Mane Tangles

My mare's mane can tangle itself into a mess quickly.  I'm not sure why it is that one of our horses always has a nice mane and the other looks like she's been tying knots in it over and over.  Does anybody know why that is and what can be done to change it?

Do I need to look for some type of leave in conditioner or hair vitamins or something like that?  I love long manes, especially on her because she has a black and white mane.  It's very pretty.  I've found the only way to reduce tangling though is to keep it shorter which is definitely not my preference.  Anybody have any ideas?