Cattle Today

Cattle Today

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Dr. Steve Blezinger

It's been said time and time again that the producer must remember that the bull is 50 percent of the herd. To a large degree this is very true since he makes up 50 percent of the genetic base of the calf crop and is also 50 percent involved in the rebreeding process. What tends to make us believe that he's involved to an even greater degree is the bull to cow ratio which is commonly 20 to 40 to 1. When you consider that one bull may be responsible for the breeding of as many as 40 cows, that makes for one tired bull but also shows the importance of insuring the bull or bulls you are using are sound and up to the job.

I tend to see two extremes in a lot of situations. Producers that go out and practically break the bank buying bulls for their herd and the other guys that go out and buy the sale barn specials. This results in either a significant amount of risk for the first producer if he purchases exceptionally expensive animals which may not be appropriately suited for the herd he has. He might be better suited to make the genetic improvements he wants by implementing an A.I. program. Don't get me wrong, I believe in buying the best genetics possible but I also believe in common sense and matching your bulls to the cow herd. The other producer will end up producing calf crops which are marginal at best and then will complain about the price he receives for these calves. This is a situation where, in most cases, you get what you pay for.

Regardless of the circumstances, producers must be aware of certain factors which affect how well a bull will perform. Remember also that the factors discussed are even more important if you have implemented a limited breeding season (i.e. 60-90 days) and all the breeding must take place in this period of time. Let's look at some of these various factors.

Factors Affecting Bull Fertility and Performance

There are 5 general factors which can affect bull fertility and performance including: 1) structural soundness, 2) capability of the reproductive organs, 3) quality of semen, 4) level of libido, and 5) plane of nutrition. All of the factors are important and a deficiency in any category will have a negative impact on fertility.

1) Structural Soundness

General health and structural soundness are important aspects of fertility. Poor health can affect libido, mating ability, and semen production and quality. Structural soundness, including functional feet, legs and associated joints, is critical for the bull to effectively travel the breeding pasture and service females in heat. Any disease condition which impairs the mobility of the bull will hinder reproductive performance. A conformational problem commonly associated with poor breeding performance is extreme straightness of the rear leg (post-legged). A moderate angle in the hock joints is necessary for the bull to thrust properly after mounting. Proper angulation in the leg joints also helps to absorb the shocks produced during each step and increase the productive life-span of the bull

2) Reproductive Organs

The reproductive system is complex. Sperm is produced continuously by the testes and stored in the epididymis. The prostate gland, seminal vesicles and cowper's glands secrete the fluid component of the semen. During mating, the penis is extruded from the sheath by the straightening of the S-shaped sigmoid flexure; sperm are transported up the vas deferens to the urethra and exit via the penis.

Deep body temperatures are too warm for proper sperm production. This is the reason the testes are located outside the body core. As the environmental temperature changes, the testes are raised and lowered in the scrotum to maintain proper temperature for sperm production.

Various conditions can affect the function of the reproductive tract. If the testicles cannot move because of fat deposition, scar tissue or a small scrotum, proper temperature cannot be maintained and semen quality may suffer. Soft testicles indicate degeneration of tissue and poor semen quality. Very small testicles indicate unsatisfactory development of sperm-producing tissue. Severe frost-bite scabs, tumors or abscesses also indicate potential problems.

Infection and inflammation can occur in any of the reproductive organs. If the testicles become inflamed, the semen quality may be impaired long after the original conditions has passed, since it takes approximately 60 days for new sperm to be produced and mature.

Common penile problems include spiral deviation, persistent frenulum and penile hair rings. The most common problem is spiral deviation where the penis is twisted instead of straight. Bulls with this defect produce fewer pregnancies than normal bulls. Bulls evaluated using an electroejaculator may display spiral deviations which will not occur under natural conditions.

A persistent frenulum is heritable conditions in which the tip of the penis remains attached to the sheath and cannot be extended. It can be surgically corrected. Penile hair rings are most often seen on young bulls. A band of hair encircles the penis. If the conditions remains untreated, infection and scarring may result. Other conditions which can affect the penis include fractures, watts and scarring from previous injuries.

3) Scrotal Circumference

Measurement of the scrotal circumference of young bulls is an accurate, repeatable method to assess current and future sperm- producing ability. The measurement gives an estimate of the weight of the testes, which is directly related to the level of sperm production. Scrotal measurement is also positively correlated with semen volume and quality. Table 1 contains minimum recommended scrotal measurements by breed and age (see chart at bottom of page). Bulls with adequate scrotal development for their age have a higher probability of becoming satisfactory breeders than bulls with smaller scrotal circumferences.

4) Semen Quality

The criteria commonly used to evaluate semen quality include sperm morphology (structure) and motility (rate and percent of progressive forward movement). Semen volume and concentration can also be used.

When assessing the results of a semen test, several points should be taken into account:

In the field, correlations between semen quality traits and fertility have been low to moderate.

The repeatability of semen evaluations of the same bull over time have been low.

Semen tests results for young bulls may not be conclusive. A poor semen test on a bull less than 15 months of age is not a reliable indicator that the bull will have poor quality semen a few weeks later. Seminal qualities may improve dramatically for up to four months following puberty.

While a bull must produce some viable sperm in order to be fertile, semen quality is only one aspect of total fertility and must be evaluated in conjunction with all the other factors.

5) Libido

Libido (sex drive) is a critical component of fertility. It is independent of scrotal circumference, semen quality, body weight, growth rate or masculinity. One method of measuring libido is to measure serving capacity. The bull is exposed to a group of restrained females and the number of mounts and services completed in a given time period is recorded. This method is time-consuming and requires confinement of a number of females.

When two or more bulls are used at the same time, in the same pasture, social interactions affect breeding performance. Social rank is related to age and seniority in the herd. The most dominant bulls tend to complete the highest number of services. In this situation, the number of cows serviced may be related more to social dominance than libido as measured by a serving capacity test.

6) Nutrition

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.

During the breeding season, bulls tend to eat less feed than is required to maintain their body weight. They use body fat for energy and may lose up to 150 pounds during the breeding season. Supplying a grain mix to bulls on pasture is not always effective. Proper pre-breeding nutrition is essential to ensure the bull has adequate reserves for a successful breeding season.

During the off-breeding season, mature bulls in moderate flesh can be maintained on spring and summer pasture. During the fall and winter feeding periods, mature bulls should be conditioned for breeding. The importance of having bulls in proper condition upon start of the breeding season cannot be over-emphasized. Herd bulls must be in good condition to be fertile and sexually active. In the fall, start feeding mature bulls hay or silage plus one pound of protein supplement before they start to lose weight. Mature bulls will consume daily amounts of feed equal to 1 to 3 percent of their body weight, depending upon condition and individuality. It is suggested to evaluate body condition score of bulls prior to the breeding season. A small amount of extra feed may be needed 60 days prior to the breeding season to get mature bulls in moderate condition. Five pounds of grain daily should be ample for most bulls; however, if a bull is lacking in condition, he may require as much as 15 to 25 pounds of grain per day. It is important to realize that "hard-keeping" bulls, which require excessive amounts of grain to maintain moderate body condition, will likely sire offspring that are also "hard-keepers" and probably should not be used for breeding due to the economics involved with feeding such animals.

When conditioning bulls remember that excessive fat deposits in the scrotum may interfere with temperature regulation. 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 hand, large breed yearling bulls starting the breeding season with minimal levels of backfat, may have poorer semen quality than similar bulls carrying a moderate level of backfat. The nutrient requirements needed to optimize reproductive performance in breeding bulls needs more research.

Cow to Bull Ratio

One last area of concern is the cow to bull ratio. To produce a calf every 12 months, the cow must be bred within 80 days alter calving. In order to achieve high pregnancy rates in a restricted breeding season, bulls should not be overworked. Table 2 shows that number of cows which a bull of average fertility can be expected to impregnate in a 60-day breeding season on pasture (see chart at bottom of page). Cow numbers may be increased by about 30 percent if cattle are kept in a confined area or are presented to the bull only when in heat.


Proper selection and management of the beef bull will increase the probability of a successful breeding season. A complete assessment of potential fertility includes an evaluation of structural soundness, reproductive organs, semen quality, libido and nutritional status. Selection for increased scrotal circumference should increase inherent fertility in both male and female progeny.

Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, Texas. He can be reached at P. O. Box 653 Sulphur Springs 75483, by phone at (903) 885-7992 or by e-mail at


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