I don't usually do it, but then again, most of the horses I ride have very short manes. It doesn't hurt the horse and I see no harm in it. I used to ride a thoroughbred with a medium length mane. If I felt off balanced then I'd grab his mane. One time he went into a bucking fit and I grabbed onto his mane to keep on. I ride English, so I don't have a saddle horn to grab onto.

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It is not 100% necessary. I can get on a horse without grabbing its mane but I don't think grabbing a chunk of mane hurts the horse so if you do its not being cruel. Anyway if you did hurt the horse it would probably throw you off.

you can grab on to the mane. it does not hurt the horse in anyway. I will hold on when every I fell of balance.

I don't think anything is wrong with it. It doesn't hurt them at all. But I don't do it that often.