I look at it like fencing, when you build fence you brace the corners to keep the pull of the fence from pulling the corner post over. The bars are the brace and the heels are the post. I'm sure they bear some weight but more importantly they support the heels against the forces of the landing phase of the stride and load bearing. Have you ever seen a hoof with underrun, crushed heels that has straight bars? Posted via Mobile Device
Subbing to this as I have always been unclear on it myself.
My personal opinion is that the bars are semi-load bearing. That is, they may make limited contact with the ground, basically just barely touching on impact.
If a horse was in the wild and had their hooves worn down by natural forces (stones, bedrock...) it seems to me that the bars would have to make full contact to wear if they are as hard as the walls. However, they are definitely not as durable as the walls, so to me that implies that they would shed kind of like the sole.
I like Mike's conception. Best one I've heard so far ever.
Here is a picture of one of my horse's hooves, she ripped the heel all the way up to the coronary band. I cut as much of the loose heel off as I could(red arrow), notice how much thicker the bar of the injured heel is than the opposite side(green arrow). That bar became fully load bearing to compensate for the lost heel support. Sorry it wasn't a better picture, I had taken it with my phone by myself.
Sorry to double post, but this now leads me to a new question. Say you were trying to take the heels down over time (a la my newly barefoot in back horse). If you take down more heel and leave the bars alone, won't the bars now be taking all the load and be subsequently crushed?
I treat the bars pretty much as I treat the walls. I believe they're there for a reason, so should not be removed. I believe as with the walls, they help with weightbearing, but that they are not a primary weightbearing structure. Eg. Standing a healthy footed horse on concrete, generally the outer walls, quarters and most of the sole & bars will be non-weightbearing. However, as horses haven't evolved for & don't tend to live & work solely on flat, hard surfaces, the entire base of the hoof is effectively weightbearing. As with the walls I think it depends on environment, work, etc as to how close they should be trimmed to the sole.