Personally, Id rather put effort into finding and pushing for non GMO feeds. Almost impossible to find as far as Im aware and I hate them and do not wish to support their use at all if I have a choice. I want a choice.
Horse feeds should be safe because horses in the wild would forage for wild grains. Blood and urine tests are all that are needed to determine if a particular formula is supplying the horse's needs. With dogs, that's a whole different matter because dogs are meat eaters that have been put on grain diets and what goes in to some brands would stand your hair on end.
For what its worth, if any product advertises itself as "not testing on animals" what that means is that they used known sources or outsourced testing. FDA requires most products we use on our hair, eyes, etc be safe and not kill us. This means animal testing. They can outsource testing so the company itself is not doing the testing or they can use known sources. Known sources are products that were tested years ago. There are a few genuine replacement tests one of which uses horseshoe crab blood for testing coagulation. There are modified LD50 tests where you use 10 dogs or 10 mice instead of 100. At the end of the day, if you decide to go "cruelty free" you had best plan to stop using the following items among others: all medication, make up, shampoo, medical supplies (band aids, casts for broken bones, cleaning supplies) and most dyes that go into food. I heard all this from a leader in animal research and honestly it makes sense. If we did not test things we would seriously injure ourselves and still gasoline should be used topically.
With dog foods Purina does a lot of testing, most of this testing is giving dogs free choice food for life and seeing which one the prefer. The dogs that are tested are class B animals which means they are bred for research. There are only a few class A dealers, I believe four total. Class A animals are pets that are taken from shelters and sold into research, from which any animal that is microchipped is disallowed and some are illegal in some states. Anyway, the point is that these dogs were bred for the specific purpose of seeing how their life health is affected by eating one kind of food. They are not foi gras geese.
Most companies, even small ones outsource their testing. This is because the 1 million they might spend in testing is much cheaper then then 100 million they loose if someones pet dies because it at their poorly researched product. At the end of the day, if you want to know where food came from grow it yourself.