Monsanto Co. and BASF AG, a German competitor, on Wednesday promised to work together to deliver higher-yielding crops to meet increasing global demand. The companies announced a $1.5 billion research and development collaboration that, over a decade or more, could deliver hundreds of varieties of genetically modified corn, soybeans, cotton and canola. The seeds would allow farmers to boost yields on limited cropland to meet the world’s requirements for food, animal feed, fiber and renewable fuels.