Black Walnut: A tree native to northeast US and Canada. Compound leaves, spherical fruit with a green, thick husk. Toxic to horses when it is included in bedding; as little as 5% in the shavings can be toxic. Ingested nuts or hulls have also been implicated as causing signs. Signs of toxicity are seen within 24 hours after exposure, and include depression, inappetance, stocked-up limbs, rapid onset of laminitis. Increased temperature, heart rate, respiratory rate and mild colic may also be seen.