Weeds have always been a part of both agriculture and non-agriculture systems. The subsistence global economy based on nomadic, food gathering and fishing of late-stone age had been gradually displaced by a sophisticated agricultural economy. The nature of weeds and how they interact with human activities form the basis of the discipline of weed science. Weed may poison livestock or seriously slow their weight gain, they can cause allergic reactions in humans, such as hay fever and dermatitis. They are troublesome in industrial areas, irrigation and drainage systems, highways and railroads. Weeds reduce crop yield, quality drastically by competition and interference. Although weed science is an ever-expanding discipline, it has always involved an understanding of weed biology and ecology.