Farmers had unsold inventories equal to 5 per cent of the record harvest at the end of August from 10 per cent a year earlier, traders estimate. That would be the lowest since 2011. Production may drop 3 per cent to 1.65 million tonnes in the year starting next month, or 27.5 million 60kg bags, the traders say.

Futures in London have advanced 19 per cent this year on speculation that consumption will outstrip supply. The robusta shortfall will be 1.9 million bags and the arabica deficit is poised to be 6.9 million bags in the 12 months from October, according to Volcafe. Drinks makers have been buying more of the cheaper robusta after their discount to the arabica variety widened as much as three-fold this year.

"There's very little coffee left among farmers," said Phan Hung Anh, deputy director of Dak Lak-based Anh Minh, Vietnam's largest private exporter by volume. "The beans are mainly with quite wealthy people, so domestic prices have to reach about 43,000 dong (HK$15.73) to 44,000 dong a kilogram for them to sell."

Robusta futures traded at US$2,011 a tonne on NYSE Liffe yesterday. Arabica, the variety favoured by Starbucks, has risen 66 per cent to US$1.837 a pound this year. The premium of arabica over robusta climbed as high as US$1.166 a pound this year from 34 US cents in December.

The widening gap with arabica had encouraged more roasters to buy robusta, Kona Haque, the head of commodities research at ED&F Man in London, had said.

"Ever since arabica supplies tightened in 2011 due to problems in Colombia and Brazil, roasters have been investing in processes that have allowed them some flexibility in their blends, which they can now tweak, depending on the arbitrage," she said.

While reserves have dropped in Vietnam, inventories are increasing on the London bourse. Stockpiles with a valid grading certificate in warehouses monitored by Liffe expanded 3.6 per cent to 87,100 tonnes in the two weeks to September 1.

"Liffe robusta coffee futures will remain supported by weather risks across the arabica variety, but stocks are building ahead of the Vietnam harvest," Rabobank analysts including Tracey Allen in London wrote in a report. They forecast futures will average US$2,000 in the third quarter and ease to US$1,900 in the fourth quarter.

Farmers in Vietnam had 84,000 tonnes of unsold inventories at the end of August from a record 1.7 million-tonne harvest, traders estimate, compared with 150,000 tonnes a year earlier in a crop of 1.5 million tonnes.

Warehouse stockpiles in and around Ho Chi Minh City fell 60,000 tonnes to 235,000 tonnes from the end of July to August 26, "confirming that less and less coffee is available up-country", Tong Teik, a company owned by RCMA Commodities Asia, wrote in a report this month.

The market, including arabica and robusta, will have a shortage of 8.8 million bags in 2014-2015, the most in nine years, from a surplus of seven million this season.