Monsanto Co. has found fertile ground in genetically modified crops and a global agricultural boom, but the world's largest seed maker also harvests its share of controversy.

To critics, it is a proponent of risky food and a symbol of a food system dominated by a few giant companies at the expense of small farmers. An attempted price increase in 2010 set off a backlash among farmers, and some investors have questioned the long-term viability of Monsanto's products as some weeds have proven resistant to its top herbicide and...