I did some research over the weekend about plants which are toxic to horses. The more reading I did, the more I was convinced that Nico is eating something slightly toxic. One of the other ponies has a similar diarrhea problem, which makes me suspicious. Right now the horses are out on super overgrazed pastures. I can picture Nico and this other pony munching on weeds when their hay runs out. Also, the Biosponge, which absorbs toxins in the gut, helped a lot when he was on it, but lost it’s effectiveness as soon as he was taken off of it. This all really makes sense if it’s some kind of poisoning, so I hope some research can fix this problem once and for all.
I rode Nico this evening after five days off and he was an angel. An angel who forgot how to back up, but overall very good. He bends, he trots and canters in a moderate, reasonable manner, and he had a great attitude about working. Also, he’s developed a fantastic walk as long as I’m driving with my seat.
After riding, I washed Nico’s butt for the five thousandth time and he acted like I was killing him. Finally I got annoyed enough and growled, “Stand…” at him and he did a lot better. I think sometimes he just needs a little reminder of what’s expected of him.
I put Nico out in his new field (the trainer switched everyone’s fields to see if Nico and the other pony’s diarrhea clears up) and then grabbed my camera. I took pictures of every plant that Nico could get access to. After coming home and looking over things, I’m pretty sure that there’s a significant amount of Milkweed in Nico’s pastures (yes, it’s in both the old and the new pasture).
Here is the plant I took a picture of this evening:

Here is a picture of Milkweed from the internet:

As far as I can see, it’s the same plant, minus the seed pods. And here’s what Purdue Vet department has to say about the Milkweed plant:
MILKWEEDS
Asclepias spp.
(milkweed family)
TOXICITY RATING: Low to moderate. Milkweeds are unpalatable, and have variable toxicities. Death is not likely unless large quantities are consumed.
ANIMALS AFFECTED: All animals may be affected. Sheep are most at risk, but cattle, goats, horses, poultry, and pets are also at risk.
DANGEROUS PARTS OF PLANT: Stems, leaves, roots.
CLASS OF SIGNS: Gastrointestinal irritation (primarily vomiting and diarrhea), incoordination, tremors, heart problems, respiratory difficulty, death.
PLANT DESCRIPTION: Milkweeds, such as common milkweed, Asclepias syriaca (fig. 26), get their name from the thick, sticky, milky sap that oozes out of cut or torn leaves, stems, and fresh pods. The usually solitary stems of milkweed grow 1 to 5 feet tall and bear opposite (sometimes whorled), sometimes fleshy leaves with entire margins. Flowers emerge in umbrella-like clusters and range in color from pink to rose-purple to orange or white. The fruit (fig. 26A) is a pod with “tufted” seeds. A dozen species of milkweeds grow in Indiana woods and swamps, but most commonly in dry soils of fields and road-
sides. Dogbanes (Apocynum spp.), which are easily confused with milkweeds, are found in the same habitats and may cause similar poisoning.
SIGNS: There are several different types of milkweeds with varying degrees of toxicity, with the whorled milkweeds being the most toxic. Milkweed plants are considered unpalatable and are eaten only when other forages are not available, and may also be found in hay and processed feeds. The primary toxicants are cardiac glycosides that cause gastrointestinal, cardiac and respiratory problems and can cause death if enough is consumed. Resins (especially galitoxin) in the milky sap may also contribute to the toxicity of milkweed. In ruminants, the first signs are incoordination, muscle tremors and spasms, bloat, increased heart rate, breathing problems, and occasionally death. Horses are very reluctant to eat this plant, and its toxicity is only rarely reported: colic, diarrhea, abnormal heart rate and rhythm, rarely death. In animals that are capable of vomiting (pigs, dogs, cats, humans), this is the first sign to develop and is beneficial in that further absorption of the toxin is lessened. Horses cannot vomit, and vomiting is not generally observable in ruminants (if vomiting occurs, the contents still remain in the rumen), therefore toxic signs will be worse in these species.
FIRST AID: There is no antidote if an animal consumes milkweed. It is important to limit further ingestion of the plants or contaminated feed. If the animal recently consumed a large amount of the plant, a veterinarian should be called so that the gastrointestinal tract can be emptied and supportive care provided. Small tastes of the plants tend to result in minor oral irritation, and serve as a deterrent to further consumption, and these little nibbles typically do not require treatment.
SAFETY IN PREPARED FEEDS: Milkweed is toxic both fresh and dried, therefore hay, silage, green chop, and processed feeds that contain milkweed are never safe for consumption.
PREVENTION: Animals will avoid milkweed as long as there is sufficient forage available. Care must be taken to avoid incorporation of milkweed into prepared feeds and hay, and these feeds should be discarded.
So if you read all of that, it sounds like the Milkweed (or Dogbane) could be causing Nico’s GI symptoms. I doubt he’s eating a ton of it, but if he’s nibbling on it because he’s bored out in the overgrazed pasture, I think it might possibly cause his mild/moderate diarrhea without any of the more serious symptoms described above. He’s out 12 hours every day (which I am very happy about), and I can definitely see him eating poisonous plants that normal horses won’t touch. It was evident last night when I was taking pictures that the horses were bored and looking for things to eat, and Nico is so desperate to be eating something every second that I’m sure he’s at least tasted the weeds in his pasture.
I have a riding lesson tomorrow morning at 9:30am, so I’ll take along some gardening gloves and go out and attack the weeds after my lesson. Also, some larger pastures are being hayed this week, so eventually he will be out on better pasture where he will have more grass to eat instead of weeds. I’m crossing my fingers and hoping this will do the trick!
And, despite extensive searching, I can’t figure out what this weed is:

Any ideas? It’s obviously been feasted on by someone.