How sensible is it, putting the horses out to pasture on a early-spring morning? Most people realise now that it is not the frost on the grass that causes laminitis but the build up of fructose that plays the main role. It is now also known that the dangers are not restricted to the days of early spring but that the fructose levels can become dangerously high at other times of the year.
Sunlight is the motor that drives growth – if there is sun but there is a shortage of either food or moisture or the temperature is too low, then the plant cannot grow and the sun energy is stored as fructose. If there is a store of fructose in the plant and all three basic factors are fulfilled, then when the sun no longer shining, the plant is still capable of growth and will use the fructose to do so.

So when is it dangerous? If the ground is not sufficiently fertilised and there is a shortage of food; if there is a dry spell and there is insufficient moisture in the ground; if the daytime temperature remains below 15˚C. The latter is possibly clarified in the following table:

Weather Plant Metabolism Fructose Levels
Night Temp Day Temp Sun Photosynthesis Growth morning afternoon evening night
<0˚C <0˚C No No None 3 3 3 3
<0˚C <0˚C Much Much None 5 5 5 5
<0˚C <15˚C Much Much Low 5 4 4 3
>5˚C <15˚C Much Much Low 1 3 4 2
>15˚C >15˚C Much Much High 1 2 3 2
>15˚C >15˚C No Little Much 1 1 1 1
The risk on a scale from 1 (low) to 5 (very high)

BUT! This is only part of the story… When the grass has been fertilised, there should be enough food – but more growth means more grass which in itself may not contain so much fructose, but when all added up together does. We also need to look at the insulin resistance of the individual horse: some horses do seem to be more sensitive than others; often the simpler breeds – the so-called cold blood types – are indicated as being the most sensitive. It is clear that the most (and most intense) cases of laminitis occur in the springtime. Oft this is the result of horses being kept off any grass between November and April. And even in the summer, they will be found more often stabled than outside and this means the metabolism of the horse is completely unused to grass. Laminitis is caused by insufficient ability to purge the body of toxins (fructose is actually not toxic in itself but for the metabolism of the horse it is); for instance, mares that have recently foaled and where the placenta has been (partially) retained, can also suffer acute* laminitis as a result of the toxins that are released by the (decaying) retained placenta. Various medication can also cause laminitis in sufficiently high doses – phenylbutazone, or “Bute”, comes immediately to mind. In conclusion: horses belong outside, 24/7, should be exposed to sufficient variation in grass, hay and other grain-free vegetation and should not be “well fed” during the winter months; in this way, the metabolism is constantly exposed to all manner of various feedstuffs whereby change will have a much less (intense) effect and a natural loss of weight in the winter period forms a break in the insulin resistance cycle. Sadly, this is not always possible for horses in livery yards and therefore it is essential to ensure the horse is given regular exposure to those things that are so dangerous – this will give a certain level of resistance. Additionally, horses should never be fed muesli or essentially any form of commercial feed – this is a source of damaging sugars (even so-called “sugar-free” products), disrupts the natural functioning of the alimentary canal and also causes physical damage to the fine hair-like structures in the intestines; the producers of horse feeds and many dietary “experts” will contradict this, but certainly for the feed producers, they have a genuine desire to sell their products! Under certain circumstances, this disruption can be enough to trigger laminitis. A working horse has a much reduced risk; not that the risk is completely eradicated, but when through work the horse can process and excrete the fructose, then the risks are much smaller. This problem is obviously not completely solved since a return to a sedentary existence in a grassy pasture will increase the risks once more.

*Acute is a medical term that describes a rapid onset – it does not refer to severity.

This page was originally published in Dutch in a slightly different form in March 2014 on the Kobolt Equine website.