GMO crops make it possible to use less herbicide and insecticide and yet increase yields. BT corn, for example, resists corn borer. Soybeans are easy to kill with spray. Fields of Roundup Ready beans are largely free of weeds. The Roundup does a good job of killing the weeds. Farmers would do the best they could with tillage and what pre emerge sprays were available before Roundup ready beans existed. Crews of migrant workers used to be a common sight. The crews were walking the beans with hoes to hand weed the fields. Crews were usually a group of men. I have seen whole families out there. Grandma and grand kids included. That's not necessary anymore. Soybeans are legumes. A normal crop leaves behind something like 45 pounds of nitrogen for the following year's corn crop.
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Glyphosate binds tightly to soil. It can persist in soil for up to 6 months depending on the climate and the type of soil it is in. Glyphosate is broken down by bacteria in the soil.
Glyphosate is not likely to get into groundwater because it binds tightly to soil. In one study, half the glyphosate in dead leaves broke down in 8 or 9 days. Another study found that some glyphosate was taken up by carrots and lettuce after the soil was treated with it.