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Problem Grazing Species in your Field

Here's some information on some of the  problem species of plants commonly occurring in horse pastures.


Rye Grass

rye grass

Rye grass has an easily recognizable arrangement of seed heads

What about ‘Zero-endophyte’, ‘Novel endophyte’ or ‘Low-endophyte’ rye-grass?

These are Rye-grasses that have been artificially modified to either eliminate the endophytes altogether (Zero-endophyte) or just the most toxic endophytes while retaining the less toxic ones (Low-endophyte) for protection against insect damage to the grass.
None of them are suitable for horses!

If it is a small amount scattered through your pasture amongst your other grasses, it won't be a problem however do not sow it!

Rye-grasses are meant for rapid weight-gain and milk production in commercial livestock which do not have a long lifespan.

There are multiple reasons why rye-grass causes serious health and behaviour problems in horses even when the endophytes have been removed.

They are

  • Prone to serious mineral imbalances

  • Too high in NSC’s

  • Store their sugars as indigestible fructans

  • High in fluorescing pigments.

Facts About Rye Grass

Rye grass harbours ‘endophyte’ fungi which produce myco-toxins under certain conditions, mainly those of spring and autumn. The endophytes grow up as the plant grows and collects in the seed-head so that the next plant will also contain endophytes. Mycotoxin production increases with rising temperatures and seed-head development of the plant in summer and then again in Autumn.
Whatever myco-toxins were present in the grass when it was baled will remain in the hay made from that grass.

It is important to understand that endophytes and their myco-toxins are ‘on the plant’s side’. In particular they deter insects from eating the grass. They therefore help protect the grass from insect damage and grazing by herbivores thereby increasing the grass’s chance of seeding and reproducing.

It is well known that in late summer/Autumn the rye-grass endophyte produces Lolitrem B, a mycotoxin which affects the CNS (Central Nervous System) of livestock consuming it. Livestock become un-co-ordinated, staggering in their hind-quarters, sometimes even falling over. It is like they are drunk. Of course they are accident prone when affected and liable to fall into fences or ditches.

The other major myco-toxin produced by the endophytes in the rye-grass is called ‘ergovaline’. It is equally, if not even more harmful as it has a vaso-constricting effect on blood vessels to various organs. It is well known to cause ‘heat stress’ in livestock. Horses can be seen sweating just out on the paddock when it isn’t even particularly hot and they have not been exercising at all.

If the vaso-constriction happens to the uterus then it can cause abortions, if it happens to parts of the intestine then it causes ‘hind-gut necrosis’ ie death of a section of the digestive tract which of course manifests as serious, usually fatal colic. (I have heard of a horse that was successfully operated on, where the dead section was removed and the horse recovered).

Make it your business to be able to recognise these grasses. Rye-grass is easily identified by it’s dark green long, narrow leaves which are shiny on the back and also by it’s long, narrow seed-head with alternately arranged seeds on either side.

dairy rye

Above: Rye grass is easily identified from a distance because the backs of the leaves are the ones that shine in the sun. Only rye grass has this feature.
The field above is 100% rye grass.


Clover is the cause of a wide array of health and behaviour problems, some of them so common we think they are normal, some way more severe causing frustration, accidents, loss of confidence in people, and unnecessary suffering and euthanasia of horses.

Without a doubt, these pastures directly impact your safety, enjoyment and pocket!!

Clover is very high in starch (complex sugars), contains phyto-estrogens which can upset hormones, contains photodynamic pigments which lead to mud-fever and sunburn and there is a very strong correlation between clover and head-flicking.
Consequently it is not on the list of good plants for horses!
Clover is a...

  • Major cause of mud-fever & sun-burn

  • Turns mares into nymphomaniacs

  • Turns geldings into stallions

  • Major contributor to head-flicking

  • Major contributor to obesity & laminitis

  • Cyanogenic under certain conditions

Sub-terranean Clover

The name comes from its unique underground seed development, different to other clovers. It is a small leaf clover and has little white flowers in the spring.

It does well in poor-quality soil where other clovers cannot survive. Sub-terranean clover is self-fertilizing, unlike other clovers and alfalfa (lucerne), which are pollinated by insects, especially honeybees. Hence it’s predominance in over-grazed ‘horse-sick’ paddocks.

It was used extensively as fodder in South and Western Australia but its high Phytoestrogen content caused serious reproductive problems in livestock grazing it especially sheep. Low estrogen strains have since been developed for this purpose.

It will cause reproductive problems in horses too especially when the horses have nothing else to eat!

red clover flowers

Red Clover above with below, sub-terranean clover

subterranean clover flowers
Dangerous barley grass seed heads

Barley Grass seed heads at the dangerous stage!

Barley Grass

Don’t let this plant go to seed and spread, zap it with roundup as the seed heads are forming (for easily identification).

Barley grass is a real hazard in hay, we have had to call the vet to extricate the sharp seed-heads from our horses tongues when we unknowingly fed hay containing barley grass - an expensive lesson!

Another option is to map out where it is occurring, recognise the plant  and graze it down BEFORE it sets seed.


Far preferable to rye-grass!

We used to worry more about Paspalum in horse pastures than we do nowadays. The main problem is the ‘mouse-dropping-like’ ergots that develop on the seed-heads over summer which can potentially cause excitability, trembling and staggers.

However the leaves and stems are perfectly safe for horses to eat. (when not short and green as with other grasses). Fortunately the seed-heads are not very palatable and relatively large quantities of them have to be ingested to induce symptoms. Hence cases of horses developing symptoms are actually rare.

Ingestion of the black ergots is more likely to be accidental because they do tend to stick to the horses muzzle and legs.

In fact Paspalum is considered desirable as part of the grass sward on many livestock production farms in northern NZ.

It is a sub-tropical grass, low oxalate and grows well over the summer months.

Nevertheless no-one wants to end up with their paddocks full of Paspalum.

A plant survives in a pasture only because conditions are favourable for it. This applies to both desirable and undesirable plants.

Paspalum (like the catsear plant) tends to become dominant in lower-fertility soil environments. If you consult a grass/pasture specialist for advice on how to rid your pastures of Paspalum, they will therefore suggest improving the fertility of the soil for the purpose of encouraging other species to grow.

The chief deficiency which limits grass growth is nitrogen so you are likely to be advised to apply nitrogen to the soil ‘get rid of’ the Paspalum.

NOT what we would advise because of the unforeseen consequences - as happened recently to a horse owner who contacted us. They had been advised to apply urea to the paddocks (back in the Autumn) – the effect was two-fold:

1. Other species (including rye-grass) became more prevalent and the Paspalum less so. Whilst this seemed like the desired result at first, the reality was that the grass became totally unsuitable for the horses grazing it.

2. The horse became more and more anxious, developed extreme separation anxiety, dangerous to even lead and obviously unrideable! Definitely not the desired result and exactly the symptoms that we see in horses grazing on fertilised dairy pastures.

Reason: Urea is pure nitrogen. Horses, being mono-gastrics without a rumen, are NOT adapted to high nitrogen diets! This is also ONE of the reasons we do not sow clovers when renovating horse pastures and why we go to great lengths to eliminate it from pasture.

INSTEAD of altering soil fertility, a better option is to OVER-SOW with the grasses that thrive in the same soil conditions that Paspalum does. (See last post). This means you are introducing grass species which will dilute the Paspalum and give your horses more variety in their pasture.

As with all grasses, the nutritive value declines as it matures. For pasture grass meant for horses, WAIT until after the young, green and vegetative stage and graze it when it is more mature when it is lower in potassium, nitrogen and sugars.

Paspalum can also be made into hay but should be cut before flowering to avoid baling ergot-infected seedheads.

Horses evolved to be thrifty – they are meant to derive a little bit of nutrition from a LOT of mouthfuls spread over the day. The lower the nutritive value of the pasture the longer they can spend out there without becoming ‘grass-affected’ or developing EMS and/or laminitis.

Contrast this with pasture growing on fertile soils – it is far too ‘rich’ for the equine species, they get far too much nutrition with each mouthful and it results in health and behaviour problems!

We would choose Paspalum over rye-grass any day!

Paspalum seed heads

Paspalum seed heads

field of mature paspallum

Field of mature paspalum.

lupin flowers

Lupin flowers


Lupin is a legume.
Some, but not all lupin species contain quinolizidine and piperidine alkaloids.

Poisonous lupins have their effect in several different ways, first, they are teratogenic, affecting the blood supply to the uterus and also interfering with the normal calcium metabolism which can mean systemic bone development disorders characterised by malformed joints and cleft palate.

The second negative affect of lupin alkaloids is neurotoxicity, characterised primarily by muscle tremors.

A third problem, often confounded by the first two, arises from alkaloids produced not by the lupin itself, but by a fungus that grows on lupin pods and stems.

Lupin is a nitrate accumulator which is another reason to avoid access to the plant.

Lupins are very palatable to horses with the young leaves and shoots being both the most tasty AND the most poisonous.

Although some species are non poisonous, it is best to remove any lupin from the horses grazing as they are not easy to tell apart.

NOTE: Lupin seeds are harvested and fed as a source of protein. As they are very high in crude protein (38%) they should only be fed in small quantities. They are also extremely high in Manganese.


Lucerne has a high phyto-estrogen content. In this respect it is similar to clovers and has the potential to upset the cycling of mares and cause other reproductive problems.

Yet another issue is the fact that lucerne, like clover, is high in photodynamic (fluorescing) pigments which are the real cause of sun-burn and mud-fever.

Not so widely known is the most important fact that lucerne is high in potassium and extremely low in sodium. Being a legume it stores sodium in the root nodules so there is virtually none in the above ground portion of the plant.

More about Lucerne here

lucerne flowers

Lucerne flower heads

field of cat's ear

A toxic field of cat's ear - do not put your horses out on this!

Cat's Ear

Driving around you will see lots of paddocks that look like a sea of yellow flowers.

They are the flowers of the 'Cat's Ear' or 'false dandelion'. Dandelions have a thicker single flower on a single thicker stem whilst the Cat's Ear has a branched stem with multiple little flowers.

Cat's Ear is very palatable to horses and is a major cause of stringhalt. It is another reason to do the annual broad-leaf spray off as the same spray that gets rid of clover will also eliminate this potential nasty.

cat's ear flower

Cat's ear close up

Buttercup flowers

Buttercups taking over your pasture is indicative of poor drainage. They are potentially harmful when they first grow but are no longer toxic after a hard frost or when dried in hay.

The consumption of freshly growing buttercups may cause:

  1. Irritation and little sores or blisters around the mouth area

  2. Colic like symptoms and/or diarrhea

Spraying it out with MCPA and then improving drainage or aerating the soil will help prevent buttercup from taking over, otherwise you’ll end up with all buttercup and no grass!


Capeweed flowers

In a drought, ‘flat-weeds’ like Cape Weed and Cat’s Ear proliferate. Cape weed  is more prevalent in Australia than NZ.


It has deep tap-roots and thrives in the dry conditions. Sometimes it is not long before it has become the predominant plant in the pasture.

These weeds with their large succulent leaves are very palatable so horses love them.

Unfortunately there is an aspect to these plants, yet to be identified,  that causes problems with the nerves which control the hind limbs resulting in hyperflexing - or 'goose stepping' characteristic of stringhalt.


This is a classic example illustrating that horses don’t ‘know’ that a plant is ‘bad’ for them. They consume them because they like the taste.
Capeweed has also been indicated in cases of stringhalt.


Dock seed heads

Dock becomes a feature of horse-sour, over grazed pasture. While it is an oxalate plant, because it is not very palatable, horses would very rarely eat enough of it for this to be a problem.

Dock is easily eradicated with a broad leaf spray, along with clover and other undesirable broad leaf 'weeds'.

Ragwort flowers

Ragwort contains pyrrolizidine alkaloid compounds that are poisonous to most animals.

Normally horses won't eat it except when there is little else to eat. It is potentially a problem when occurs in hay.

 The alkaloid compounds in ragwort can damage the liver when eaten. The toxic effect builds up over time, causing irreparable damage.

This means that your horse will get just as ill from eating small amounts of ragwort over a long period of time as it would do from eating a large quantity in one go.

The horse may become lethargic or behave abnormally. They can develop photosensitisation, where areas of pink skin become inflamed and painful when exposed to sunlight, like serious sunburn. They can also lose significant amounts of weight, even though they may be eating well.

Eventually they may go blind, have to fight for breath, start to wander or stagger or be found pressing their head against a wall.


The symptoms and subsequent death can come about so quickly that owners have sometimes found their horse dead without warning.

Never leave uprooted ragwort in the field where the horse can get to it - it becomes more palatable when it has wilted or dried!

Poison Hemlock

Poison Hemlock contains an array of peperidene alkaloids - chemicals similar to nicotine.

These chemicals cause initial stimulation followed by severe depression of the central nervous system, resulting in paralysis and respiratory failure.

All parts of this plant are toxic, especially the leaves before flowering and the flowers and seeds!

Poison Hemlock is also teratogenic  ie: causing abortion in mares.

It is easy to confuse Poison hemlock with the more common Carrot Weed (Queen Anne's Lace).
The leaves of Carrot weed have a hairy underside, matte, fern-like, and smell like parsley, whereas Hemlock has leaves that although similar in shape,  are larger, hairless and shiny.
Hemlock stems have purple spots, are hairless and waxy whereas Carrot weed stems have white hairs with none of the purple colouration.

The difference between hemlock and carrot weed

The difference between Poison Hemlock and Carrot Weed above.

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