Herbicides, also commonly known as weeds killers, are pesticides used to kill unwanted plants. Selective herbicides kill specific target plants, while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic herbicides which are made up of natural plant hormones. Herbicides used to clear waste ground, industrial sites, railways and railway embankments are not selective and kill all plant material with which they come into contact. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat. Excess and deficiency in the use of everything is bad. Here we will try to elaborate the effects of herbicides on the living things and their environment so that the judicious use of herbicides can be adopted.

Health effects

Herbicides have widely variable toxicity. In addition to acute toxicity from high exposure levels, there is concern of possible carcinogenicity, as well as other long-term problems, such as contributing to Parkinsons disease. Some herbicides cause a range of health effects ranging from skin rashes to death. The pathway of attack can arise from intentional or unintentional direct consumption, improper application resulting in the herbicide coming into direct contact with people or wildlife, inhalation of aerial sprays, or food consumption prior to the labeled pre-harvest interval. Under some conditions, certain herbicides can be transported via leaching or surface runoff to contaminate groundwater or distant surface water sources. Generally, the conditions that promote herbicide transport include intense storm events (particularly shortly after application) and soils with limited capacity to adsorb or retain the herbicides. Herbicide properties that increase likelihood of transport include persistence (resistance to degradation) and high water solubility.

Phenoxy herbicides are often contaminated with dioxins; research has suggested such contamination results in a small rise in cancer risk after exposure to these herbicides. Triazine herbicides exposure has been implicated to increased risk of breast cancer, although a causal relationship remains unclear. Herbicide manufacturers have at times made false or misleading claims about the safety of their products.

Well known chemical manufacturer company (Monsanto) has agreed to change its advertising campaign after pressure from New York, Attorney General Dennis Vacco; Vacco complained about misleading claims that its glyphosate-based herbicides, including Roundup, were safer than table salt and “practically non-toxic” to mammals, birds, and fish. Roundup is toxic and has resulted in death after being ingested in quantities ranging from 85 to 200 ml, although it has also been ingested in quantities as large as 500 ml with only mild or moderate symptoms. Herbicide parquets is suspected to increase risk of Parkinsons disease. All commercially sold, organic and nonorganic herbicides must be extensively tested prior to approval for sale and labeling by the Environmental Protection Agency. However, because of the large number of herbicides in use, concern regarding health effects is significant. In addition to health effects caused by herbicides themselves, commercial herbicide mixtures often contain other chemicals, including inactive ingredients, which have negative impacts on human health. For example, Roundup contains adjuvants which, even in low concentrations, were found to kill human embryonic, placental, and umbilical cells in vitro. One study also found Roundup caused genetic damage, but the damage was not caused by the active ingredient.

Ecological effects

The environmental impact includes effects of herbicides on non-target species. Over 95 per cent of herbicides reach a destination other than their target species, because they are sprayed or spread across entire agricultural fields. Runoff can carry pesticides into aquatic environments while wind can carry them to other fields, grazing areas, human settlements and undeveloped areas, potentially affecting other species. Each herbicides class comes with a specific set of environmental concerns. Such undesirable effects have led many herbicides to be banned, while regulations have limited and/or reduced the use of others.

Commercial herbicide use generally has negative impacts on bird populations, although the laboratory studies have overestimated negative impacts on birds due to toxicity, predicting serious problems that were not observed in the field. Most observed effects are due not to toxicity, but to habitat changes and the decreases in abundance of species on which birds rely for food or shelter. Herbicide use in silviculture, used to favor certain types of growth following clear cutting, can cause significant drops in bird populations. Even when herbicides which have low toxicity to birds are used, they decrease the abundance of many types of vegetation on which the birds rely. Herbicide use in England has been linked to a decline in seed-eating bird species which rely on the weeds killed by the herbicides. Frog populations may be affected negatively by the use of herbicides as well. The herbicide atrazine can turn male frogs into hermaphrodites, decreasing their ability to reproduce

Scientific uncertainty

The health and environmental effects of many herbicides is unknown, and even the scientific community often disagrees on the risk. For example, in 1995 a panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D herbicide had divided opinions on the likelihood 2,4-D herbicide causes cancer in humans. As in 1992, studies on phenoxy herbicides were too few to accurately assess the risk of many types of cancer from these herbicides, even though evidence was stronger, that exposure to these herbicides is associated with vertebrate immune systems. Furthermore, Atrazine also have similar impacts on vertebrate immune system.

The writers are associated with the Department of Agronomy, University of Agriculture, Faisalabad, Pakistan. They can reached at <nabeelahmadikram@gmail.com>

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