What is one of the problems with improving the food supply by genetically engineering better crops?

Some crops are genetically engineered to resist certain herbicides, which means the herbicides they are engineered to be resistant to can withstand having the herbicide sprayed directly on the crop. A known problem with these crops is that after a number of years, more herbicide is needed to control weeds. Because of this, more herbicides are sprayed on the crops and end up in foods. The safe limit for glyphosate (Round-up) in foods has been raised by the EPA in the United States and the use of Glyposate on crops has doubled in the years since Glyphosate resistant crops have been commercially grown.

Other crops are genetically engineered to produce an insecticide in the plant itself. Though the insecticide is suppose to target certain insects, two concerns are that non-targeted insects are being killed and people's and animal's health may be affected when those crops are eaten.

Generally, some believe horizontal transfer of the engineered genes in the crops may occur. Another problem that has occurred is weeds have developed resistance to the herbicides crops have been engineered to resist. The absence of long-term studies concerns others, making some feel that they are the long-term study.