FEMA is, Ag isn't.
Ag is big though. It runs the foodstamp program. Foodstamps were originally justified as a way to deal with surplus agricultural production.
One clue to subservience level is the term "Department" which generally indicates a cabinet level agency, with the exception of the military branches, which are "Departments" under the Department of Defense (and that mainly as a carry over from when the Department of the Army and Department of the Navy really were independent departments). Under departments are usually agencies, adminstrations, or bureaus. For example, Federal Bureau of Invesitagion, Drug Enforcment Agency, and Bureau of Alchol Tobacco, Firearms, and Exlosives, all under the Justice Department.
The Secreary of Agriculture is Tom Vilsack. Here is the org chart for the Department of Homeland Security.
As you can see, agriculture isn't there, and Ag has it's own departmental web page.