Walk every metre before a single hoof goes in
Do not survey a new field from the gate and call it done. Walk it in overlapping strips, the way you would mow it, then walk the boundaries, the ditch lines, the corners behind the shelter and the shaded ground under every tree, because that is exactly where the dangerous plants and the dumped rubbish collect. Take gloves, a rag fork or a spade, and a bag or bucket for what you pull. A small field deserves an unhurried half hour; a big one deserves a whole morning, and either is cheap compared with what one missed plant can do.
Learn ragwort at every stage, not just in flower
Most people only recognise ragwort in July, standing waist-high with flat clusters of yellow daisy flowers — by which time it has usually been in the field for a year. The plant spends its first season as a flat rosette of deeply lobed, almost frilly leaves hugging the ground, easy to walk past in spring grass. Both stages matter, because horses mostly avoid the bitter growing plant but will eat ragwort readily once it is wilted, dried or baled into hay, when it turns palatable and stays just as poisonous to the liver. Dig plants out with the whole root, bag them, and carry them off the field entirely: pulled ragwort left lying on the grass is more dangerous than ragwort still growing. Wear gloves for the job, and never burn or compost the plants anywhere horses can reach the remains.
Hunt the supporting cast of poisons
Ragwort is the famous one, not the only one. Hemlock grows tall and hollow-stemmed along ditches and damp edges, foxglove colonises banks and woodland margins, and bracken takes over rough, neglected ground. Under oak trees, look for fallen acorns in autumn; under sycamores, look for the spinning seeds and seedlings, which are linked to a serious and often fatal muscle disease in grazing horses. Check along garden fences for clippings thrown over from the other side — yew above all, which can kill in very small amounts — and ask neighbouring gardeners kindly but firmly never to dump prunings into the field.
Look for the dangers that are not plants
A new field hides physical traps as well as poisons: rabbit holes and badger scrapes in the gallop lines, old fencing wire rolled into the hedge bottom, broken glass and tins around any spot where bonfires have been lit, and sharp offcuts near gates where work has been done. Check the water supply too — a trough fed from a clean source, no stagnant pond with a scummy surface that horses can drink from, and no blue-green algae bloom on standing water in hot weather. Finally, walk the fence itself: a plant-safe field with a broken boundary is not a safe field at all.
If the field has already been grazed
If horses have been on the field before your check and you find poisonous plants in quantity, keep them off it until it is cleared, and watch the horses rather than shrugging. A horse that goes dull, loses its appetite, develops diarrhoea or shows skin that blisters in sunlight after grazing a suspect field needs a vet call promptly, with the plant you found named in the call, because that detail shapes what the vet does next. Assigning the first deep check — and the standing weekly re-walk of boundaries and ditch lines — as a named duty in KinsLe's Care Teams means the field gets inspected even in the weeks when you are away or busy, and nobody assumes someone else has done it.