[1] This area may need to be cleaned, starting at a young age after breeding and the birth season, but particularly in geldings.
[2][3][4] Not only can smegma, a waxy substance that includes dirt and dead skin cells, accumulate, but some geldings (and occasionally, stallions) may also form a "bean", a hardened ball of smegma inside the sheath or even the urethra that, in extreme cases, can interfere with urine flow.
To begin, a gentle stream of warm water is run into the sheath from a hose or a large, needleless syringe.
[1] The process requires either that the horse "drop" his penis or that the groom reach up inside of the sheath to pull the penis gently from the sheath by the glans (head) in order to apply cleaner and carefully clean the entire region.
[1] The "bean" is often found in the urethral diverticulum, a pocket adjacent to the opening of the urethra, so that area must also be checked.