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Weed-wiping may also be used, where a wick wetted with herbicide is suspended from a boom and dragged or rolled across the tops of the taller weed plants. This allows treatment of taller grassland weeds by direct contact without affecting related but desirable shorter plants in the grassland sward beneath.

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The method has the benefit of avoiding spray drift. In Wales , a scheme offering free weed-wiper hire was launched in in an effort to reduce the levels of MCPA in water courses. Herbicide volatilisation or spray drift may result in herbicide affecting neighboring fields or plants, particularly in windy conditions. Sometimes, the wrong field or plants may be sprayed due to error. The use of herbicides as a chemical weapon by the U.

Herbicides have widely variable toxicity in addition to acute toxicity arising from ingestion of a significant quantity rapidly, and chronic toxicity arising from environmental and occupational exposure over long periods. Much public suspicion of herbicides revolves around a confusion between valid statements of acute toxicity as opposed to equally valid statements of lack of chronic toxicity at the recommended levels of usage.

For instance, while glyphosate formulations with tallowamine adjuvants are acutely toxic, their use was found to be uncorrelated with any health issues like cancer in a massive US Department of Health study on 90, members of farmer families for over a period of 23 years. 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 labelled preharvest interval.

Under some conditions, certain herbicides can be transported via leaching or surface runoff to contaminate groundwater or distant surface water sources.

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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.

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Phenoxy herbicides are often contaminated with dioxins such as TCDD [30] [ citation needed ] ; research has suggested such contamination results in a small rise in cancer risk after occupational exposure to these herbicides. Herbicide manufacturers have at times made false or misleading claims about the safety of their products. Chemical manufacturer Monsanto Company agreed to change its advertising after pressure from New York attorney general Dennis Vacco ; Vacco complained about misleading claims that its spray-on glyphosate-based herbicides, including Roundup, were safer than table salt and "practically non-toxic" to mammals, birds, and fish though proof that this was ever said is hard to find.

The risk of Parkinson's disease has been shown to increase with occupational exposure to herbicides and pesticides. 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. Commercial herbicide use generally has negative impacts on bird populations , although the impacts are highly variable and often require field studies to predict accurately. Laboratory studies have at times overestimated negative impacts on birds due to toxicity, predicting serious problems that were not observed in the field.


Herbicide use in silviculture , used to favor certain types of growth following clearcutting , 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.

Frog populations may be affected negatively by the use of herbicides as well. While some studies have shown that atrazine may be a teratogen , causing demasculinization in male frogs, [44] the U. Environmental Protection Agency EPA and its independent Scientific Advisory Panel SAP examined all available studies on this topic and concluded that "atrazine does not adversely affect amphibian gonadal development based on a review of laboratory and field studies.

The health and environmental effects of many herbicides is unknown, and even the scientific community often disagrees on the risk. For example, a panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions on the likelihood 2,4-D causes cancer in humans. Weed resistance to herbicides has become a major concern in crop production worldwide.

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Plants have developed resistance to atrazine and to ALS-inhibitors, and more recently, to glyphosate herbicides. Marestail is one weed that has developed glyphosate resistance. Weeds that can resist multiple other herbicides are spreading. Few new herbicides are near commercialization, and none with a molecular mode of action for which there is no resistance. Because most herbicides could not kill all weeds, farmers rotated crops and herbicides to stop resistant weeds.

During its initial years, glyphosate was not subject to resistance and allowed farmers to reduce the use of rotation. A family of weeds that includes waterhemp Amaranthus rudis is the largest concern. The use of other killers to target "residual" weeds has become common, and may be sufficient to have stopped the spread of resistance From through researchers discovered 13 different weed species that had developed resistance to glyphosate. But since then only two more have been discovered.

Weeds resistant to multiple herbicides with completely different biological action modes are on the rise. Dow, Bayer CropScience, Syngenta and Monsanto are all developing seed varieties resistant to herbicides other than glyphosate, which will make it easier for farmers to use alternative weed killers.

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Even though weeds have already evolved some resistance to those herbicides, Powles says the new seed-and-herbicide combos should work well if used with proper rotation. Resistance to herbicides can be based on one of the following biochemical mechanisms: [51] [52] [53]. Careful observation is important so that any reduction in herbicide efficacy can be detected.

This may indicate evolving resistance. It is vital that resistance is detected at an early stage as if it becomes an acute, whole-farm problem, options are more limited and greater expense is almost inevitable. Table 1 lists factors which enable the risk of resistance to be assessed. An essential pre-requisite for confirmation of resistance is a good diagnostic test. Ideally this should be rapid, accurate, cheap and accessible. Many diagnostic tests have been developed, including glasshouse pot assays, petri dish assays and chlorophyll fluorescence.

A key component of such tests is that the response of the suspect population to a herbicide can be compared with that of known susceptible and resistant standards under controlled conditions. Most cases of herbicide resistance are a consequence of the repeated use of herbicides, often in association with crop monoculture and reduced cultivation practices.

It is necessary, therefore, to modify these practices in order to prevent or delay the onset of resistance or to control existing resistant populations. A key objective should be the reduction in selection pressure. An integrated weed management IWM approach is required, in which as many tactics as possible are used to combat weeds. In this way, less reliance is placed on herbicides and so selection pressure should be reduced. Optimising herbicide input to the economic threshold level should avoid the unnecessary use of herbicides and reduce selection pressure.

Herbicides should be used to their greatest potential by ensuring that the timing, dose, application method, soil and climatic conditions are optimal for good activity. In the UK, partially resistant grass weeds such as Alopecurus myosuroides blackgrass and Avena spp. Patch spraying, or applying herbicide to only the badly infested areas of fields, is another means of reducing total herbicide use.

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When resistance is first suspected or confirmed, the efficacy of alternatives is likely to be the first consideration. The use of alternative herbicides which remain effective on resistant populations can be a successful strategy, at least in the short term. The effectiveness of alternative herbicides will be highly dependent on the extent of cross-resistance. If there is resistance to a single group of herbicides, then the use of herbicides from other groups may provide a simple and effective solution, at least in the short term.

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For example, many triazine-resistant weeds have been readily controlled by the use of alternative herbicides such as dicamba or glyphosate. If resistance extends to more than one herbicide group, then choices are more limited.

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It should not be assumed that resistance will automatically extend to all herbicides with the same mode of action, although it is wise to assume this until proved otherwise. In many weeds the degree of cross-resistance between the five groups of ALS inhibitors varies considerably.