Not all carbs are created equal
This is NOT about calories, or the amount of Digestible Energy. We need to focus on the FORM of those calories - on each individual carb fraction. Sugar causes glucose and insulin surges that may trigger a cascade of effects in animals with abnormal glucose metabolism. Too much starch may spil into the hind gut, causing a bloom of lactic acid producing bacteria. Fructan is getting a lot of attention these days, because it too, can cause disruption of bacterial population dynamics in the equine cecum. For more information on fructan.....
Even if a greater amount of DE and similar amount of total carbohydrate is fed to ponies in the form of alfalfa, blood glucose levels will be 30% higher from a meal of oats comprising less DE. Plasma insulin increased with glucose entry rate. [Argenzio cit. ] Some forms of carb are safer than others. It’s the sugar, starch and fructan that we need to limit. Even if your horse is not an easy keeper, you can STILL fatten a horse without having to resort to a high sugar and starch (grain and Supergrass) diet by choosing feeds higher in safer forms of carbohydrates like pectin and hemicellulose that ferment more slowly and do not cause a glucose/insulin surge.
The dairy industry have come up with some practical methods for measuring sugar and starch in the ration, and they’ve found that by limiting Non Structural Carbohydrates to < 38-40 % of the ration, the incidence of laminitis is greatly reduced in producing dairy cows. Here’s a site that describes the difference between NSC and NFC. It's in reference to cattle ration formulation, but remember that dairy folks are way ahead of the horse folks on the learning curve about carbohydrates in rations.
Pectin is another type of carbohydrate found in the NFC fraction. It has been found to be a safe source of calories for laminitic horses. Pectin is very low in grass, but is found in alfalfa at levels of 10 -15% of dry matter (Hatfield pdf). Pectin can be very high in feeds like unmolassed beet pulp and soy hulls, and for that reason these ingredients are often main components of feeds designed for laminitic horses. If a horse is intolerant of sugar or starch, but cannot maintain enough weight on hay alone, feeds containing beet pulp, soy hulls, and/or mature alfalfa are often beneficial as a safe alternative to grain or grain products.
The Laminitis Trust in England has developed a strategy very similar to the dairy industry for developing feeds safer for laminitic horses. The scope and intent of the recommendation is to be commended, although the allowable % of NSC may vary in individual horses. Because nutrient content varies so much we need to get actual analytical tests done rather than rely on book values. I’d very much like US feed manufacturers to adopt a similar certification process.
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