Description Zesterra® is an all-natural supplement designed to support horses during times of stress, such as hauling, heavy training, foaling and during times of extreme heat or cold. Zesterra® is safe for all classes of horses including lactating mares and foals. Zesterra® stimulates appetite and water consumption safely and naturally by balancing the pH of the stomach. It can be used as a flavor enhancer for horses that reject water from new sources when being hauled to new destinations.