The penis of the bull is different from many species in that it contains little erectile tissue but is constantly rigid. In a nonerect position, the bull's penis forms an S-shaped curve. During erection, at the time of ejaculation, the penis straightens out and extends about a foot beyond the prepuce. They become motile after ejaculation, a process that results from muscular contractions of the ducts that lead from the testis and connect into the penis. Secretions of the accessory glands not only provide a means of transporting sperm, but also provide essential nutrients for sperm metabolism.
Seminal plasma is rich in fructose, the major source of energy for sperm.
A mixture of 20 percent sperm and 80 percent seminal plasma is formed. This mixture is called semen. A bull can ejaculate from five to 20 billion sperm in five to eight ml of semen. Normally, only one sperm can fertilize an egg. With natural service, a bull may ejaculate seven to 10 billion sperm cells just to breed a single cow.