MEXICO CITY, Nov 9 (Reuters) - Mexico's government cannot make purchases of yellow corn from the United States because it does not want genetically modified (GM) corn, President Andres Manuel Lopez Obrador said on Wednesday, amid pressure from its top trade partner over the future of the imports."There is a market for it, but the government cannot make a purchase because we do not want GM," Lopez Obrador said, citing a lack of scientific investigation into its effects.Like most countries, the overwhelming amount of corn purchases in Mexico are made by private companies, including major commodities traders like Cargill (CARGIL.UL) and Bunge.Mexico is ready to halve its U.S. imports of yellow corn when the decree goes into effect and is considering direct agreements with farmers to secure non-GM yellow corn imports, the country's deputy agriculture minister said in October.Mexico is mostly self-sufficient in white corn, used to make the country's staple tortillas, but has for decades imported huge quantities of yellow corn, mostly from U.S. suppliers, for its massive livestock industry as well as the production of industrial foods like cereals and sauces."