Because Hair is a posteffect getting a correct depth pass can be a real pain. Also correct reflections can get you a lot of headaches. There are thirdparty plugins who can help you with this kind of problems but if you want to stay with a inhouse solution, here's a way to solve a hair specific task with the extrude deformer – making grass. Excuse the lame render speed :-)
Hair is a great module nevertheless, it's exceptionally fast when it comes to render particles.
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