Beef Cattle Bull Scrotum and Testicles

       The testicles have two functions; producing sperm and testosterone. The testicles are located outside of the body cavity in the scrotum. Normal sperm formation occurs at a temperature below normal body temperature. The scrotum acts as a control for the temperature of the testicles. It is done by means of a temperature sensitive layer of muscle located in the wall of the scrotum. The muscle relaxes when hot and contracts when cold. In warm temperatures relaxation will increases the length of the scrotum, thus moving the testicles away from body heat and in cold weather, the scrotum shortens and the testicles are held close to the warm body.

     One common cause of low fertility in bulls comes from abnormal testicle and scrotal sack development. The testicles should be symmetrical, about the same size, and freely movable in the scrotum. Small size or degeneration often affects one testicle only and is a serious defect.

     You will usually find about three basic scrotal shapes in beef bulls. They may be referred to as a normal or bottle shaped scrotum, straight sided scrotum and wedgeshaped scrotum. Those having a normal scrotum with a distinct neck generally have the best testicular development. The normal scrotum offers the best opportunity for temperature control of the testicles. Often bulls with straight sided scrotums are only moderate in testicle size. The straight-sided neck of the scrotum is generally the result of fat deposits that may impair proper temperature control. As bulls mature and lose weight they will may develop a more normal scrotum. Wedge shaped scrotums are pointed toward the bottom and hold the testicles close to the body wall. Bulls with this wedge shaped scrotal configuration will have undersized testicles and won't usually produce semen of adequate quality.

     When palpated the consistency of the normal testicle is much like a firm rubber ball. Extremely hard testicles indicate possible infection and very soft indicates degeneration. The epididymides, a structure that surrounds the testicles and transports semen to the sex glands can be palpated. Defects seriously affect fertility. The neck or upper part of the scrotum can be examined. Intestine will be found in the upper part of the scrotum if a severe inguinal hernia is present. This is most common on the left side. Sometimes large fat deposits in the upper part of the scrotum can resemble an inguinal hernia.

     Testicular size or the amount of sperm producing tissue is estimated through the use of scrotal circumference.

     Scrotal circumference is an accurate and highly repeatable measurement when obtained by use of a flexible centimeter tape slipped over the bottom of the scrotum and pulled snugly to the point of greatest diameter of the scrotal sac with the testes fully descended. Testicles that are not fully descended may have wrinkles in the scrotum that will inflate the measurement. It is important to get the testicles descended in cool weather (below 50 degrees F) if accurate results are to be obtained. If below 32 degrees, bulls should be evaluated in a warmer environment. The thumb and finger of one hand are placed on the side of the scrotum cradling the testes rather than grasping either the front or back or neck of the scrotum. How much scrotal circumference is enough? In one study, the probability of a beef bull having satisfactory seminal quality increases until about a scrotal circumference of 38 cm. After that point no additional improvement is apparent.

     Bull age has the greatest effect on testicular development in young bulls from 6 to 36 months of age. There is rapid testicular growth in young bulls (6 through 16 months of age) and tremendous range in testes size for bulls of the same age within breed. Thus scrotal growth goes up fast and then flattens out rather than a straight line growth. Scrotal circumference increases from 2 to 3 centimeters between one and two years of age for most breeds.

     There is a high correlation between scrotal circumference and sperm output. In yearling bulls research has shown that as scrotal circumference increase, motility, percent normal sperm, volume, sperm concentration, and overall sperm output increase while percent abnormalities decrease.

     Another interesting observation is that scrotal circumference of the bull is highly correlated with age at puberty in half-sibling heifers. Heritability estimates for female reproductive traits are generally low, while heritability estimates of testicular traits are moderate to high. It has been estimated that for every one centimeter increase of a sire's scrotal circumference over the population average, one can expect a four day decrease in the age at onset of puberty in heifer offspring. Sires with above average scrotal circumference should produce female offspring that reach puberty sooner and have greater lifetime reproductive potential.


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