Executive vice president of strategy and operations Carl Casale, 48, is due to take over from Crews from September 1 with Crews continuing to work as special assistant to the CEO until November 30.
Crews said: "This company has a bright future with strong investments in places that can make a difference for farmers, our people and our shareowners. After careful consideration with my family, I've decided this is the right time to cap my long career and ensure a successful transition as the company embarks on its next decade of innovation."