Any feeds or supplements designed for cattle should NEVER be fed to horses. Many additives included in cattle supplements can harm or even kill horses. For example ionophores (monensin, laslocid, or salinomycin) are common ingredients used in cattle feeds to promote growth. These, however, can be lethal in horses. Each year horses are killed due to ionophore toxicity from eating supplements intended for cattle. Another common ingredient in cattle supplements is nonprotein nitrogen (urea). Too much urea can kill horses.