This condition (azoturia) is usually caused from excessive lactic acid build up in the muscles after exercise.
Horses that have a tendancy to do this should be fed as much good quality hay and roughage (preferably grass hay and not alfalfa) as they want. The carbs (grains) should be limited. If your horse is only ridden once or twice per week she should not be fed more than 4 pounds of grain per day and even less is better.
Many horses tye up due to dehydration, decreased glucose, electrolyte imbalances, vitamin and mineral deficiencies and even fluctuations in hormones.
Make sure to provide plenty of fresh water every day. If you live somewhere where the water buckets tend to freeze, condsider buying a heated water bucket.
If you are feeding excessive amounts of grain for calories, consider lowering the amount of grain fed and supplement with vegetable oil (up to 1.5 cups per day) or beet pulp or rice bran.
Make sure that your horse has a mineral block available at all times, or consider adding electrolytes to her diet. You should be able to get a packaged electrolye source from your feed store or vet.
Horses that suffer from low thyroid hormone also tend to tye up easier. Talk to your vet about doing a thyroid test.
Make sure that you warm your horse up before heavy exercise and provide an adequate cool down period after (lots of walking).
If you have not already spoken to your vet I suggest a call. Make sure that she is drinking plenty of water.
I hope this helps, let me know if you have further questions.
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My answer should not take the place of your Vet's recommendation.
Vicki CVT
