There are no herbs to expel retained afterbirth in mares.
I do however have a brood mare mix which tones the uterus prepares for foaling, The Brood Mare Mix.
Ideally you would begin this treatment pre-pregnancy through to post-pregnancy.
This blend of herbs is designed to improve or restore fertility and normalize the mare's heat cycle, to protect and develop the foetus, and to tone up the uterus for ease of foaling.
The overall aim of this treatment is to help the mare produce a strong healthy foal with no problems each time, while prolonging and facilitating her breeding career by maintaining her own reproductive and general health and condition.
Alternatively you can make up a raspberry leaf tea by boiling a cupful of raspberry leaves in a jug of water, cool off divide into 3 portions & use as a drench 3x daily.
This will condition & tone the uterus