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 …