We noticed many beautiful yellow fields between Corvallis and Salem. What are they grown for? Areas in Oregon?
The leaves in the picture down look like turnip leaves. While the flowers are similar, mustard has leaves that are more serrated along the edges (see the Wikipedia article on white mustard for some example photos).
Turnip seed is grown on more acres here than mustard seed. Most of it is used to plant cover crops and forage for livestock, with a sizable share of the seed sold in the mid-west. See this news story: Those yellow fields of spring.