Complementary feed
Complementary feed is a feed which, by reason of its composition, is sufficient for a daily ration only if used in combination with other feed. It is defined in Article 3(2)(j) of Regulation (EC) No 767/2009.
How is it different from complete feed?
Complete feed (Article 3(2)(i)) is sufficient for a daily ration on its own. Complementary feed rounds out other feed, so it is given alongside the animal's usual food, not instead of it.
How do you recognise it on a label?
The type of feed is a mandatory label particular (Article 15(a)): "feed material", "complete feed" or "complementary feed".
What may the label of such a feed claim?
Article 13 allows objective, verifiable and scientifically substantiated information about composition and nutritional characteristics. Article 13(3)(a) forbids attributing to a feed any effect on a disease.
Sources
Guide: How to read a dog and cat food label: a guide to Regulation (EC) No 767/2009
Educational information about the rules, not veterinary advice.