Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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Dr. Getty
Greetings Pam,
Thank you very much for writing about your experience.
When feeding hay free-choice, there are a few things to keep in mind.
-- The hay should be a grass hay, since offering alfalfa as part of the free choice mixture can result in too much of this calorie-rich legume. Alfalfa is an important component of the hay ration because it boosts the overall protein quality. However, depending on the horse's need, it should be fed between 10-30% of the total hay supply, and therefore, is best to feed separately, rather than part of a grass/alfalfa hay mixture.
-- A horse that is prone toward insulin resistance should, as you know, be limited in the amount of non-structural carbohydrates (NSC) consumed. This includes starch and simple sugars. There is no way to tell if a grass hay is low in %NSC simply by looking at it. So, having your hay analyzed is essential when feeding it free-choice to a horse that is prone toward obesity (and hence, insulin resistance). It is possible that your hay has an NSC level that is too high to be fed free-choice. The solution is not to limit feeding, since this results in cortisol release (stress hormone), which further exacerbates insulin resistance, leading to weight gain. Instead, consider having your hay analyzed by your local county extension service or by contacting Equi-Analytical Labs: www.equi-analytical.com. Look for a total %NSC less than 12% (%NSC = %WSC + Starch).
-- Check the %NSC of the concentrated feed you're using. Many "low starch" feeds are lower in starch than an oat-based feed, for example, but not low enough for an insulin resistant horse. NSC should be less than 12%.
You mentioned that you now have pasture available. If your nighttime temperatures are lower than 40 degree F, the grass is too high in sugar and starch to be safe. Once it warms up at night, allow grazing only during the early morning hours. The most "dangerous" time is late in the afternoon on a sunny day, because the grass synthesizes sugar and starch throughout the day due to sun exposure.
Best regards,
Dr. Getty 
Author of Feed Your Horse Like A Horse
Where are you from? Bayfield
How did you locate this forum? CO