Age at Puberty � An Overview

Puberty is typically defined as the age at which reproductive function is initiated in animals. This corresponds to changes in hormone levels and the ability to function sexually in order to reproduce. More specifically, puberty can be defined in bulls as the age at which the animal is first able to produce an ejaculate containing 50 million sperm with a minimum of 10% motility (capable of motion). It is related to age, body weight and testicle weight. Age and body weight at puberty vary across breeds as shown in Table 1. However, it is interesting to note that scrotal circumference at puberty (an indicator of testicle weight) remains fairly constant at 28 to 29 cm. In most cases bulls may exhibit initial sexual interest about three weeks prior to puberty. Subsequently they attain mating ability about six weeks after puberty. Although bulls that have reached puberty can breed, reproductive capacity increases as the bull continues to mature. The reproductive development and performance of young bulls has gained attention as beef breeders attempt to accelerate improvement of economic traits in cattle and reduce costs by using younger sires. This is not quite as much of a problem if the young bull is simply used to collect semen and used in an A. I. Program, but if used in natural service, this issue can become more of a problem.

Nutrition as it Relates to Puberty

As we have discussed repeatedly, proper nutrition is necessary for good reproductive performance. Balanced amounts of protein and energy are required for sperm production and the physical activity associated with breeding. Adequate amounts of vitamins and minerals are also important in reproduction.

In young, growing bulls it is important to recognize that the onset of puberty can be directly affected by the nutritional plan these young cattle are on. Inferior nutrient supplies will delay growth and development of the animal as well as the onset of puberty. Subsequently, cattle on an optimal plane of nutrition, i.e. the diet meets all their nutrient needs, will experience growth rates and pubertal on-set as quickly as genetic predisposition allows.

Another nutritional factor that plays a role in bull performance is that of intake during the breeding season. During the breeding season, bulls tend to eat less feed than is required to maintain their body weight, primarily because they have other things on their mind. They use body fat for energy and may lose 150 pounds or more during the breeding season. Supplementing with a high-energy grain mix to bulls on pasture is not always effective. Proper pre-breeding nutrition is essential to insure the bull has adequate reserves for a successful breeding season and can tolerate the weight loss without detrimental effects.

Still another issue to consider is that of potential excess conditioning. In these cases scrotal fat deposits can be a problem. Excessive fat deposits in the scrotum may interfere with temperature regulation. While the degree of body fat required to adversely affect sperm production has not been well defined, extreme fatness has been associated with low serving capacity. On the other band, large-breed yearling bulls starting the breeding season with minimal levels of subcutaneous fat may have poorer semen quality than similar bulls carrying more moderate levels of fat cover. The nutrient requirements needed to optimize reproductive performance in breeding bulls needs more research.

Issues Concerning Scrotum and Testicles

Temperature Control: The testicles have two functions: producing spermatozoa, and producing the hormone, testosterone. The testicles are located outside of the body cavity in the scrotum. This is essential for normal sperm formation that occurs at a temperature several degrees below normal body temperature. The scrotum is important for controlling the temperature of the testicles (thermoregulation). This is done by means of a temperature sensitive layer of muscle (cremaster muscle) located in the wall of the scrotum. This muscle relaxes when hot and contracts when cold. In warm temperatures relaxation increases the relative length of the scrotum, thus moving the testicles away from body heat. In cold weather, the scrotum shortens and the testicles are held close to the warm body.

Scrotal/Testicle Shape: A common cause of low fertility in bulls is abnormal testicle and scrotal sack development. The testicles should be symmetrical, nearly the same size, and freely movable in the scrotum. Small size or degeneration often affects one testicle only and is a serious finding.

There are three basic scrotal shapes in beef bulls. These are the "normal" or "bottle-shaped" scrotum, "straight-sided" scrotum, and "wedge-shaped" scrotum. Bulls 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 thermoregulation (note previous discussion). As bulls mature and lose condition, they will often 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 scrotal configuration have undersized testicles and seldom produce semen of adequate quality.

Consistency Of The Testicle: The consistency of the normal testicle is much like a firm rubber ball. Extremely hard testicles indicate infection (orchitis) and very soft ones indicate degeneration. The epididymides, the structure that surrounds the testicles and transports semen to the accessory sex glands can be palpated. Defects of this structure 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.

Scrotal Circumference: Testicular size or the amount of sperm producing tissue is estimated through the use of scrotal circumference. Table 2 reflects the scrotal circumference sizes of bulls of different breeds at different ages. 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 F, 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. The question has been asked: 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

Effects Of Age on Scrotal Circumference: 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 is curvilinear (goes up fast and then flattens out) rather than linear (a straight line). Scrotal circumference increases from 2 to 3 centimeters between one and two years of age for most breeds. Some data from Colorado State University suggests a value of 0.026 centimeter per day for an adjustment factor.

Scrotal Circumference and Male Fertility: There is a high correlation (.81) between scrotal circumference and sperm output. In yearling bulls, researchers have noted that as scrotal circumference increase, motility, percent normal sperm, volume, sperm concentration, and overall sperm output increase while percent abnormalities decrease. It has been estimated that for every 1 centimeter increase of a sire's scrotal circumference over the population average, one can expect a 0.25 increase in scrotal circumference in male offspring.

Scrotal Circumference and Female Fertility Traits: An interesting fact is that scrotal circumference 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. The variation of one centimeter increase in testicular circumference on reduction in age at puberty has been from 0.75 to 10 days. It is well accepted that sires with above average scrotal circumference should produce female offspring that reach puberty sooner and have greater lifetime reproductive potential.

Conclusions

Attention to details such as age at puberty and scrotal circumference and other testicular factors can play an important role in the fertility and overall performance of the young bull. These components should be a strong deciding factor when selecting replacement bulls or developing those from your own herd.