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A study1 from 2017 has been frequently posted in forums and social media, which is supposed to prove that horses at risk of laminitis can be put out to pasture without any problems, as long as the field is regularly topped. Unfortunately, it is often the case that only the keywords in the headline or the summary are read, without anyone bothering to actually read the study in full. The facts would then present themselves somewhat differently.

To do this, you first need to understand how studies are generally being put together. Especially in complex biological systems such as a pasture habituated by a herd of horses, there are an infinite number of variables that can have an influence on the result. In order to find out whether the measure you want to investigate is actually the cause of the measured values, you have to try to turn as many variables as possible into constants. If you end up with just one variable that you specifically change, then this must be the cause of the measured result.

This starts with the fact that horses of only one breed are usually used, which is why almost all tests in Germany are carried out on warmblood horses and in the USA on Quarter Horses. It is impossible to say whether the Shetland Pony, the Trotter, or the Shire Horse, with their somewhat different metabolism, may react completely differently. In feeding studies, a grass monoculture roughage is usually used to minimise the influence of different plant species. As everyone knows, herbal plants in particular have a considerable influence on the metabolism.

In the cited study, an attempt was made to reduce the variables in the vegetation by placing the horses (6 adult stock-type geldings) on meadows sown exclusively with tall fescue (Lolium arundinaceum). This is absolutely correct in terms of the design of the study, as the variables of the different plants and horse types are already excluded.

The sugar content in the grass and the blood sugar levels in the test horses were then measured at different grass lengths (which were achieved by topping). It turned out that there were lower sugar levels in the topped grass and lower blood sugar levels in the horses on the previously topped areas compared to the horses on the tall grass (topped to a length of 30-40cm)

The result could now lead you to think that you simply top your meadow and then the sugar content will be fine. But the devil is in the detail.

Tall fescue plays no or only a very minor role in Central European pastures. The picture shows why:

Tall fescue with scale
© Die Gute Pferdweide

Despite growing to a height of almost 80 cm, this typical specimen has just two nodes. This means it can grow twice as tall. Hence its name: Tall fescue. Take a look at your field and analyse how many grasses there are that are around 1.50 metres tall when in flower. Probably not too many.

If you top this grass down to a height of 10 cm, the remaining stalk will no longer contain any chlorophyll, which is easily visible from the pale colour in the picture. Chlorophyll is the green leaf pigment and is responsible for photosynthesis, i.e. the production of sugar from water and carbon dioxide. So far, the results of the study are conclusive, because it is not surprising that there is hardly any sugar left in this short, almost chlorophyll-free piece of grass in the days after topping.

On closer inspection of the stalk in the picture, however, you can already see fungal growth up to approx. 18cm. The Midwest of the USA has a territorial climate. This means either dry or wet, making it ideal for tall fescue. The more or less permanently moist maritime climate of Central Europe, on the other hand, favours fungal diseases on tall fescue. In this typical young plant in the picture, the fungus has already progressed to a height of 18cm. Even if you have sown your pasture exclusively with tall fescue and topped it before grazing, the horses would ingest little sugar but a lot of mould, which is not good for their health either.

As tall fescue fortunately does not play a major role on our pastures, the study is factually correct, but not transferable to the conditions we are dealing with in Europe. If you look at our meadows, which mostly consist of different types of grasses, most of which remain significantly smaller in growth, then you notice that they can have high sugar contents in the lowest 10 cm.

Therefore, such a recommendation is rather dangerous for our mixed meadows. If the remaining stalk is still green after topping, the plant immediately starts producing sugar again, causing the sugar levels to rise rapidly, as the plant wants to grow again and needs sugar building blocks to form cellulose as a scaffold substance.

Based on practical experience, a conversion of the sward to pure tall fescue is absolutely not recommended in our climatic conditions due to the risk of fungal infestation in the lower part of the plant.

It is also not advisable to graze our meadows with mixed plant populations shortly after topping, as the (shorter) grasses that are mostly native to Europe can immediately start producing sugar again.

The only way to manage the horses appropriately, especially horses at risk of laminitis is by paying close attention to the correct height of the grass growth (lengths) and if necessary, the additional use of a gazing muzzle.

Written by guest author Helmut Muß, ‘Die gute Pferdeweide’

Sources
Siciliano PD, Gill JC, Bowman M, Effect of sward height on pasture non-structural carbohydrate concentrations and blood glucose/insulin profiles in grazing horses, Journal of Equine Veterinary Science (2017), doi: 10.1016/j.jevs.2017.06.004.

Helmut Muß