Chemical Control of Weeds and its Implications

A weed is an undesirable plant that grows on cultivated ground and which provokes crop yield loss. Since ancient times, farmers have applied different control methods to eradicate them. However, they are still present in every agro-ecosystems. In the past mechanic control was the only practice known until the use of the first chemical methods. The beginning of chemical techniques was based on the use of inorganic salts such as sodium chloride. The first herbicide was an organic substance called DNOC (4,6-dinitro-o-cresol), which was discovered during the 1930s in France. Since then, more than 200 active principles have been developed and approved. With this progress, labor hand requirements for weed control have been reduced. Nowadays, herbicides dominate the pest market and represent more than 40% of total world pesticide amounts used.

Herbicide use is associated with reductions in crop yield loss due to weed interference and an increase of 30% in total production, reflecting an economic benefit to the whole society. However, extensive herbicide use has been related to several problems. Among them the following topics could be highlighted:


 * Environmental behaviour of herbicides: the life of a herbicide does not end in the weed, as much as 55% of an applied herbicide is lost in other environmental sinks (i.e. soil, atmosphere and bodies of water) and it can provoke toxicological effects on wildlife.


 * Human risk: the capacity of a herbicide to cause adverse effects in humans depends on its active ingredient and its formulation. Although the manipulation of pesticides requires strict safety norms, the exposure of a person to pesticides is possible due to truck and aircraft spills during their application.


 * Impact on biodiversity: Herbicides used to control target weeds also affect other plant and algal species that support ecological functions in agro-ecosystems. These species provide shelter and food for many types of beneficial insects (i.e. pest controllers), birds and wild animals. Thus, loss of biodiversity alters trophic chains, genetic pools and ecological services of agro-ecosystems.


 * Herbicide-resistant weeds: the wide and continual use of an herbicide leads to the selection of herbicide-resistant weeds. These plants have specific mechanisms that allow them to reduce toxic effects by limiting herbicide absorption or transport into the plant, degrading the active principle, compartmentalizing into vacuoles or changing the target enzyme of the herbicide. Currently, the cases of herbicide-resistant weeds have been increasing and the success of chemical control is at risk.

These impacts on the agro-ecosystems can be minimized when herbicide applications are combined with other control techniques such as cultural, mechanical and biological methods in programs of integrated weed management.