) species are a highly abundant group of 500 to 600 species of slow-growing, deciduous, hardwood trees; 60 of which are native to North America. Oaks are classified into two groups--red and white oaks. Both groups produce acorns, which are nuts with a tough leathery shell that fall from trees in the autumn season. Red oaks have pointed leaves with bristle-tipped lobes and white oaks have rounded lobed or large regular teethed leaves. Leaves are often clustered at the ends of twigs.
The buds, twigs, leaves, and acorns from oak trees can all be potentially harmful to horses if eaten.
Most poisoning cases have involved horses eating young, immature leaves in the spring and/or freshly fallen acorns in the autumn, as these contain the highest concentrations of condensed and hydrolyzable tannins. Members of the black and red oak species tend to have higher concentrations of hydrolyzable tannins than other oak species. Horses typically must eat very large quantities for 2 to 5 days straight to develop signs of toxicity. In horses, oak toxicity is characterized predominately by gastrointestinal (GI) disease, and less commonly, kidney disease and hepatotoxicity. Affected horses often present with varying degrees of colic and diarrhea.
Horses are most at risk of oak toxicity when they are turned out in pastures with little grass and access to fallen acorns from oak trees. Some horses may develop a taste for acorns, and actively seek them out in pastures.