Any chemical used to destroy or inhibit plant growth, especially of weeds or other undesirable vegetation. The concepts of modern herbicide technology began to develop about 1900 and accelerated rapidly with the discovery of dichlorophenoxyacetic acid (2,4-D) as a growth-regulator-type herbicide in 1944–1945. A few other notable events should be mentioned. During 1896–1908, metal salts and mineral acids were introduced as selective sprays for controlling broad-leafed weeds in cereals; during 1915–1925 acid arsenical spray, sodium chlorate, and other chemicals were recognized as herbicides; and in 1933–1934, sodium dinitrocresylate became the first organic selective herbicide to be used in culture of cereals, flax, and peas. Since the introduction of 2,4-D, a wide variety of organic herbicides have been developed and have received wide usage in agriculture, forestry, and other industries. Today, the development of highly specific herbicides that are intended to control specific weed types continues. Modern usage often combines two or more herbicides to provide the desired weed control. Worldwide usage of herbicides continues to increase, making their manufacture and sale a major industry.