Unlike the spring, autumn is not generally seen as being fraught with danger.
In spring, we know that dewy mornings, balmy afternoons and young grass can provoke a life-threatening bout of laminitis. We know that the Ragwort (Jacobaea vulgaris), just starting to show itself, is a possible killer too.

Conversely, the autumn is a season of tranquility, falling leaves and a general slowing down of nature…

But the autumn has its hidden dangers too. In terms of grass growth, the autumn can be a repeat of the spring with dewy mornings and balmy afternoons leading to new growth; grass in autumn can be as highly charged with sugars as grass in spring presenting exactly the same dangers. Since grass, sugars and laminitis are so well covered elsewhere, not least of which here on the Sabots Libres blog in Spring is in the Air and Autumn is in the Air, we will not cover the problems of laminitis in this article.

Acer pseudoplatanus

A less known problem is Atypical Myopathy, a commonly fatal disease caused by consumption of the seeds of trees and shrubs in the family Sapindaceae. Ironically, the Horse Chestnut is one of these family members but probably the most dangerous is the sycamore and more specifically Acer pseudoplatanus.
Note – that the distribution, as shown on the map in this linked article, is well outside the natural habitat and covers most of temperate Europe.

The seeds contain Hypoglycin A (HGA) which slows or stops energy production in the muscle cells. It is effectively a fatal form of equine rhabdomyolysis (ER), the disease also traditionally known as Monday-morning sickness.
AM can be treated but is far from successful and it remains frequently fatal. Clearly avoiding infection is a much greater guarantee than treatment. Ensure thus that horses are not in meadows in which, or on the periphery of which, sycamore trees grow; when riding out, pay attention to any places you may stop and allow the horse to graze. The biggest problem is that AM is most often detected too late; the damage is already done and almost always irreversible.

Another problem of the autumn, and particularly after such a dry summer as that which 2022 presented large parts of Europe with, is the attempt to gather one last harvest of hay while in the throes of an Indian summer. Grass has been growing again and in some regions is presenting just enough growth to harvest; the longer periods of warm sunshine between rainy spells makes the drying of hay a distinct possibility despite being so late in the year. The problem here is that plants rarely seen in the spring and summer hay harvests have started to show prevalence such as Datura (Datura stramonium) – also known as the thornapple, jimsonweed, devil’s trumpet, moonflower, devil’s weed, hell’s bells – and the narrow-leaved Ragwort (Senecio inaequidens).
Datura is a member of the Solanaceae and as such is related to the potato, tomato, aubergine, sweet peppers etc., but also the deadly nightshade. It is highly toxic containing hallucinogens and respiratory depressants; it can cause arrhythmia and is commonly fatal. Happily, horses are not known to voluntarily eat Datura.

Datura (Thornapple, Hell's Bells...)
Narrow-leaved Ragwort

Narrow-leaved Ragwort has much the same toxicity and danger its common cousin (Jacobaea vulgaris) – see Ragwort & Co. –, but, unlike the apparently addictive J. vulgaris, horses are not known to eat this narrow-leaved variety which has a completely different, lance-shaped, leaf structure.
However, both these plants can be unintentionally harvested and dried in hay, a process rendering them tasteless – and deadly. It is therefore of the utmost importance that in the event of haymaking very late in the season, these plants must be isolated and avoided.