The short list to hunt for first
Three hazards outrank everything else in a British-style horse pasture. Ragwort is the slow one: its toxins damage the liver a little with every mouthful, it is bitter enough that horses usually avoid the growing plant, and it loses that bitterness when dried — which is why ragwort in hay is so dangerous, and why the plant must never be left lying in the field. Yew is the fast one. Every part of the tree is poisonous, a small quantity can be fatal, and horses will eat wilted clippings they would never touch on the tree, so a yew anywhere near a fence line — or a neighbour who might toss clippings over it — is an emergency waiting for a windy day. Third are sycamore seeds and spring seedlings, linked to atypical myopathy, a serious muscle condition that strikes grazing horses in autumn and spring. A sycamore overhanging the field means a seed-clearing routine in the seasons when they fall and germinate.
The second tier worth removing
Beyond the big three, work through the field systematically. Oaks are fine as shade trees until a heavy acorn year, when fallen acorns and oak leaves eaten in quantity can cause gut and kidney trouble — graze horses away from a laden tree or clear beneath it. Bracken poisons slowly over a season of nibbling and should be cut back or fenced off where it crowds the grazing. Foxglove, hemlock and the nightshades appear along ditches, banks and muck-heap edges and are all worth pulling before they seed. Garden shrubs are a quieter risk: laurel, privet and rhododendron in boundary hedges are poisonous, and their clippings are the real danger, because wilting concentrates a horse's willingness to eat them. Buttercups in a flush are mildly irritating and usually avoided while growing; they are a sign of a sour, overgrazed field more than a poisoning emergency.
How to remove them so they cannot still be eaten
Removal done carelessly can raise the risk instead of ending it. Dig ragwort out whole, root and all, wearing gloves — the toxins can pass through skin — using a fork or ragwort tool rather than snapping the stem, since a broken root resprouts. Bag or bucket every plant as you go and take it off the pasture entirely to be burned or disposed of where no animal can reach it; never leave pulled plants wilting on the grass, because a wilted ragwort is sweeter to a horse than a growing one. Cut bracken and ditch plants should be gathered the same day for the same reason. For trees, you do not always need a chainsaw: fencing horses back from a yew or a heavy-dropping sycamore, and sweeping seeds from a small sacrifice area, can manage the risk while you arrange proper removal.
Keeping the pasture clear
Plants move in, so this is a routine, not a one-off purge. Walk the whole field, including the hedge bottoms and the ditch banks, whenever you check fences — and always after hedge cutting, storms or deliveries of hay from a new source, since a bale cut from a ragwort field carries the danger indoors. Learn the seedlings as well as the flowering plants; ragwort rosettes in spring are the cheapest version of the problem you will ever deal with. If a horse is found eating yew, or shows sudden weakness or distress after grazing near fallen sycamore seed, treat it as an emergency and call your vet immediately rather than waiting to see — with the fast poisons, speed is the treatment. Make the walking easy to remember: on KinsLe, Care Teams lets you set that pasture plant check as a recurring duty, assigned and ticked off season by season, so the field gets inspected before anything dangerous gets eaten.