A campaign promise that helped bring Thailand's Prime Minister Yingluck Shinawatra and her political party to power in July elections is roiling the global market for rice, Asia's staple food that is now eaten by nearly half the world's population.

They promised their government would alleviate poverty in the countryside and raise rural income levels by buying unmilled rice, known as paddy, from Thailand's eight million rice growers at 15,000 baht ($480) per ton — double the pre-election price.

The program to increase the minimum guaranteed price for farmers, which has just taken effect, seems tailor-made to keep more supplies at home and drive up the export price.