My horse had Blue Seal only throughout his life. I retired him to another barn which fed Purina but would feed the Blue Seal if I bought it, which I did. I got into a conversation with the manager of the local Blue Seal company store a few years ago. She told me Blue Seal is manufactured in Vermont. The production schedule is set up so that horse feed is not manufactured following something for another animal that has ingredients that can be toxic to horses.
Blue Seal has been in business in New England for over 150 years. They are now associated with Kent. Our store manager has been at this for years. She really knows her stuff, including competing products. .