Contact your veterinarian and have a complete physical done on the horse. Make sure that the animal is fit enough to begin retraining. A horse that has stood idle for a long time is likely to require extensive conditioning through regular non-riding exercise before it will be ready to begin actual riding.
If you are in the market for a new horse, you may find that a previously trained horse currently on pasture rest can be a good deal. As long as the horse has had solid training beforehand, you should be able to bring that horse in from the pasture and retrain it to be a solid riding animal once again. The amount of time this retraining takes depends on a number of things, from the horse itself to the type of riding you plan to do. Some horses can go from pasture pet to show horse in a matter of weeks, while others may require six months or more of retraining.
Exercise the horse to increase its fitness level. You can longe the horse, or ask it to move around in a round pen to build its muscles back up and get it fit enough for regular riding. The work on the longe line also gives you a good training opportunity, allowing you to direct the horse's movement using the longe whip and your body language. To longe the horse you simply attach a longe line to the halter and ask the horse to move around you in a large circle, using the whip to urge the horse on and get it to change directions. The round pen works the same way, replacing the longe line with a small round pen in which the horse can move freely at the direction of the handler.
Tie the horse to a sturdy post and place the saddle on its back. Get it used to the feel of the saddle on its back again after a long time off. The horse should not have forgotten how to ride, but the feel of the saddle may be somewhat unfamiliar.
Allow the horse to move around on the longe line or in the round pen with the saddle on its back. The more work you can do with the horse saddled, the better off you will be.
Place the bridle on the horse and ask a helper to hold the animal while you prepare to mount. Step lightly in the stirrup and bounce up and down to get the horse used to your weight again. Continue to do this several times, then swing up into the saddle. Ride the horse lightly around a small enclosed area like a round pen or riding ring.
Assess the things the horse still remembers by laying the rein on the animal's neck to see if it still remembers how to turn and neck rein. If the horse was properly trained before, it should remember the things it has learned and get back into riding rather quickly. The amount of time needed to take the horse from pasture rest to full riding depends on a number of factors, including the horse's condition, how long it has been at rest and your own level of skill as a trainer.
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