I think the percentage of animals being euthanized is 50%. Euthanasia is the practice of intentionally ending a life to relieve pain and suffering. Animal euthanasia is the act of putting an animal to death or allowing it to die by withholding extreme medical measures. Reasons for euthanasia include incurable (and especially painful) conditions or diseases, lack of resources to continue supporting the animal, or laboratory test procedures. Euthanasia methods are designed to cause minimal pain and distress.
Euthanasia is distinct from animal slaughter and pest control although in some cases the procedure is the same. In domesticated animals, this process is commonly referred to by euphemisms such as "put down" or "put to sleep".