Here’s the answer: Genetically Modified Organisms (GMOs), like corn and soybean crops, designed to be glyphosate-resistent. Glyphosate is a widely used herbicide, like Roundup.
In recent years, agricultural practices have expanded ten-fold, and farmers have an understandable desire to be more efficient and productive with their yields. This means not spending time and energy (read: money) on manually removing misplaced plants, like milkweed between rows of corn. Glyphosate, like Roundup, was developed and is now sprayed across entire fields to rid the area of anything other than the desired crop. The crops themselves are genetically modified to resist the glyphosate. However, a simple breeze might allow the Roundup to carry outside of the crop area, so that even milkweed along fence rows will die.
How much do you know about GMOs? Click this infographic to enlarge: