O.K.Balch, D.Butler and M.A.Collier
Balancing the normal foot: hoof preparation, shoe fit and shoe modification in the performance horse EVE Manual No 4 The Equine Hoof Dec 1998
"The bars can be trimmed below the level of the ground surface of the finished wall and contoured to match the concaved adjacent sole. Excessive trimming of the bars is discouraged as bars may act as internal struts to inhibit heel contraction in the absence of normal frog pressure (Balch et al. 1991)"
Lameness in Horses - Weight-bearing structures of the foot
"The wall, bars, and frog are the weight-bearing structures of the foot. The sole should not bear weight except for a strip about ¼ inch wide, or less, inside of the white line. The bars should bear weight and in shoeing should not be removed but lowered only enough to allow fitting of the shoe. The bearing surface of the wall should be level with the frog for even distribution of the weight."
Farriery - A complete illustrated guide
"The bars appear on the bearing surface of the foot as convergent ridges which are fused with the sole and united by the frog. By not completing the circle of the wall they allow for expansion of the foot and being part of the wall take weight and provide extra bearing surface and strength at the heels. They must be allowed to grow and develop normally. If they are cut away, as is sometimes practiced, the foot is deprived of support at the heels and contraction results. The are of the sole between the wall and the bars is referred to as the "seat of corn"."
The following 7 quotes were taken from "A Table With All the Trimmings" - An in-depth analysis of seven trimming techniques ranging from barefoot to traditional. (I believe this was an American Farriers Journal supplement.
"bar structure is retained to provide stability during flexion and distortion."
"Purchase or surface area at the angle of the bar is encouraged."
"...the bars are level with the live sole..."
HoofTalk Natural Trim
"The bars are tapered downward to blend in at the sole plane."
Proper Trimming of a Sound Horse
"Very carefully, begin trimming the solar concavity at the tip of the frog. Trim to where the frog and sole horn meet until the beginning of the bars"
"Bars and sole have not been weakened by excessive paring or rasping."
Dr Ric Redden
Four Point Trim
"Leave everything the horse needs and leave nothing he doesn't need, which means I leave all sole and bars."
Natural Balance Trim
"The bars are trimmed to the height of the outer wall or not at all, except in cases where they are rolled and are flat with the sole. At that time, they should be removed enough to encourage more upright regrowth."
Michael T. Savoldi
Universal Sole Thickness
"Trimming to uniform sole thickness simply means trimming a hoof wall to the same plane of the healthy sole". (no mention of the bars)"
Dr Robert Bowker
Physiological Trimming for a Healthy Equine Foot
Bowker also emphasizes that the bars of the foot should not be removed. "They are there for a reason! Leave the bars so that they are a little bit lower (shorter) than the hoof wall - with a 'little bit' meaning a fraction of a millimetre. But obviously if the bars have overgrown the horn they need to be trimmed but not removed - again this is a matter of common sense."
In regard to the bars there are two important considerations. First they must protrude only very slightly above the level of the sole and must not reach the level of the walls; where they meet the frog they should be about 1cm shorter than the wall to allow the sole to draw flat on weightbearing. If the bars protrude so far as to become level with the wall even in a non-weightbearing situation they prevent the expansion of the hoof capsule on weightbearing keeping the sole high (like a pillar in a cathedral) and push it higher as they grow also pinching the frog near the frog corium.
All of this causes contraction prevents the hoof mechanism and causes bruising of the solar corium under the navicular bone (commonly diagnosed as the heel pain of navicular syndrome).
Also the bars must not come to be pressed flat onto the sole: this results in a firm merging with the sole horn and may cover large parts of the sole making it inflexible (and causing pressure onto the solar corium reducing circulation there resulting in poor horn production and/or bruising). If this condition persists for a long time anyone finally attempting to remedy this deformity by removing the laid-down bars may find no sole horn left under them (but once the unnatural pressure is removed sole horn will quickly regenerate).
EVJ Vol 21 Number 1 Jan 1989 The relationship of frog pressure to heel expansion
"Lungwitz (1884) suggested that as the foot took maximum weight depression of the second phalanx would compress the digital cushion which would thus expand sideways forcing out the hoof cartilages and the hoof wall at the coronary band. At the same time pressure from the ground would act upwards against the frog causing it to spread and force the bars outwards resulting in expansion of the heels at the ground surface. Lungwitz (1884) stated that if frog pressure was absent only limited expansion could take place and that in his experiments the feet with no frog pressure had upright heels and that the hoof walls made an unsatisfactory angle with the ground."
"Anatomically the hoof wall is essentially a truncated cone with a segment removed from the posterior aspect. Because the hoof walls slope out towards the ground surface of the foot loading the wall will result in a force which causes the ground surface of the foot to spread outwards. This action is restrained by the internal structures of the foot. It is quite probable that this anatomical shape of the hoof wall is one factor contributing to expansion of the heels in weghtbearing and slight variations in hoof shape would partly account for inconsistent results seen in these experiments. Lungwitz (1891) suggested that the second phalanx pressing down on the cushion of the heel might squeeze the heel between phalanx and frog with a resultant outward pressure from the hoof cushion spreading the heels. A third factor which cannot be ignored is that the bars of the foot which run up to join the frog also give some support to the heels. Excessive pressure on the frog could conceivably result in the bar being pulled inwards and upwards the resultant force being transmitted to the hoof wall via the sole and junction of the bar to the heel of the foot."