Poultry and poultry products shall be exempt from the provisions of the Federal Food, Drug, and Cosmetic Act [21 U.S.C. 301 et seq.] to the extent of the application or extension thereto of the provisions of this chapter, except that the provisions of this chapter shall not derogate from any authority conferred by the Federal Food, Drug, and Cosmetic Act prior to August 18, 1968.