Weeds, in general, precede crops on farming lands and are major yield reducing factors. In ecological terms, most annual weeds are r-strategists, establishing populations with high relative growth rate (r). Thus, they produce numerous viable seeds and their populations quickly build up, in an exponential pattern. Weed competition with crops reduces agricultural output (quantity and quality), and increases external costs by spreading them across farm boundaries. It is also a major constraint to increased farmers’ productivity, particularly in developing countries where weed control claims 20 to 50 % of farmers’ time and keep smallholders in a vicious circle of poverty.