California Code, Vehicle Code - VEH § 34500.3

(a) The department shall adopt rules and regulations that are designed to promote the safe operation of vehicles, regarding cargo securement standards.  The regulations adopted pursuant to this section shall be consistent with the securement regulations adopted by the United States Department of Transportation in Part 393 (commencing with Section 393.1) of Title 49 of the Code of Federal Regulations, as those regulations now exist or are amended in the future.

(b) Regulations adopted pursuant to subdivision (a) do not apply to a farmer transporting his or her own hay or straw, incidental to his or her farming operation, if that transportation requires that the farmer use a highway, except that this subdivision does not relieve the farmer from loading and securing the hay or straw in a safe manner.