Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hello Alli,
I can see that you have been doing your research. Let me simplify things for you a bit.
Non-digestible carbohydrates that you mention are the fibrous portion of grass and are digested in the hind gut by the bacterial flora that live there. Grass, being a roughage, is high in this type of carbohydrate and it provides calories for the horse for energy (to do work and keep vital organs functioning).
Non-structural carbohydrates, on the other hand, are the sugars, starchs, and fructans that you mentioned and they are digested in the foregut (small intestine), providing calories, as well. However, too much NSC can cause laminitis in some cases and can interfere with the insulin response in insulin-resistant horses.
The NSC content is highest in the afternoon once the grass has been exposed to sunlight. But, if your horses are healthy, and not overweight, there is no need to restrict their time on pasture.
Grass, being higher in water than hay, needs to be consumed at a larger rate to get the same amount of solids. It also is higher in many vitamins because hay is dried and exposed to the air for a long period of time (hence, vitamins are destroyed).
So, that's a summary in a nutshell. How this affects your particular horses, can vary depending on many factors. So, if you would like to discuss this further regarding your particular horses, please feel free to contact me for an appointment.
All the best,
Dr. Getty 
Where are you from? Bayfield, CO