If you pick your courses carefully, you maybe able to get away with changing majors costing you one or two classes. Pick classes that cover both majors, and try to avoid classes that only cover one. Switch as soon as you can, and keep a high GPA during the quarters before you switch. High GPA varies depending on which major, and how full they are.
Since you decided to only be within agriculture, it shouldn't be too hard to do that. Check out this page:
Cal Poly CAFES : Advising : Changing/Choosing a Major It is information for changing majors to the ag department. Read the page and requirements carefully, and bookmark for your future reference.