All barding encumbrances listed in UA are reduced to 40% of their listed value when equipped. If not being worn by the mount, encumbrance is as listed.Ex1)Here is the reasoning:
Rules for barding were introduced in 19832) and reprinted in UA in 1985. They are basically broken – mainly, the barding is far too heavy. Encumbrance was dramatically increased from the OD&D numbers, to the point where medium and heavy horses may not even be able to move at all.3) This was brought up in Dragon #118, but via a forum submission which was apparently not replied to by TSR.
| Barding | AC mod | AC type | Eq. cn. | Uneq. cn. | Base MV“ and Mi./day (L/M/H Warhorse) |
|---|---|---|---|---|---|
| Leather | -1 | 8 | 640 | 1600 | 21/17/15 |
| Scale | -2 | 6 | 1000 | 2500 | 15/15/14 |
| Chain | -3 | 5 | 1400 | 3500 | —/12/13 |
| Plate | -4 | 3 | 2000 | 5000 | —/—/12 |
Don't forget, MV is halved if the mount's first encumbrance threshold has been met. If the mount's maximum encumbrance has been reached, it is unable to move.