This depends on both the metabolic state of the horse(s) and the suitability of the pasture...
Unsuitable pasture will cause metabolic (health, movement and/or behaviour) issues in any horse.
How ‘close to the brink’ of metabolic dysfunction the horse is will have a bearing on how long his metabolism will cope with unsuitable forage.
This is why previously ‘bomb-proof’ horses can turn into ‘fire-breathing dragons’ virtually over-night when sold to a home with unsuitable pasture. Or why a horse who was on a high Lucerne diet at his previous home suddenly becomes a head-shaker at his new home.
On the other hand, a horse who has serious metabolic issues can be given access to suitable, what a farmer would describe as ‘poor quality’ grass (meaning relatively low potassium, nitrogen, sugars) and he will tend to get better over time – he may need some nutritional assistance but he will improve.
Pastures most likely to cause problems include…
• Dairy pasture – rye/clover
• Any of the high production grasses – avoid any strain of rye-grass – endophyte-free or not
• Lush meadows (as in the above picture) – horses are mono-gastrics which this means their diet should be predominantly GRASSES.
• Short or lush GREEN pasture – especially when growing in the shade
• Fertilised pasture
• High oxalate grasses
• Short, stressed over-grazed or heavily frosted pasture
• Mono or duo-culture swards
• Grass growing on land which tends to stay wet all year round
• Green shoots after drought breaking rain
• Grass growing on land that has been flooded or after a fire
• Brand new grass not previously grazed by other livestock or cut for hay and then left to harden off
Least likely to cause issues are…
• Pastures growing on slightly acidic soils
• Mature grass (as opposed to vegetative)
• Pasture on dry stony ground
• Unimproved pasture
• Tussock country
• Grasses grown with a long rotation eg regenerative agriculture
• Large variety of GRASSES (as in the species listed here - Desirable Species seen in the picture above)
• Dry grasslands
The QUANTITY of grass relative to the size/metabolic state of the horse also needs to be taken into account.
A lot of the advice people are given is aimed at promoting the health of the pasture (eg sowing clovers amongst the grasses for its nitrogen fixing properties) without understanding the consequences for the health of the horse and safety of the rider.
The ‘health’ of the pasture does not necessarily correlate with the health of the horse. For instance horses grazing ‘healthy pasture’ are still very prone to laminitis, EMS, head shaking, tight muscles, behavioural issues etc.