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Bull Breeding Manual

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Society for Theriogenology Manual for Breeding Soundness Examination of Bulls 

Second Edition (2018) – Jennifer H. Koziol, DVM, MS, DACT; Chance L. Armstrong, DVM, MS, DACT -- 147 pages, full color

Chapter 1: Factors Evaluated by Observation of Breeding Behavior

Chapter 2: Physical Examinations

Chapter 3: Examination of Scrotum and its Contents

Chapter 4: Scrotal Circumference

Chapter 5: Transrectal Examination

Chapter 6: Examination of the Penis and Prepuce

Chapter 7: Semen Collection

Chapter 8: Evaluation of Semen Quality

Chapter 9: The Breeding Soundness Evaluation Form

Chapter 10: Venereal Diseases of the Bull

Chapter 11: Management of Beef Bulls

Updated Bull Breeding Soundness Exam Standards


In 2016, a committee of SFT members was formed to review bull breeding soundness standards. The committee, with support from the SFT Executive Board, adopted several updates to BBSE standards with regards to classification of defects, with two primary changes: 1) abandoning a primary and secondary system; and 2) no longer considering distal droplets or abaxial implantation of the tail as morphological defects. Additionally, the committee voted to remove gross motility from the form, although no changes were made to progressive motility standards. New BBSE forms that reflect these changes can be ordered from the SFT office and the updated eBSE software will be released late fall/early winter of 2019. Below are a compilation of the changes.

Classification of sperm abnormalities as head, midpiece, and tail

Although the primary and secondary sperm classification system has been widely accepted and used, the criteria for classification of defects as primary or secondary varies and the perceived significance of primary and secondary abnormalities differs widely among examiners. The most appropriate definition of a primary abnormality is one that originates during spermatogenesis, whereas a secondary defect originates within the epididymis and a tertiary defect is caused by iatrogenic or environmental conditions. All head defects such as acrosomal defects, pyriform heads, microcephalic and macrocephalic sperm and nuclear vacuoles are considered primary abnormalities. Additionally, the Dag defect, mitochondrial sheath defects, and coiled principal pieces have their origins in spermatogenesis and should be classified as primary.

Several morphologic defects may have multiple origins, allowing them to be classified as either primary or secondary. For example, proximal droplets may be the result of disturbance of spermatogenesis (primary) or a disturbance of epididymal function (secondary). Detached heads may result from a defective formation of the basal plate which connects the sperm head to the midpiece capitulum (primary), or may occur due to abnormal epididymal function (secondary).

Classification as primary or secondary indicates the suspected origin of the defect but does not necessarily correlate with the effect of specific defects on fertility. Adverse conditions may simultaneously affect both epididymal function and spermatogenesis, suggesting that primary and secondary defects can be of equal importance as indicators of disturbance of reproductive function. With improved understanding of cause and effect of various sperm abnormalities, assigning greater importance to primary defects is being abandoned.

None of the currently utilized classification systems fully meets the needs of field andrology. Due to this lack of functionality SFT adopted a differential counting scheme that groups defects under categories of head, midpiece and principal piece (tail) abnormalities. Furthermore, space is provided on the form to describe the most common abnormalities noted under the respective headings. A minimum of 70% morphologically normal sperm is still needed to be considered a satisfactory potential breeder.

The goal for examining sperm morphology is to determine the percentage and types of sperm abnormalities present in a sample and construct a record of sperm morphology (spermiogram). The spermiogram may be used to predict potential fertility of semen produced at time of sampling, as well as to determine possible cause of an abnormal spermiogram. Knowledge of the probable cause of abnormalities within the spermiogram will allow the practitioner to develop a prognosis, which will aid the client in making an informed decision about the future of the bull in question.

Distal droplets are no longer considered abnormalities

Distal droplets, characterized as cytoplasmic droplets located just proximal to the annulus, have not been implicated as a cause of substantial impairment of fertility in natural service, even when present in high percentages in the ejaculate. Consequently, sperm with distal droplets and no other discernable defects should not be counted as abnormal in bulls intended for natural service.

In a Swedish study, bulls that produced high percentages of sperm with distal droplets in their ejaculates had AI non-return rates similar to their cohorts with low numbers of distal droplets in their ejaculate. Another study using Zebu bulls suggested that distal droplets were age-dependent, with immature bulls displaying higher percentages, whereas mature and older bulls had a lower prevalence.

Normally, when semen is mixed with seminal fluid and sperm become motile, droplets are shed from the distal midpiece due to increased fluidity of the sperm membrane over the droplet region. At the distal midpiece region, the membrane stretches to form a stalk and then ruptures, releasing a non-membrane bound remnant of the spermatid cytoplasm. Much of the background debris in semen smears results from disintegrating and shed droplets.

Some hypothesize retention of the distal cytoplasmic droplets may be due to a lack of a hemolytic factor, specifically, phospholipid binding protein (PBP), present in seminal fluid. Another theory regarding lack of PBP is that some bulls release insufficient seminal fluid at ejaculation, thereby increasing distal droplets in the semen sample. Interestingly, when semen with a high percentage of distal droplets is incubated for 15-30 minutes or the sample is gently agitated, many retained distal droplets are released.

Abaxial implantation of the tail is no longer considered abnormal

In bulls, presence of sperm with abaxial implantation of the tail does not have a detrimental effect on fertility and such sperm should be considered a normal morphological variationThe combined results of experiments to determine the effect of abaxial tail placement on fertility indicate that sperm with abaxial tails fertilize ova at a normal rate and are not associated with embryonic death.


Abaxial attachments of the sperm tail to the base of the nucleus occur in low to very high proportions of sperm, either alone or in conjunction with accessory tails. Sperm with abaxial tails usually have an empty implantation fossa. Abaxial tails do not appear to originate as a result of extrinsic factors. Bulls producing sperm with abaxial tails often do not display other sperm abnormalities in high percentages and presence of the abaxial tails remains constant over prolonged intervals. When ejaculates containing sperm with abaxial tails in the absence of other sperm abnormalities are produced, bulls generally have grossly normal testes and produce semen with density and progressive motility, which can have good post-thaw motility and viability following cryopreservation.


Sperm Motility

Assessment of mass motion is in­fluenced by 3 factors: concentration, per­centage of progressively motile cells and velocity of progressively motile sperm. Semen with fair concentration may have 80% rapidly, progressively motile sperm without wave motion, whereas highly concentrated semen may have only 50% motile sperm but still exhibit some slow wave motion. In addition, semen with very good concentration and a high percentage of progressively motile sperm may have little or no wave motion if the speed of sperm progression has been diminished by chilling the semen or if in­terval from collection to examination is prolonged. Consequently, gross motility must be carefully interpreted and determination of individual progressive motility is pre­ferred for assessment of semen quality. Therefore, gross motility has been removed from the BBSE formNo changes were made to 30% progressive motility as the minimum needed to be considered a satisfactory potential breeder.


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The Society for Theriogenology is an organization of veterinarians dedicated to animal reproduction, whose mission is to promote standards of excellence in reproductive medicine, to provide outreach and education to veterinarians, and to foster continual improvements in theriogenology.

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