Spring Grass Seeds - The Hidden Health Hazards in Your Paddock
Spring is beautiful from the gate. Finally there is new green grass! But Spring is also infuriating up close. As the grasses shoot and the seedheads rise, horses start encountering plants that don’t just annoy them - they can cause neurological disease, gut upset, skin reactions, mouth injuries, and full‑body photosensitisation. Most owners know some spring grass seeds are bad, but very few know which plants cause which symptoms. Once you understand the patterns, the symptoms you are seeing makes perfect sense.
The biggest offenders in Australian spring are endophyte‑infected ryegrass, paspalum, ergot‑infected grasses, clover, and the needle‑type grasses like speargrass and barley grass. Each causes a distinct clinical picture. Some are a problem because of ingestion and others from skin contact.
Flatweed and dandelion also cause Australian stringhalt. You can read more about that here.
See this article for pictures of common pasture grasses.
Spring Grass Seeds Containing Endophytes
Endophytes are microscopic fungi that live inside certain pasture grasses, especially perennial ryegrass and tall fescue. They don’t sit on the surface like mould, they live within the plant tissue, in a symbiotic relationship where the grass feeds the fungus and the fungus protects the grass from insects, drought, and heat.
The problem for horses is that many endophytes produce toxic alkaloids. These compounds are biologically active - they interfere with nerve signalling, blood vessel function, thermoregulation, and hormone production.
In tall fescue, the endophyte Epichloë coenophiala produces ergot alkaloids, which are potent vasoconstrictors. These compounds narrow blood vessels, disrupt thermoregulation, and, most critically, suppress prolactin. Pregnant mares are especially vulnerable: they may carry foals longer, develop thickened placentas (“red bag”), produce little or no milk, and deliver weak foals.
These alkaloids survive drying, so hay can carry the same risks as pasture. Hot, dry summers and early autumn spikes increase toxin levels, making outbreaks more common.
Endophyte‑infected perennial ryegrass is the classic cause of ryegrass staggers or "grass tetany". The fungus inside the plant produces lolitrem B, a neurotoxin that interferes with nerve conduction. Horses don’t need to eat much to show signs. The earliest symptoms look behavioural: a horse that startles at shadows, becomes hypersensitive to touch, or suddenly loses confidence under saddle. As exposure increases, you see head tremors, muscle quivering, difficulty coordinating the hind end, and the unmistakable “staggers” gait.
Ergot‑infected grasses (including rye, barley, and some wild cereals) produce ergovaline, a potent vasoconstrictor. Horses exposed to ergot show cold extremities, swelling in the lower limbs, lameness that worsens in cold weather, and in severe cases, sloughing of the tips of ears or tail. It’s rare but devastating, and it appears in wet springs followed by sudden heat - perfect conditions for fungal infection of seedheads.
Photosensitisation - It's not Sunburn!
The easiest mistake to make in Spring is confusing sunburn with photosensitisation, because at first glance the skin looks similar. Sunburn is mechanical damage from UV light on unpigmented skin. It’s local, predictable, and heals quickly once the horse is kept out of harsh sun.
Photosensitisation is a completely different process: it’s a biochemical reaction triggered when the liver is stressed by plant toxins or fungal spores - clover, capeweed, panic grasses, and some mouldy hays. The liver fails to clear photodynamic compounds, and the skin reacts violently to normal sunlight.
Horses with photosensitisation show swelling, crusting, peeling, and intense pain on white areas, often around the muzzle, eyelids, and pasterns. They may become head‑shy, reluctant to graze, or stand in the shade even on mild days. Sunburn improves rapidly with shade and barrier creams; photosensitisation does not, because the underlying problem is hepatic (relating to the liver). If the skin keeps worsening despite sun protection, the plant is the culprit, not the weather.
Slobbers
Clover is the quiet troublemaker. White clover and subterranean clover can host slaframine, a mycotoxin produced by the fungus Rhizoctonia. Horses exposed develop the classic “slobbers”: ropes of saliva, wet chins, damp feed bins, and sometimes mild colic signs. Clover also contributes to photosensitisation when it harbours fungal spores that damage the liver. Horses show crusty white noses, swollen eyelids, peeling skin on the muzzle and pasterns, and an intense dislike of sunlight. As above, owners often blame “sunburn”, but the liver is the organ in distress.
Some moulds in hay (especially Fusarium species) can irritate the oral mucosa enough to cause excessive salivation, but this is not true “slaframine slobbers”.
Fresh buttercups contain protoanemonin, which irritates the mouth and can cause drooling, lip swelling, and mouth pain. But horses rarely eat them unless grazing pressure is extreme, and the toxin is destroyed when the plant dries, so hay is safe.
Skin Contact Seed Problems
Paspalum causes trouble through its sticky, irritant seedheads. When they mature, they lodge in eyelids, nostrils, lips, gums, and sheath folds. Horses show sudden head‑shyness, excessive blinking, tearing eyes, rubbing their face on their forelegs, or refusing the bridle. Ingestion can irritate the gut, causing loose manure or mild colic signs. It’s not a toxin problem, it’s mechanical irritation from seeds designed to cling.
Speargrass, barley grass, and other needle‑type grasses cause the most dramatic seed injuries. Their seeds penetrate skin, mucous membranes, and soft tissue. You’ll see swollen eyelids, small draining abscesses around the lips or gums, unexplained mouth pain, or a horse that suddenly refuses the bit. Seeds can migrate deeper, causing infections that take weeks to resolve. These are mechanical injuries, not toxins, but they can be just as debilitating.
All of these conditions appear in Spring because the biology of the plants changes as they mature. A paddock that was safe in winter becomes risky in September ad October simply because the seed heads have emerged or the endophyte load has increased. Horses don’t need to gorge themselves, a few mouthfuls, or even simple contact, can be enough.
The most important thing for owners is identifying those grasses in your pasture and recognising the symptoms early.
Once you know which plants cause which symptoms, the next step is simply protecting the horse while the season runs its course. Seedheads can be managed by mowing or slashing before they fully mature, which dramatically reduces the number of irritant seeds that end up in eyes, nostrils, lips, and sheath folds.
Horses that are prone to eye injuries or tearing benefit enormously from a well‑fitted fly mask, which acts as a physical barrier against paspalum, barley grass, and speargrass seeds.
For horses reacting to ingested toxins - ryegrass endophytes, clover slaframine, or fungal spores - using a mycotoxin binder such as Mycosorb helps reduce the background load and often settles mild neurological or gut signs while you adjust their grazing.
Any skin that’s already irritated from photosensitisation or seed contact generally responds rapidly to Red Healer ointment, which supports the barrier layer and reduces inflammation.
None of these tools fix the plants themselves, but they make the horse far more comfortable while the Spring biology does what it does.
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