What might be a negative result of growing a cropthat is genetically engineered to resist weed killer?
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Answer:
For HRCs, risks can be considered qualitative estimates which combine the likelihood and severity of both immediate and delayed adverse effects to human health, the environment and the farmer’s economy. The likelihood and severity of each unwanted effect associated with HRCs depends on the crop, the HR trait, the local weed flora, climatic conditions and farm management practices and can only be estimated on a case-by-case basis (Madsen et l. 2002b).
In glyphosate-resistant crops, optimal weed control often requires sequential applications with glyphosate, and the timing relative to weed emergence is important (Swanton et al. 2000). When glyphosate is sprayed 2-3 times annually at high rates it imposes a high selection pressure on the weed flora. In 5-8 years this may cause shifts in weed composition towards species that naturally tolerate glyphosate (Benbrook, 2001; Shaner, 2000) and other herbicides may be needed to control these weeds (Shaner, 2). Payne and Oliver (2000) suggest implementing conventional, post-emergence herbicides in the weed control programme in glyphosate-resistant soybean to assist the control of glyphosate tolerant species, such as Sesbania exaltata (Raf.) cory, Ipomoea spp. or Amaranthus rudis Sauer. Furthermore, it may become difficult to control volunteer crops in subsequent years. If farmers grow glyphosate-resistant varieties of both corn and soybean in a soybean-corn rotation, then glyphosate cannot control the volunteer corn, which can be a serious weed problem in soybean (Shaner, 2000).
Gene-flow from crops to other crops or related species is another route to the development of resistant weed populations in the field. Once the resistance gene is present in crop volunteers or related weed species then it is expected that the same weed control practices (consistent sprayings with herbicides having the same mode of action), which cause herbicide resistance to occur in naturally tolerant/resistant weed biotypes, will lead to a rapid build-up of HR-weeds and volunteers.
Increased herbicide use is considered a risk in some parts of the world although the effects on human health or the environment are seldom specified in details, but derived effects from pesticide-use such as ground-water pollution and pesticide residues in vegetables, for example, have caused public concern. There seem to be two major explanations why herbicide use in HRCs may increase. One reason is that a high level of crop tolerance may enable the farmer to increase doses to achieve an improved weed control without harming the crop (see Figure 2). The other reason is problems with tolerant/resistant weeds and volunteers, which require farmers to increase dose or mix herbicides with different modes of action to maintain an acceptable level of weed control.
Biodiversity within the field may be influenced if the herbicide, to which the HRC is resistant, is used at a higher level of efficacy than hitherto in order to achieve an improved weed control. Furthermore, weed species respond differently to different herbicides or other weed-control measures and a shift in prevailing species is very likely. If the growing of an HRC is taking place at the centre of genetic origin, then changes in the diversity of the indigenous species and risks of diminishing the genetic diversity of these species is a hazard (FAO, 2001). It is, however, very unlikely that HR crops will cause erosion of genetic diversity of wild species outside the cultivated land, because the trait is associated with the use of herbicides which are not being applied in the wild, and a HR trait does not confer selective advantage unless the herbicide is used (Poulsen, 1995; Madsen et al. 1998). Therefore, there is a low risk of erosion of the genetic diversity of wild species growing in natural environments.
Despite these concerns, some uses of GM crops, e.g. herbicide resistant sugar beet, appear to be safe so far as ecological risks are concerned, when these are judged by ordinary scientific standards (Madsen and Sandøe, 2001).
Finally, it must be emphasized that risk assessment is only one of the steps leading to the final approval of an HRC, which implies a political decision. First, an ‘acceptable level’ of risk is not an objective term and second, risk assessment of HRCs is associated with a high degree of uncertainty with regard to the magnitude and likelihood by which potential hazards associated with the HRC may occur (Madsen et al. 2002b).