Fritz's nervousness and digestive troubles have prompted me to learn more about feeds, and in particular NSCs - non-structural carbohydrates. We have a number of senior horses at our barn, and also a number who are insulin resistant, so I'm always trying to learn what works best for them from a nutritional point of view. I'm certainly no expert, but will try to summarize what I understand. NSCs consist of both sugars and starches and are the "quick-digest" carbohydrates that can raise blood glucose levels; structural carbohydrates are "slow-digest" carbohydrates that are digested in the hindgut. Here is a link to a good description of what NSCs are and how they function in the horse's diet, from Triple Crown feeds - read this first as it will help you, I hope, to better understand the rest of what I'm saying (I'm not endorsing Triple Crown, but their web site has excellent information). This link also has a good table of the NSCs for various feedstuffs.
I think the first think to remember is to look at the total picture - all the things your horse is eating - forage (grass and hay) and other feedstuffs as well as vitamins and minerals. It's important, I think, not to become so obsessed with one aspect that you lose sight of the total picture. And the information you have about what your horse is eating won't be perfect or complete, even if you regularly test your hay and grass, and get the best information you can about the other feedstuffs. All this means is that you can only do the best you can do, and it isn't necessary to be perfect.
I've asked Jason from Paradigm Farms (where our Lily and Norman are retired), who is an expert on these matters, to comment, and here's part of what he had to say (and check out the feeding and nutrition posts from Paradigm that are listed on my sidebar) - I've edited, so if you have corrections or comments, Jason, please add to this:
For most horses at most stages of their lives, a moderately high NSC feed MAY not be hurtful. However, if a horse has suffered ulcers or is prone to digestive upset it may be a wise idea to go the low NSC route. Too much NSC in the gut leads to acidosis which in turn is a stressor that leads to a predisposition for a whole bunch of maladies including ulcers, founder, laminae problems, colic, etc. Horses evolved to eat grass in small meals, many times per day. Even the lushest of grass pastures is generally lower in NSC's than most grains. Because of this, more often than not for horses in work I recommend an increase in the quality and quantity of hay before an increase in grain feeding. A "quality" horse hay in this instance would probably be a really soft grass mix containing moderate levels of protein (<=14%) and relatively low levels of fibre (ADF <=35 and NDF <=50). This sort of hay can be a trial to find . . . . Too much protein actually leads to a process called protein catabolization which steals energy away from where it is needed to break down the excess protein into nitrogen which is excreted as urea. It is not a healthy process; in humans it's why those who go on a very high protein diet DO lose weight and DO look terrible when they've accomplished it ! NSC's are basically carbohydrates which lack a fibre portion and which are rapidly available and rapidly digested in the gut. Their metabolic purpose is to provide a quick burst of energy while the structural CHO portion of the diet is digested and absorbed.To simplify, most feeds are made up of a percentage of each of these: protein ingredients (soymeal, corn distillers grains, etc.); grain (starch) ingredients (corn, oats, barley, wheat, etc.); fibre ingredients (beet pulp, alfalfa meal, soyhulls, etc.) and mineral and vitamin ingredients. [T]he only real control we have in selecting a feed for older or IR horses is to select one that has a relatively small grain portion and which achieves much of its energy from fat.
One thing that is interesting to me is the table of feedstuffs contained in the Triple Crown link. I was surprised to learn that oats, corn and barley are the highest in NSCs, and therefore feeds that contain these in their top few ingredients are likely to be high in NSCs. Jason points out that the particle size is important too - whole grains are lower in NSCs than finely ground grains, but this aspect is hard to take into account when dealing with commercial grains.
And here's another table of various widely-available feeds and their NSC values, and another table that lists Purina feeds. I have not independently verified these NSC values, and some of them may be out of date - the best way to find the NSC value for a feed you are using is to call the manufacturer - many of these numbers are not otherwise available. But always keep in mind that it's the total picture that needs to be kept in mind - what are your horse's energy needs - how much work are they in and how old are they; what other foodstuffs are in your horse's diet; does your horse have any metabolic issues, like tying up or insulin resistance that might affect what you feed; and what are your alternatives.
I'm still in the process of digesting (so to speak!) all of this and thinking about whether I'll recommend any long-term changes in Fritz's diet to his owner or whether I'll be making any changes to the feeds we use.