Remarkably effective at keeping deer, rabbits and other herbivores from browsing in yards and gardens, this powerful, long-lasting repellent works so well because it triggers their instinctive fear of predators. It consists largely of dried, powdered blood from agricultural sources. When mixed with water, it emits an odor that is innocuous to humans, but to most mammal pests it suggests recent carnivore activity, generating an immediate flight response.