Article on metabolic issues.

Why the grass is not always greener…

Horses eat grass, right?  Right… but it may not be what is best for them particularly if your horse has metabolic issues.  There is a lot of overlap between the metabolic conditions of PPID (Cushings), Insulin Resistance (IR) and Equine Metabolic Syndrome (EMS) and so correct diagnosis is important as they are treated differently.

Equine Metabolic Syndrome (EMS) describes a characteristic collection of clinical signs and changes in equids.  Affected animals are typically obese, but not always and have abnormal fatty deposits in the neck and tail head areas.  Chronic and acute laminitis is common and the horse may also be insulin resistant.  EMS may be the end result of an inability to properly metabolise dietary carbohydrate and can occur at any age from around 5 years old with native breeds being most commonly affected, but the underlying reason why some horses develop it and some do not, is not fully understood.  However, there does seem to be a genetic disposition, with affected horses possessing a ‘thrifty’ gene that enabled their wild ancestors to survive in harsh environments.  Our modern environments with plentiful nutrient dense food stuffs has caused this genetic survival mechanism of increased efficiency of energy metabolism to become maladaptive resulting in increased occurrence of this debilitating condition.

Excess body fat secretes inflammatory substances known as cytokines which can damage the hypothalamus, the portion of the brain that recognizes leptin.  Leptin normally tells the brain that enough food has been consumed and to stop eating; this is the body’s way of maintaining normal weight.  In the leptin damaged horse (as a result of long term EMS or insulin resistance) the horse keeps on eating, his appetite is never satisfied and so weight increases; a vicious circle.  The common practice for EMS horses is to severely restrict food intake but often this does not remedy the situation as food deprivation results in a stress response on a cellular level acting to raise cortisol levels.  Raised cortisol levels results in even more fat being stored by cells.  A long term and more appropriate solution would be to offer suitable low calorie low sugar forage on a free choice basis.  This will act to stabilise the hormonal response to restricted forage and allow the horse to return to a favourable weight.  This may take longer than if forage is restricted to 1-1.5% of bodyweight but the end result will be a long term solution. 

What is vital however is that carbohydrate intake is minimised and exercise is increased.  However if laminitis is also involved medical therapy may also be required due to the inability to exercise the horse.  Our good quality British grass has high levels of carbohydrate in the form of sugars, as has good quality hay.  The non-structural carbohydrate (NSC) content of forage should be determined by analysis and should be below 10% of the forage dry matter.  Well-made haylage should have the analysis available and be below 10% combined sugar and starch, whereas hay may need to be soaked to remove excess sugar, however this may be an unreliable method to use due to the range in quality of hay and the time spent soaking.  Soaking both hay and haylage will help to reduce calorie content but some late cut haylage such as our Timothy with a DE content of around 9MJ/kg and combined sugar and starch of just 5.1% may be suitable. 

Insulin Resistance is a reduction of the horse’s ability to respond appropriately to the hormone insulin.  Obesity, as a direct result of excessive calorie intake is the primary cause of IR and it is this excess fat or ‘adipose tissue’ that as we touched on earlier produces a range of hormones and inflammatory mediators (cytokines).  So, again the message is to bring about a gradual long term reduction in weight whilst reducing the inflammatory stress response through appropriate free choice feeding of low calorie forage.

Pars Pituitary Intermedia Dysfunction (PPID or Cushings) is caused by an enlargement of the pituitary gland leading to increased production of several hormones causing adrenal glands to increase production of cortisol. EMS may be a predisposing factor for PPID and so affected animals should be monitored for the onset of PPID.   PPID animals are characterised by long curly coats, unexplained bouts of laminitis, lethargy, decreased immune function, abnormal fatty deposits above the eyes and muscle wastage.  This condition can be controlled by medication.

For all of the above conditions it is vital that the horse receives balanced broad spectrum vitamins and minerals.  It is particularly important to watch the amount of iron your horse consumes as excess iron exacerbates IR and forages (pasture/hay/haylage) generally contain substantial amounts; it is critical that it is balanced with other copper and zinc and so therefore mineral supplements must be chosen carefully.

The take home message is feed a horse like a horse; think moorland and savannah containing plentiful low grade fibrous herbage fed on a trickle like basis.  Do not let your horse get overweight and exercise regularly.