Over the last 10-20 years or so, the concept of “grass affected” horses has taken hold. This label covers an enormous range of symptoms (excitable behaviour, neurological issues, itch, colic, aggression, musculoskeletal issues… pretty much anything aside from traumatic injury or infectious disease!), all apparently caused by grass.

The big picture explanation of this is that horses did not evolve to eat grass like they do now, because they evolved in semi-arid grasslands and steppes. We typically think of the Przewalski’s horses in Mongolia living in places that look like this…

But the Mongolian grasslands can also look like this…

And sometimes in between like this…

It’s not always dry, mature, stalky grasses.

This presumption also discounts the fact that horses migrated and did extremely well all over Europe in a very wide range of climates that ranged from sub arctic, through temperate, semi arid and even sub tropical over the 100,000s of years that they were hanging about. This very complicated paper suggests that at the dawn of domestication horses in Europe were much more environmentally adaptable than has been previously believed based on Asian horse fossil data.

My point is, that while horses are very well designed to survive on coarse, very fibrous grasses – they are equally designed to cope with the new young growth that occurs in every environment at certain times of year, and apparently able to cope with temperature/warm and moister environments also, which would support more nutritious grasses for a longer growing season across the year.

Of course, wild horses in some areas had to cover much larger distances every day to get feed and water, faced predators, harsh winters without cosy shelters and generally had more difficult lives than our domestic horses. We have also bred extremely productive modern grasses in the last century which contain more nutrition than wild grasses ever had. This combination can spell disaster in certain situations – particularly in those breeds that are closer to their primitive types. Insulin resistance was beneficial to these horses to make the most of the good times, but is another topic altogether!

But the majority of horses that tend to be labelled “grass affected” seem to be (in my experience) the more “modern”, lighter breed horses that are further away from the primitive horse – thoroughbreds, warmbloods, quarter horses. The sorts of horses that originated in the less harsh parts of the world, that would have been less exposed to harsh conditions and had access to better feed more often – so (and I’m just theorising here) should be more tolerant of modern grasses.

So, the evolutionary background of this theory is, in my opinion, a bit shaky.

When I first started hearing about “grass affected” horses (when I was 19 and had a headshaker), it was all about high potassium levels in the grass (and lucerne hay).

Potassium is an extremely important mineral and electrolyte in the body - it's involved in the maintenance of acid-base balance, osmotic pressure (fluid control) and a crucial ion involved in neuromuscular excitability (electrolyte = conducts electricity when dissolved in water).

Luckily for horses, potassium is ABUNDANT in their natural diets, and according to the NRC Nutrient Requirements of Horses, it is "normal" for potassium intake to greatly exceed the estimated minimum requirements.

I strongly agree with this - any forage based diet I've every analysed has anywhere between 5-10 times more than the text book potassium requirement of between 25-50g. This is because all forages typically contain at least 1-2% potassium - if a horse consumes an average of 10kg of this forage over a day, its daily intake will be a MINIMUM of 100g. Most grass and hay analyses I see actually have between 2-3% potassium, which ups the daily intake to 300g, just for the base forage portion of the diet!

Again, luckily for horses, they are very efficient at excreting potassium through the kidneys - and the more that is consumed, the more the rate of excretion is increased through urine, faeces and sweat. This is a totally normal process. Sweat is a huge factor in potassium loss, and potassium deficiency can be a problem in heavily exercising horses, especially in hot humid climates - as horses do not have very good mechanisms to conserve potassium.

So the horse's system is very prepared for the "excess" of potassium it naturally encounters day to day. The NRC suggest that the maximum amounts listed are in fact nowhere near levels that could cause toxicity, and potassium excess has not been reported in the scientific literature to date.

There is a genetic syndrome - hyperkalemic periodic paralysis (HYPP), which causes horses to be very sensitive to potassium, and it is recommended to keep their diets under 1% potassium (100g - note this is still 2-4 times higher than the minimum requirements).

Interestingly, veterinary experts in treating HYPP do not recommend restricting grazing fresh pasture, as even though it can be high in potassium per kg DRY MATTER, the fact that pasture is usually 80% water means that the rate of consumption is much slower than a dry feed/hay, and thus will not produce high blood potassium levels.

And finally - falsely high serum potassium levels are EXTREMELY common due to blood sample handling errors. I previously worked in a hospital pathology laboratory, and every day there would be samples that needed to be recollected or the potassium result discounted - this is due to haemolysis of the sample, which is where the red blood cells begin to break down and leak their contents into the serum. This can occur due to collection technique, if the sample is not correctly agitated post collection, or if it is left for too long before spinning to separate the red cells from the serum.

Horse blood is even more prone to this issue than human, as they have very high levels of potassium in the red blood cells - so even a very slight amount of haemolysis can result in an elevated potassium result.

So, back to “grass affected horses”. It is true that rapidly growing grasses have higher potassium levels than dormant ones, and that lucerne tends to be a little higher than grasses, but the difference means little in the context of ALL forage being high in potassium! This rather scientific theory of excess potassium causing all manner of behavioural and neurological problems simply doesn’t stand up in healthy horses.

Over the years, I have noticed that “grass affected” could also apparently be caused by high protein/nitrogen/nitrates. It’s explained that modern grasses have high levels of protein because this is beneficial to ruminant livestock, who can make better use of high protein levels and convert ammonia to protein via the rumen bacteria. Horses, on the other hand, cannot. But non-protein nitrogen contained in forage is rarely ammonia, but nitrates/nitrites (the biochemistry between these two is super complicated), which are produced under rapid growth that follows stress – not normally in healthy pastures.

Nitrates are plant compounds found in many types of plants which can convert to nitrites in the horse's digestive system. Nitrites can inhibit oxygen use in animals and can be toxic, as well as interfering with iodine.

Typically, horses are not considered very sensitive to nitrate/nitrite toxicity (compared to ruminants such as sheep and cattle), and acute toxicity is quite rare, however there are concerns that breeding stock and sensitive horses such as those with laminitis may be affected by lower concentrations of nitrates that are occasionally found in pasture and hay.

Nitrate can accumulate in pastures or hays that have had excessive nitrogen fertiliser application and young, lush growth (particularly after drought conditions) can also be quite high.

Nitrates can definitely be a problem at certain times and could be behind some odd symptoms – but they are not “normal” in grasses – this is like blaming all hay for causing respiratory issues because sometimes the bales are mouldy…

Potassium and protein are the main supposed villains of this story, but “grass affected” is also explained to be caused by high calcium/low magnesium in young grass, high sugars/starch, mycotoxins, phytoestrogens and more. All of these things can cause problems, but to blame all “lush” grass for them (and have a convenient supplement with mystery ingredients that fixes it…) is in my opinion pseudo-science. There is NO scientific evidence to support this theory as a whole.

Having had this long rant, you may be thinking “but my horse definitely shows symptoms in spring/on green grass and is better when I take them off”. I 100% believe you – and in most cases it can be explained by the horse being exposed to a sudden change in diet – suddenly higher sugars, mineral imbalances, gut microbiota trying to digest new things! Especially in a horse that may not have a balanced, diverse diet that supports mineral balance and enough gut microbial diversity to promote resilience to change. And of course, there can always be individuals that are, for want of a better word, "allergic" to various things, and there's no reason to think a horse could not be allergic to rye grass, or clover, or brome, or whatever species - but it's unlikely to be a widespread (and not clinicially reported) phenomenem.

These sudden changes will be more extreme when you have pastures that are stressed, single or few species, and grown on unhealthy soils (synthetic fertiliser etc), and if you have a horse that is struggling with other underlying health (especially gut) or stress issues. Taking the horse off pasture for good is one solution (although many hays can have the same “problems”), but providing diverse forages and ensuring minerals are correctly balanced may, in the long run, make the horse more resilient to the changes in pasture it will see year in, year out, and was designed to deal with.