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He says he applies about 800 tonnes of sewage sludge every year to fields where he grows maize destined for animal feed.

Last month, Malawi blocked imports of flour, rice, ginger, bananas and maize from Tanzania, and other countries, saying this was to protect local producers.

Malawi, which shares a border with Tanzania, has blocked imports of flour, rice, ginger, bananas and maize from its northern neighbour.

But as the two countries work on a trade deal, experts say Washington now wants to push "big-ticket" farm exports - wheat, cotton, corn and maize - to narrow its $45bn trade deficit with India.

“There’s a disturbing level of contamination of native maize with genetically modified traits,” said Timothy Wise, a researcher at the Global Development and Environment Institute at Tufts University.