea bread baskets Russia and Ukraine was swiftly followed by a summer heatwave in the European Union. Dry weather now also threatens crops in another important exporter, Australia.

Experts predict that by the end of the season, the eight major exporters will be left with 20 percent of world stocks - just 26 days of cover - down from one-third a decade ago.

The USDA estimates that China, which consumes 16 percent of the world's wheat, will hold 46 percent of its stocks at the beginning of the season, which starts around now, and more than half by the end.

The 126.8 million tons China is estimated to hold is up 135 percent from 54 million five years ago.

A repeat of the 2007/08 crisis, which forced many countries to limit or ban exports, is unlikely in the absence of other drivers at the time, including $150-per-barrel crude oil.

The recent three-year high for wheat prices of $5.93 a bushel on the Chicago Board of Trade pales in comparison to the high of $13.34-1/2 a bushel in February 2008.

China started stockpiling wheat in 2006, setting a guaranteed floor price to ensure food security and stability.

At around $9.75 a bushel as of last week, Chinese prices are now so high that they cannot sell internationally without incurring a major loss.

Rabobank analyst Charles Clack said he expects China to continue building stocks into next year but in the long term it would look to reduce reserves by curbing domestic production, reducing imports or conducting internal auctions.

Government wheat reserves now total nearly 74 million tons, according to Shanghai JC Intelligence Co, most of it from 2014-17 but a small amount as old as 2013.

Sylvia Shi, analyst at JC Intelligence, said China would continue to import wheat it cannot produce in sufficient volumes to help meet a growing appetite for high-protein varieties for products like bread and other baked products as diets become more Westernized.