Trouble for Horses in Paradise: Toxicity and Fatality Resulting from the Consumption of Indigofera spicata (Fabaceae) on Oahu Island

Abstract

This is the first account of fatal toxicity in horses resulting from grazing on the pasture plant creeping indigo, Indigofera spicata, on the island of Oahu, in the Hawaiian Islands. A survey in the town of Waialua on the north shore of Oahu island indicated that creeping indigo is common and abundant on grazing lands during the rainy season and requires intensive chemical and physical control measures. Four pastures were surveyed where ranchers reported mortality of more than 17 horses since 2020. We document these incidents to alert state animal and livestock support officials, groups monitoring invasive species, and horse owners regarding the problem of this noxious weed and to support breeders with information to confront its invasiveness. Herbicide treatment is not economically feasible, and breeders opted to physically uproot the plants from the paddocks and restrain horses to clear pastures as they were eliminating the plants. We urge state officials for a long-term control strategy to reduce the problems associated with this weed.