We purchased a stallion for breeding at nine years of age. He had bred before, but curiously had very few offspring for his impeccable credentials. He was a lovely stallion, very well-behaved, and a perfect gentleman to work around. When we bought him we were advised that he was an over-enthusiastic breeder and that we should breed him by natural cover at pasture at the beginning of the breeding season.
So for his first mare, we proceeded as advised, putting the stallion and mare together in a small paddock where we could intervene if necessary. We rapidly realized that we had a problem breeder. This horse would get so excited that prior to ever penetrating the mare, he would flower and his already large penis would become dinner plate-sized. He was then physically unable to enter the mare. It looked as if this had been a long-standing problem for him as he was obviously incredibly frustrated by this. The harder he would try, the more frustrated, excited, and angry he would get, until he would be running that mare down, teeth bared, and viciously trying to attack her. The mare was so frightened by him she was gumming at him in submission. Needless to say, he was removed.
We tried breeding him in-hand and found that he was pretty much uncontrollable to hand breed. He would rear up and lunge uncontrollably and frantically at the mare and try repeatedly and unsuccessfully to penetrate her and would not back off when directed.
So, we decided to approach this problem methodically. We did a lot of work with him at halter so that he learned to listen to us. We would work him quite hard before we bred him (e.g., free lunged at a strong canter or gallop for 20 minutes or so) to work the edge off and get him breathing fairly heavily. This really helped to tone down his sexual excitement. We felt that if he was a bit short of breath, he would be more concerned about getting air to breathe as opposed to being rough on the mare.