We had 200 bales of first-cut grass hay delivered in September. In the last few weeks, several of our horses have developed sores on gums and tongue. What is causing this and, if it is the hay, why did it take so long to develop?

Mouth sores and ulcers can be caused by several different factors, including certain weeds and debris that might be found in hay. These causes include:
• Splinters, thorns, stickers in hay
• Toxic plants in hay or turnout area (e.g. Foxtail)
• Phenylbutazone (Bute) toxicity
• Blister beetle toxicity: associated with alfalfa hay and extreme colic and founder symptoms
• Vesicular stomatitis: a highly contagious virus that lasts about two weeks and results in loss of appetite due to a sore mouth.
The horses should be examined by a veterinarian to determine the potential cause of the mouth sores. If the horses have vesicular stomatitis, they should be kept at home and isolated from other healthy horses until the virus has fully run its course. Horses should not travel for at least 30 days after the ulcers are gone.
If the horses are being exposed to toxic weeds in the hay, the entire load of hay should be returned to the seller and replaced with clean bales. Many weeds will only grow in certain areas of a field (wet spots, outer edges, etc.), so it is common to have weeds in some bales and not in other bales from the same hay field and cutting.
Check your turnout area as well for weeds. If horses don’t have access to edible forage such as pasture or hay, they are much more likely to “taste test” weeds—even the prickly kind! Some weeds known to cause mouth sores are buttercup, mayweed chamomile, and downy brome (cheat grass). In addition, exposure to wilted oak leaves and acorns may also cause mouth sores. Foxtail can also cause sores in the mouth and should never be fed to horses.