A new report published in US Department of Agriculture suggested that majority of US corn and soybeans in US are being genetically manipulated to withstand certain chemicals and climate conditions and saying that they could cause a major environmental risk:
Because glyphosate is significantly less toxic and less persistent than traditional herbicides …the net impact of HT crop adoption is an improvement in environmental quality and a reduction in the health risks associated with herbicide use (even if there are slight increases in the total pounds of herbicide applied). However, glyphosate resistance among weed populations in recent years may have induced farmers to raise application rates .Thus, weed resistance may be offsetting some of the economic and environmental advantages of HT crop adoption regarding herbicide use. Moreover, herbicide toxicity may soon be negatively affected (compared to glyphosate) by the introduction (estimated for 2014) of crops tolerant to the herbicides dicamba and 2,4-D.
On the other side, one study said that GMO could be helpful for the environment. It said that crops genetically modified to poison pests can deliver significant environmental benefits:
Broadly speaking, the deployment of Bt crops may favour biocontrol services and enhance economic benefits not only in Bt crop fields but also in the whole agricultural landscape.
Furthermore, there is no scientific consensus regarding the environmental safety of the GMOs:
A report by the British Medical Association concluded that with regard to the long-term effects of GM foods on human health and the environment:
‘many unanswered questions remain’ and that ‘safety concerns cannot, as yet, be dismissed completely on the basis of information currently available’.
The report called for more research, especially on potential impacts on human health and the environment.
Adding also this part: there is no consensus on the environmental risks of GM crops:
As with GM food safety, no scientific consensus exists regarding the environmental risks of GM crops. A review of environmental risk assessment approaches for GM crops identified shortcomings in the procedures used and found ‘no consensus’ globally on the methodologies that should be applied, let alone on standardized testing procedures . Some reviews of the published data on Bt crops have found that they can have adverse effects on non-target and beneficial organisms - - effects that are widely neglected in regulatory assessments and by some scientific commentators. Resistance to Bt toxins has emerged in target pests , and problems with secondary (non-target) pests have been noted, for example, in Bt cotton in China ,.
To conclude, no scientific consensus is reached regarding the environmental safety of GMOs, takeaway evidence: http://www.enveurope.com/content/pdf/s12302-014-0034-1.pdf