Increasingly, human foods are being labeled with figures on their glycemic index (GI) and this concept is also being used to categorise horse feeds. There have been a number of recent studies of the GI of horse feeds, but some of these have produced conflicting results and more work is needed before the position becomes clear.
The term “GI” is a measure of the amount of glucose in the blood after a meal. The glucose is derived from the digestion of starch and sugar in the small intestine and secondarily from the fermentation of fiber and overflow of starch or sugar in the large intestine. It is measured as the area under which the blood glucose curve after a meal, with a standard amount of starch and sugar. For comparative purposes, oats is given a value of 100 and other feeds are compared to oats as a standard.
In general, feeds that have a higher starch and sugar content have a higher glycemic index and hence stimulate higher insulin responses. However, if the starch is not well digested in the small intestine, a lower glycemic index would be expected.
Because of its lower pre-caecal starch digestion, raw or cracked maize, which has a higher starch content than oats, is rated as a lower GI feed. If you apply the same logic, heat processing of grains such as barley and maize, will dramatically increase the pre-caecal digestion of the starch, which should also increase the GI. But research in this area has produced conflicting results, and it seems that a high pre-caecal starch digestibility is not always followed by a high GI.
Recent research in the U.S. has shown that there is a relationship between abnormally high glycemic responses to feed in young horses and the incidence of clinical OCD. In the major study conducted, the incidence of OCD was recorded as both clinical and sub clinical OCD that was treated by surgery, and averaged over 10%. Figures this high in yearlings are seen in countries with x-ray repositories such as Australia and the US.
So, if you are looking to minimize the occurrence of OCD, will the use of a highly digestible feed such as Capstone Stud Time be risky? Not at all, if it is used sensibly! Whilst the micronising process could increase the GI of the barley and maize in Stud Time, the feed includes a blend of other ingredients such as lupins, sunflower seeds, full fat soyabean, lucerne chaff and the Life Time balancer pellet which are lower GI ingredients.
Sweet feeds that contain very large amounts of molasses have a very high sugar content and tend to be eaten rapidly, and these feeds are often shown to have very high GI. Stud Time has a much lower molasses content than other popular breeding feeds used in South Africa and also has added oil, which will slow stomach emptying and reduce the GI of the overall feed.
Meal size and rate of intake are important contributors to GI. As lower quantities of the highly digestible Stud Time are needed for
optimum growth and condition, smaller meal sizes are expected when compared with conventional feeds. Slower rates of intake of the hard feed, which occur when chaff is added, can also dampen GI responses.
Body condition has a key influence on the glucose and insulin responses to a grain meal, and horses that are fat tend to have an exaggerated glucose and insulin response. Carrying extra body condition and weight is a key risk factor for the development of OCD. This has been shown in a number of studies, including the major GI and OCD study.
Remember that micronizing grains is a major advance in terms of protection of the sensitive balance of micro-organisms in the hind gut because little soluble carbohydrate enters the hind gut disrupting normal gut, function, lowering hind gut pH, and potentially leading to colic and/or laminitis.
In summary, it’s not as simple as saying that just because a feed contains highly digestible micronised maize and barley, it must be a high GI feed and have an increased risk of OCD when fed to your horses. Capstone Stud Time is the best feed available for breeding horses in South Africa, and if used as recommended, will give great results. Remember that feeding is only one of the risk factors in relation to OCD and is not the major cause!