Feature: Weed awareness

FEATURE

Weed awareness

Weeds can be a hazard. To find out just how hazardous, RACHEL ROAN spoke to equine specialist Dr Cristina Rosales.

When it comes to weeds that could harm your horse, the list is long, and it can be confusing for horse owners to know what they should be wary of. Dr Cristina Rosales is an equine specialist at Barwon Equine Hospital in Victoria. She has a fascination with weeds and an interest in toxicity, so I asked her for some advice. Here’s her list of the ten most common offenders:

The terrible ten

Flatweed Dandelion: This weed causes stringhalt, where the horse loses the ability to walk in both hind legs, and in addition, may become a ‘roarer’. Dandelion poisoning isn’t fatal, but treatment takes time. With six to twelve months of rest, good pasture, and medication to relax muscles and nerves, there is a chance that the horse can return to work. However, depending on the level of toxicity, they may never return to high level performance work.

St John’s Wort: Hard to eradicate, the seeds of St John’s Wort can remain dormant in soil for up to 20 years, and their tubular roots and runners add to the removal problem. Even when poisoned, the roots often survive and regrow. While it’s not lethal, it can make your horse unwell. The toxin in the plant gets processed in the bloodstream and travels to the skin. When exposed through unpigmented skin, the toxin interacts with UV light to cause sunburn, making the horse prone to skin infections. It also causes gastro-intestinal irritation.

Paterson’s Curse

Paterson’s curse is easy identifiable due to its distinctive purple flower. It occurs in all Australia states, but is most common in areas of high winter rainfall. If left unchecked, it quickly becomes a problem when conditions are favourable as it can produce seeds from autumn through to spring. If ingested, it may cause fatal liver failure. “It’s tricky to diagnose unless we’ve seen the weeds, or the owner tells us that the horse has eaten them,” Cristina comments. “It can take six months before any signs of poisoning begin to show, by which time it’s too late to help the horse.”

Fireweed: A weed commonly found on the NSW’s Central Coast, Fireweed is also fatal to horses. Other than UV sensitivity, this plant predominantly causes irreversible liver and brain damage.

Oleander: This weed results in heart disease and ultimately death. Not only is oleander a danger to horses, it is

also poisonous to humans, dogs and cats, and other domestic livestock. It is extremely toxic, so even the smallest amount is harmful. It may look pretty while flowering, but it isn’t worth having on your property.

Foxglove: Found in warmer coastal areas, Foxglove produces tall spires of bell-shaped flowers. While most horses will not eat fresh foxglove, if even 100g is consumed death occurs in a matter of hours. Symptoms include diarrhoea, staggering, fitting, and collapse. All parts of the plant are toxic. Fortunately, it can be managed by planting improved pastures. If individual plants are pulled out by hand, do not burn them, and dispose of them carefully to avoid accidental poisoning.

Marshmallow Weed: Found across Australia, like Oleander and Foxglove, this weed can cause a rapid heart beat, and survival rates are low. It also impacts the skeletal muscles, causing the horse to become very weak and experience muscle tremors. Christina tells me that when a horse presents with a high heart rate, the cause is usually something other than poisoning, and it often takes a post-mortem to confirm the cause of death. Seeds found in the gut are sent to a specialised lab for toxicity testing and identification. “While doing a postmortem won’t bring your horse back, it can help prevent other deaths,” she adds.

Crofton Weed: Sudden death can also be caused by Crofton Weed poisoning. While researchers are yet to identify what this toxin is, signs of poisoning may begin with exercise intolerance and coughing, before sudden death.

Deadly Nightshade: Deadly Nightshade targets the horse’s nervous and intestinal systems, and although it’s nasty, there is an antidote that can reverse its effects. However, it’s only administered if the vet is sure the horse has ingested the plant. “We have to be certain to give medication that reverses the side effects of the toxin, otherwise the drug itself can cause significant harm,” Cristina explains. “You would want to know there was a history of the plant being in the area before you reached for that antidote.”

Yellow Star Thistle: This large, spindly weed often grows in disturbed soil along roadsides. Ingestion affects the nerves that control the horse’s ability to chew. Other facial muscles may also become paralysed, resulting in a frozen or blank expression. While a horse is still able to swallow, it will continuously bite and try to hold food in its mouth. Mouth ulcers can then form and gums will become damaged, while the horse may appear to choke, with its tongue poking out. Finally, lesions will form in the horse’s brain. To avoid the horse slowly starving to death, euthanasia is the only option.

Symptoms

For many horses, symptoms after poisoning from weeds often present like colic. “Toxicity with weeds can trick you in that way,” Cristina says. “Unless it’s one of the pyrrolizidine alkaloids, which present very differently, symptoms from toxicity can be vague.” Another major challenge with poisoning is a horse’s inability to vomit. “Any suspicion of toxins means we have to repeatedly stomach tube a horse, flushing it with salty water and magnesium to cause movement through and out of the gastrointestinal tract,” she explains. Charcoal is another remedy vets reach for, as it can help absorb toxins. However, charcoal won’t help if there are unchewed or larger particles of plant or leaf matter.

TOP: Unfortunately, survival rates from Marshmallow Weed toxicity are low (Image by Rachel Roan). MIDDLE: A large, spindly weed, Yellow Star Thistle causes paralysis of the mouth and face. BOTTOM: Signs of Crofton Weed poisoning can begin with exercise intolerance and coughing.

Prevention

While it’s important to understand which weeds are dangerous, know how to identify them, and recognise potential symptoms, Cristina suggests consulting your local agronomist as the best course of action. They will know which weeds are common to your area, and can advise on chemicals or methods of management.

When poisoning weeds, make sure you remove the horses from the paddocks as herbicides can make previously unpalatable poisonous plants appealing. Some weeds are just as poisonous even when dead or dry, so it’s important to remove them after they’ve been treated.

Cristina’s best advice is that prevention is the key to protecting your horse from poisoning. Just by following some simple management techniques, horse owners can avoid a potentially deadly situation.

Weeds can be spread through hay, vehicles or machinery from another property, or through the digestive tract of a newly arrived animal, so proactive management of your pastures, quarantining animals to prevent pasture contamination, and routine checks of your paddocks are a must. “A lot of these toxins don't have a cure,” Christina tells me, “so it’s better to check paddocks regularly for weeds, especially at the change of the season.” Weed toxicity usually varies depending on the stage of growth or time of year. Fertilisers may also cause a flush of growth in poisonous plants and increase their toxicity.

Fortunately, most horses will not eat a weed if they have an adequate food source and a balanced diet. However, in conditions where a horse has restricted access to sufficient good pasture, they will eat whatever they find.

Agrifutures Australian Field Guide to Poisonous Weeds has a comprehensive list of weeds, grasses, shrubs, trees and common garden plants that are toxic to horses.