Studs for horses are attached to the shoes to improve traction. Studs are the same whether they are for horses or ponies. It may sound both simple and obvious, but there's more to consider than one might think. It's usually when you ride on a grass track that you need to have more options.
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Grass studs come in many different shapes depending on the surface. Using studs that are too long for a harder and more stable ground increases the risk of injury and wears out the horse. Likewise, studs that are too short for damp or soft ground can pose a safety risk as the horse might not get sufficient traction. This is a very general description, but considering these aspects when putting studs on a horse before a competition will guide you well.