Predicting Bull Fertility
Livestock Update, February 2000
W. Dee Whittier, DVM, Extension Veterinarian, Cattle,Department of Large Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech
One of the major contributors to overall reproductive performance in any setting where natural service is used is fertility of the male. Since beef cattle reproduction depends so heavily on natural service assuring high bull fertility is always crucial to successful breeding seasons with high pregnancy rates. Predicting the fertility of bulls is an area of research that has been active for some time and which is ongoing. Research and experience have identified a number of factors that influence bull fertility.
Following is a list of factors that influence bull success in impregnating cows during limited breeding seasons:
In an attempt to allow trained professionals to assess the potential for the reproductive success of a bull a systematic approach to bull evaluation has been developed. This evaluation involves an assessment, performed in as objective a manner as possible, providing for the prediction of bull fertility. This procedure is termed the Breeding Soundness Examination (BSE) and has been formalized by the American Society for Theriogenology, whose members administer the examination.
The BSE is performed at a single examination, although repeated examinations may be required in some cases. It requires that bulls be restrained. It can, however, be performed with equipment that is relatively portable so that veterinarians often perform the evaluation on the farm. The BSE consists of the following procedures.
Physical examination - The bull is examined in a systematic way for any problem that would hamper his ability to impregnate cows. This examination may be rather brief or more detailed if there is a reason to suspect that there is a problem with any body system. Common areas for problems are abnormalities of the feet and legs or the eyes. A bull cannot locate and mate cows unless his feet and legs are sound.Interpreting the results of the BSE Table 1 shows the minimum requirements for passing the BSE. It should be remembered that the BSE is only a tool for predicting bull fertility and not a guarantee. Remember that bulls are not routinely evaluated for libido, mating ability or infectious disease status. Remember also that injuries or diseases may result in a bull becoming infertile quite suddenly; bulls may be sound on the day of the evaluation but become infertile soon after. Remember also that social interactions in a breeding setting may have a profound effect. A young bull added to a herd where an older dominant bull is present may fail to impregnate cows despite being reproductively normal.
Reproductive tract examination - The reproductive tract consists of the scrotum and its contents including the testicles, the penis, the prepuce and their associated structures. These structures can be examined externally both visually and by manual palpation. However, examination of the penis and entire prepuce typically requires the extension of the penis using an electroejaculator. There are also a number of internal portions of the reproductive tract which require a rectal examination. The arm of the examiner is inserted through the anus of the bull into the rectum. Because of the flexibility of the rectum the internal portions of the penis, the internal parts of the vas deferens and the accessory sex glands (the prostate and seminal vessicles) can be manually examined for normalcy.
Measurement of Scrotal Circumference (SC) - Measuring scrotal circumference is a crucial part of the BSE. Scrotal circumference has been determined to be the measurement that best predicts the output of sperm cells for bulls when multiple collections by artificial vagina are not available. The measurement technique involves the use of a circular tape. This measure is useful because there is a correlation between the scrotal circumference and the volume of semen-producing tissue that the bull posses. Since SC increases with the age and weight of the bull the circumference must be interpreted in light of the bull's age.
Semen collection and examination - Although semen could theoretically be collected using an artificial vagina, in most cases the difficulty in training bulls to use this system makes it impractical. Instead, the semen sample is collected using a device called an electroejaculator. This device employs a probe that is inserted rectally into the bull. The probe has electrodes that conduct tiny amounts of electricity to the nerves that run through the bottom of the bull's pelvis. This stimulation results in the bull achieving an erection and finally ejaculating semen. An experienced veterinarian or reproductive physiologist should determine semen quality. An examination of the reproductive tract may indicate possible abnormalities in semen quality. Bulls exhibiting normal physical capabilities may still be incapable of settling cows because of poor quality semen.
Bulls which fail to pass the BSE are assumed to be subfertile. Certainly they may sire some calves but would not be expected to perform well in a typical breeding setting. Bulls who fail the BSE at one point may later be capable of passing. Evaluators usually attempt to predict such outcomes and thus classify bulls as Unsatisfactory as a Deferred status.
Table 1. Requirements for being classified as a satisfactory potential breeder bull by the Breeding Soundness Examination system of the Society for Theriogenology.
|Minimum Recommended Scrotal Circumference||Minimum Recommended Motility is 30% of Fair (F)|
|Age||SC (CM)||Mass Activity (Gross)||Rating||Individual|
|< 15 Mo.||30||Rapid Swirling||Very Good (VG)||> 70%|
|>15 < 18 Mo.||31||Slower Swirling||Good (G)||50-69%|
|>18 <21 Mo.||32||Generalized Oscillation||Fair (F)||30-49%|
|>21 < 24 Mo.||33||Sporadic Oscillation||Poor (P)||< 30%|
|> 24 Mo.||34|
Minimum Recommended Morphology is 70% Normal Cells.
To be classified as a Satisfactory Potential Breeder requires a satisfactory Physical and Reproductive Tract Examination and minimum values for Scrotal Circumference, Motility and Morphology. Any bull not meeting minimums is classified as either an Unsatisfactory Potential Breeder or classification may be deferred at the discretion of the evaluator.