Stuff you need to know about Rye Grass

It is not surprising some people get the wrong idea about Rye Grass.

They have been advised by people associated  with the farming community that the modern strains of Rye Grass are no problem because they are either ‘low’ or ‘zero’ endophyte.
We originally understood this to be true too however, we found out the hard way that there is actually a lot more to know about Rye Grass and its effect on the horse’s metabolism. Endophytes turn out to be the least of its problems!

Here is some information to help you understand Rye Grass:


Prolific Growth

Rye-grass is considered desirable by farmers because it is very easy to grow and, being relatively high nutrient density, (high in sugars, crude protein & potassium), it facilitates rapid weight gain and maximum milk production in livestock, certainly not features we are looking for when it comes to feeding our horses!

Understanding Endophytes

Perennial Rye Grass (the most prevalent strain and tends to grow by default here in NZ) contains endophytes (fungi that live inside the plant) which produce myco-toxins under certain conditions. These myco-toxins are basically insecticides which kill insect pests like the Argentine Stem Weevil.

One of these mycotoxins – called ‘LolitremB’, is known to cause staggers in all livestock including horses especially in late Summer or Autumn.

Another is called ‘ergovaline’ and is liable to cause heat stress and reproductive problems. There are many other endophytes which produce these harmful chemicals known as myco-toxins under certain conditions.

 As these endophytes are a known livestock health hazard, strains of Rye Grass have been developed which either have no endophytes at all, or contain specific endophytes deliberately retained or inserted in order to knock the insect pests but without harming stock.
Hence the widespread belief that such strains of Rye Grass are safe forage for horses.  Not true unfortunately because, come to find out, there are multiple other aspects of rye-grass which adversely affect horses grazing it!

Toxin-binders are meant to be effective against the myco-toxins produced by endophytes in Rye Grass. However see next point:

The Most Common Cause of Staggers

If mycotoxins were the only cause of ‘staggers’ then horses grazing endophyte free grasses shouldn’t ever come down with ‘staggers’. But they frequently do.

It is very common for horses to develop ‘staggers’ when grazing ‘zero endophyte’ strains such as ‘Italian’ or AR1. This is because staggers can also be caused by mineral imbalances inherent in these grasses. These mineral imbalances (including high potassium & nitrogen, low sodium & magnesium) disturb nerve impulse transmission and muscle function and cause the hyper-sensitivity, uncoordination, ‘drunken gait’, even falling over – all symptoms of staggers very similar to those caused by the Lolitrem B mycotoxin.


Mineral Imbalances

Based on multiple forage tests we have conducted over many years, mineral imbalances are THE major problem with any variety of Rye Grass, whether it is ‘endophyte-free’ or not.

There is an obvious correlation between horses grazing rye-grass pastures, especially when there is clover present and the most serious of issues we see including laminitis, head-shaking, respiratory, behavioural, hormonal, musculo-skeletal and many more.
(This doesn’t mean other grasses aren’t culprits either but rye-grass is particularly ‘bad’, to the point it is known as ‘disaster pasture’ or ‘founder fodder’).

Potassium & Sugar Content

Rye Grass is a ‘cool season’ grass, meaning it thrives in cool temperate climates. The average potassium content of cool season grasses is between 2 – 4%, even more if it has been fertilized with anything containing potassium – eg: NPK, SuperK, Potash and others.

There are modern strains of Rye-grass which have been developed to grow cells which are double or even triple the size of normal plant cells in order to maximise nutrient content – these are known as Diploids and Tetraploids.
Banquet® II tetraploid long rotation ryegrass, Delish® tetraploid short rotation grass, Lush tetraploid Italian ryegrass, Feast®II tetraploid Italian ryegrass.
No wonder these would cause serious problems for horses. Steer well clear!


Whereas clovers and other broadleaf plants store their sugars as starch, easily broken down by enzymes, rye-grass stores its sugars as ‘fructans’ for which no mammal possesses any enzyme capable of breaking it down. Fructan is a known cause of laminitis in horses.

Fluorescing Pigments

Rye Grass is dark green and therefore high in fluorescing pigments which contribute to Photo-sensitivity (‘Mud Fever’ and ‘Sunburn’) Primary photosensitization is most commonly caused by ingestion of photodynamic plants, such as buckwheat, St. John's wort, perennial rye grass plantain, and legumes such as Lucerne, clovers and trefoil.


Annual Rye-Grass

Annual Rye Grass infected by the bacterium Rathayibacter toxicus, can cause fatal poisoning to all livestock. This occurs in Australia from time to time. Toxicity develops at flowering and seedset. Hay made from toxic ryegrass will also be toxic. Very scary stuff for horse owners!

Rye-Grass Hay

It can be difficult to find hay that contains no rye grass and luckily most horses can handle SOME rye in their hay and pasture. For those horses who are prone to being Grass Affected, soaking hay will remove 20 – 30% of the sugars and up to 50% of the potassium. (See upcoming post) Worth the extra effort for horses and ponies prone to laminitis.

Top-Foda’, a forward thinking hay producing company in Victoria, Australia produces acceptable rye-grass hay. They have learned how to produce low sugar hays which are also not high potassium. They have other varieties availaible now too.

Re-sowing Pastures

So the moral of this story is - don’t let ANYONE talk you into sowing ANY variety of rye grass or clover whatsoever. Many of the Seed/Farming companies understand what is needed for farming livestock but have NO IDEA that horses require exactly the opposite - Renovating pastures is expensive in time and money let alone having to repeat the process.