hello, just sure if any of the below will help, but here goes
1. Racehorses are trained that when rider pulls on reins, horse is supposed to go faster. So if you're on an ex-racer who hasn't been formally retrained, he might still think this, either all the time, or when he gets outside, and the excitment of everything takes over. At the end of this post I'll copy and paste a groundwork exercise you can do to retrain the brakes.
2. Lets see what other reasons could be causing this issue. Maybe too much food, and not enough turnout, for the type / amount of work your horse is doing? A lot of energy in, and not enough work to use it up can cause this type of issue.
3. Sore teeth. Horses with sore teeth can bolt. Might be worth a check vet-dentist checkup.
4. Sore back. Horses with sore backs can bolt. Ex-racehorses I'd imagine could be prone to having dodgey injuries due to the fact they might have been raced as 2 and 3 yos, when their bones weren't yet fully developed, and it could have caused damage.
5. Horse, when excited forgets you are the boss
Figure out what places you are in control (maybe his normal paddock or arena), what places are 50/50 (you have less control) and what places you have no control (grass fields).
Now make a list of all the things you can do in the safe place (arena etc), starting with the simplest thing first. Eg. I can lead horse perfectly in arena. I can ask him to backup in arena, I can ask him to do a HQ yield in Arena. I can get up in arena, I can stand still in arena, I can ask for a step back in arena.
You might have 30 things you can do well in safe place / arena, 10 things you can do well in 50/50 place (maybe roadway to field etc), and maybe just 1 thing you can do well in open field.
First of all get all these 30 things perfect (seriously) in safe place. Then pick the easiest one of these, and try it in the 50/50 place. If your horse gets it right (and its a good achievement) actually stop, get off, lead horse home. Doesn't mater if you've just spent 5 minutes out in 50/50 place. This will have a huge positive impact on your horse. He'll be thinking 'I must have done something pretty impressive to end the lesson!'.
I'd advise maybe ten minute lessons, if possible each day or if you live near horse, maybe twice a day.
What you want to do is, one task at a time, get those 30 tasks which are perfect in the arena, perfect in the 50/50 place. When they are perfect in the 50/50 place, then take the easiest one (maybe halt-backup on ground) out into the big field.
For the first riding one in the big field, I'd say walk out at edge of field, stop, and then backup. Then if horse does it, get off, lead home, end lesson.
The idea is that you build stuff up slowly, lots of small little steps, trying to improve your horses behaviour while keeping both of you safe. It'll take a few weeks, but should help, but the groundnwork retraining below will be very important too.
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GROUNDWORK EXERCISE TO RETRAIN BRAKES
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It can be useful to know how to retrain a horse who is difficult to stop, or who you constantly find yourself in a tug of war with when riding, with both of you pulling on the reins or your horse leaning on your hands a lot.
Firstly, there's a few reasons you should look into - does your horse have sore teeth, back pain, badly fitting saddle (horses can run forward to get away from any sort of pain), badly fitting bit for his mouth shape, too much high energy food, not enough time outside to gallop around by themselves, etc.
You can retrain your horse's brakes if its just a learnt thing, but if its any of the things above, its better to fix those first in order for thr retraining to work. eg. If your horse pulls cos he has a sore tooth, you can retrain him all day but he'll still pull cos his tooth is sore.
OK, so you have checked that your horse has no issues such as above, and instead it is just a training issue. So lets retrain your horse.
GROUNDWORK
Lets start on the ground in an enclosed arena or small paddock, with just a headcollar and lead rope (ideally a nice 12 foot one)on your horse.
1. Stand slightly off centre in front of your horse, looking at his face. Give him a rub. He should be standing still and relaxed.
2. Put two fingers very lightly on where his noseband usually is, across the centre of his nose.
3. Softly put a little pressure on the nose (like a tiny push with both fingers - REALLY lightly) to ask your horse to take a step back.
4. Your horse will probably either stand there half asleep, or look out at whats happeneding next door. Thats fine. He hasn't figured out your asking him to do something yet.
5. Make this *light* nose pressure rhythmical. Touch with two fingers on noseband area, then take hand away, count for three seconds, then touch again on noseband area. Continue like this for 20 or 30 seconds. remebr this is something new for your horse, so give him time to think it out. Good teachers have patience
6. After maybe half a minute of this (remember the rest in between the light pressure, these rests are REALLY important, and your horse will normally move DURING a rest) keep that *light* nose pressure, but now gently throw the end of the lead rope towards your horses chest.
This is a bit like rubbing your stomach and patting your head but if you concentrate its possible
What you are looking for is for that *split second* where your horse is thinking what you are thinking.
So look at your horse's chest, and the second you see him start to shift his weight backwards, even bore a step has been taken, stop everything and just stand quietly beside him.
Repeat this, always starting from light nose pressure only, and only slowly building up to flicking the rope against your horses chest, until you see that slight movement backwards.
What you are doing is retrainig your horse to learn is that:
Light pressure on noseband area = think (and move) backwards.
Do this for two or three days, and work up from thinking backwards, to one step backwards, to two or three steps backwards over a few days. Don't rush, be patient, and only do it for max 5 mins a day. Keep your lessons short and interesting.
RIDING - reverse
Riding will only work if you've done the groundwork first.
1. Sit up on your horse, and stand still.
2. Pick up a very light contact on the reins.
3. Wait.
4. Your horse might try to shake his head, walk forward, walk sideways, waggel an ear, etc. Hold the light contact.
5. The *split second* you feel him *start/think* abotu moving backwrds - even before he takes a step backwards, release ALL pressure on reins (ie give him the reins in horse talk), then give him a rub, relax, and just enjoy the view.
6. Repeat for no more than 5 mins for a few days, only releasing all pressure on rein when horse either thinks of going back, or when you get good takes a step backwards.
RIDING - walk-halt-reverse
RIDING - trot-walk-halt-reverse
RIDING - canter-trot-walk-halt-reverse
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Hope this is of some use