<P> Although the Green Revolution significantly increased rice yields in Asia, yield increases have not occurred in the past 15--20 years . The genetic "yield potential" has increased for wheat, but the yield potential for rice has not increased since 1966, and the yield potential for maize has "barely increased in 35 years". It takes only a decade or two for herbicide - resistant weeds to emerge, and insects become resistant to insecticides within about a decade, delayed somewhat by crop rotation . </P> <P> For most of its history, agriculture has been organic, without synthetic fertilisers or pesticides, and without GMOs . With the advent of chemical agriculture, Rudolf Steiner called for farming without synthetic pesticides, and his Agriculture Course of 1924 laid the foundation for biodynamic agriculture . Lord Northbourne developed these ideas and presented his manifesto of organic farming in 1940 . This became a worldwide movement, and organic farming is now practiced in most countries . </P>

Where did the earliest cities develop and why