The shedding blade consists of a strip of flexible metal - sometimes made of plastic - (approx. 50 cm by 4 cm), with a handle at both ends (1). It serves to wipe off moisture from heavily sweaty horses.
See also the sweat scraper. [MOT]
(1) A discarded scythe blade was often used for this (e.g. ''Nouveau dictionnaire universel des arts et métiers'': s.v. couteau).