Herbicides associations with crops

PERFLUOROALKYLPYRIDINES New developments ia trifiuoromethylpyridine technology are associated with the commercialization of numerous crop-protection chemicals as herbicides, fungicides, and iasecticides (Table 15). Physical properties for representative trifiuoromethylpyridines are fisted ia Table 13. 

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

Finally, certain triazine herbicides can be used selectively in orchards and in some horticultural crops. In this case, selectivity is not based only on physiological differences between species, but on physical selectivity associated with the location of the herbicide and the roots of the crop and weed species in the soil. Triazine herbicides such as simazine, which has very low solubility in water, remain close to the soil surface in most mineral soils. Careful application of simazine in horticultural or fruit crops can result in the herbicide being available to control shallow-rooted weed species without harming the deeper-rooted perennial species. The success of this use is dependent not only on the relative rooting depths of the tolerant and susceptible species, but also on soil conditions and other factors that may affect herbicide fate and movement. 

This appendix lists various crops, together with the insecticides and herbicides that may be associated with their production. It thus provides a starting point for determining the pesticides of interest in a particular catchment. Such a listing cannot be definitive for example, sometimes a pesticide may be used that is not specifically recommended for control in a particular crop. This situation can occur for various reasons, such as lack of access to more appropriate pesticides. 

Table A3.1 Association of insecticides and herbicides with crops and crop types... 

Chemicals considered for health-based guideline values Chemicals that may give rise to consumer complaints Chemicals potentially discharged through effluents from industrial sources Association of insecticides and herbicides with crops and crop types... 

In a similar fashion, the EIQ has been applied to Canadian canola cultivation in order to estimate the environmental impact of the adoption of herbicide tolerant canola, which includes transgenic varieties. In recent years, herbicide tolerant canola varieties (glyphosate-, glufosinate-, and imidazolinone-tolerant) have been adopted by Canadian fanners at a larges scale, accounting for 80% of the national canola acreage in 2000. The adoption of these herbicide tolerant varieties has been associated with a concomitant decrease in herbicide use on canola. The environmental impact of herbicide use on canola determined by the EIQ method decreased in correlation with the decreased amounts of herbicide applied to the crops, rather than this would be due to different intrinsic properties (EIQ) of the herbicides involved [23]. 

Apart from the insecticides and herbicides already discussed, there are many other kinds of pesticides in use. These range from rodenticides (chemicals designed to kiU mainly rats and mice) to fungicides, nemato-cides, and molluscicides (which target fungal infections of crops and animals, infestations of animals by nematode worms, and organisms such as slugs, respectively). Some of these have been associated with cases of... 

Organic Mercury. Case report studies suggest that dermal exposure to methylmercury or phenylmercury in humans can cause rashes and blisters on the skin (Hunter et al. 1940 Morris 1960). A 33-year-old male worker exposed to methylmercury nitrate dust for 2 years developed bums and blisters on his forearm (Hunter et al. 1940). These effects healed within 9 days. Sensitivity to phenylmercuric salts is shown by individuals who developed itchy, pruritic, papular eruptions or rashes on their skin following acute dermal exposure (Morris 1960). A 54-year-old woman with a family history of atopy was found to display erythema (at 30 minutes postexposure) and urticaria (at 60 minutes) when treated topically with a 0.01% solution of phenylmercuric acetate (Torresani et al. 1993). This positive reaction was associated with aggravation of facial edema and an attack of bronchospasm. The woman, who was a farmer, was believed to have been previously exposed to phenylmercuric acetate during contact with pesticides and herbicides used on farm crops. 

When herbicides are used in crop rotations, the grower is concerned about the effects of potential herbicide residues on subsequent crops. However, a key question associated with enhanced biodegradation is the potential impact of one herbicide on the subsequent performance of a different herbicide, especially if they are structurally similar. If soil... 

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