HMRC's national advice line tends not to advise and some of the above responses are correct but for the wrong reasons!
In fact many 'food' products are not zero rated, hence high profile legal cases concerning VAT and Jaffa cakes amongst other exciting things.
In the case of animal feeds, food doesnt qualify for zero rating if it is held out as pet food.
Horses are not classed as pets by HMRC so unless their food is sold specifically as branded for pet horses it will be zero rated. Spillers etc's food is generally not classed as being marketed for pets so should be zero rated.
I have attached an extract from HMRC's public notice on Animals and animal feeds, it states:
6.3 Non-pet species kept as pets
Some animals that are not pet species may be kept as pets, such as:
chickens;
horses and ponies;
rabbits;
reptiles; and
sheep.
Their food can be zero-rated unless it is packaged or held out for sale in a way that shows it is intended for a pet.
Oh well, back to the day job drafting much much more on VAT....