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The Grass Affected Horse Explained

The Full Story

Of course horses eat grass - but not all grass is suitable for horses!


The ‘Grass-Affected’ horse has health, movement and/or behaviour issues caused by ​one or more aspects of his forage.

Problems are either compounded or helped by what is added or not added in the way of hay, feeds and supplements.


There is a spectrum of indicators which range from mild ‘behavioural’ issues to very dangerous ones, from mild health issues to very serious ones.

To establish whether your horse is affected in any way, go through the Health Check-List.

Significantly, the symptoms list for PSSM is virtually identical to those on our Health Check-List, not surprising when you understand that diet and environment determine gene expression.

In fact wherever you see the words 'idiopathic', 'myopathy' or bartonella, consider that the horse is simply Grass Affected and make it a priority to first address his diet.


Summer & Autumn grazing for wild/feral horses; a very different kettle of fish to most domestic horses!

Photo Credit J T Humphrey

Such issues are outward signs of internal disturbances to the horses bio-chemistry and are therefore ‘metabolic disorders’. Resolving these issues is principally about correcting the horses environment and nutrition.

It is essential for the welfare of the horse, not to mention the safety of the handler/rider; that these ‘metabolic disorders’ are resolved BEFORE any training, riding or breeding begins.


A ‘Grass-Affected’ horse cannot process information so is not able to be in a 'learning frame of mind'.

When thus ‘Affected’ it is a waste of time, unpleasant for the horse and potentially very risky, to be trying to 'work through' any of these issues because they are not training issues. Rather they are bio-chemistry issues affecting the horse's nervous system including all his senses: his eyesight, his hearing and his proprioception, his co-ordination and ability to properly operate his muscles.

Hence not only his perception is altered, but also his awareness and his sensitivity. When he is thus affected he will be having a whole different experience to what he should be having and also to what you think he is having!

He will tend to ‘react’ instead of ‘respond’. He is not being naughty, difficult, or  resistant. He just needs to be put somewhere safe on suitable forage for the time it takes for his metabolism to return to normal again.

Neither is it a matter of tiring the horse out (the ‘wet saddle blanket’ approach) because 'grass affected' horses have mineral imbalances, not simple excess energy from high sugars and hence do not improve with exercise. They are more likely to get themselves in a lather and keep going until they drop.

For over-reactive, hyper-sensitive horses, body-work and other therapies are best postponed until the chemistry of the horse’s nervous system is back to normal. Some horses cannot stand to be touched and will be aggressive towards people; they are best untouched apart from basic care over this time, usually 7-10 days.

We say “GRASS” affected because grass is the staple forage of most domestic horses and therefore usually has the biggest influence.


*For more about this important topic, read Aspects of Pasture and also Forage & Feed

For specific help with your horse fill out the Enquiry Form

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