Herbicide, also commonly known as weedkillers, are chemical substances used to control unwanted plants. Apart from selective/non-selective, other important distinctions include persistence, means of uptake, Herbicides have also been used in warfare and conflict. Modern herbicides are often mimics of natural plant hormones which interfere with growth of the target plants. The term organic herbicide has come to mean herbicides intended for organic farming, due to herbicide resistance - a major concern in agriculture - a number of products also combine herbicides with different means of action. In the US in 2007, about 83% of all herbicide usage, in 2007, world pesticide expenditures totaled about $39.4 billion, herbicides were about 40% of those sales and constituted the biggest portion, followed by insecticides, fungicides, and other types. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat, prior to the widespread use of chemical herbicides, cultural controls, such as altering soil pH, salinity, or fertility levels, were used to control weeds. Mechanical control was used to control weeds. The first modern herbicide,2, 4-D, was first discovered and synthesized by W. G. Templeman at Imperial Chemical Industries, in 1940, he showed that Growth substances applied appropriately would kill certain broad-leaved weeds in cereals without harming the crops. By 1941, his team succeeded in synthesizing the chemical, in the same year, Pokorny in the US achieved this as well. Independently, a team under Juda Hirsch Quastel, working at the Rothamsted Experimental Station made the same discovery, Quastel was tasked by the Agricultural Research Council to discover methods for improving crop yield. By analyzing soil as a system, rather than an inert substance. Quastel was able to quantify the influence of plant hormones, inhibitors and other chemicals on the activity of microorganisms in the soil. While the full work of the unit remained secret, certain discoveries were developed for use after the war, including the 2. When it was released in 1946, it triggered a worldwide revolution in agricultural output. It allowed for greatly enhanced weed control in wheat, maize, rice, and similar cereal crops, because it kills dicots. The low cost of 2, 4-D has led to continued usage today, like other acid herbicides, current formulations use either an amine salt or one of many esters of the parent compound. These are easier to handle than the acid, atrazine does not break down readily after being applied to soils of above neutral pH. Under alkaline soil conditions, atrazine may be carried into the profile as far as the water table by soil water following rainfall causing the aforementioned contamination
Weeds controlled with herbicide
Herbicides being sprayed from the spray arms of a tractor in North Dakota.