Karen Berk of Equine Reproduction Services, Ocala, Fla., advocates increasing comfort with a hot towel. I use this method when the stallion isn't at his peak due to cold weather, or with an older stallion. I hold the AV in my left hand, and put light pressure with the right hand on the scrotum. An assistant hands me the hot towel, and I place it at that point. She has found that the warmth encouraged complete release in a stallion.
Physical reasons can affect the quality of semen.
Berk cautioned breeders of the effect of artificial lights on stallions. She collects close to 300 times a year, and has noted the differences in the stock horse breeds kept under lights for better hair coats. In the stallion no longer showing, who' s then turned out in pasture and not under lights, the fertility goes up. Hock or back pain can affect a horse's willingness to breed. He could be in pain as he shifts his weight to mount.