Bull Breeding Soundness Evaluation: Current Status

Bull Breeding Soundness Evaluation

A breeding soundness evaluation (BSE) is a relatively quick and economically prudent procedure for screening bulls in relation to potential fertility. Conducting a BSE implies a complete evaluation of all factors contributing to normal reproductive potential. Veterinarians are advised to be as thorough as current technologies allow and to consistently follow a routinely standardized procedure. Predicting which bulls would be least successful in impregnating cows represents the greatest value of the current system. That is, based on history and information collected at evaluation, bulls are categorized as being above or below thresholds for characteristics known to most affect fertility. The most recent guidelines for BSE were approved by the Society for Theriogenology (SFT) in 1992 [1,2]. The SFT also makes Bull Breeding Soundness Evaluation forms (individual bulls) and Bull Certificate of Evaluation forms (herds) available to members (SFT, 530 Church Street, Suite 700, Nashville, TN 37219: therio@wmgt.org).

As outlined, a BSE is intended to be a rapid, albeit thorough and systematic format for identifying problems affecting male fertility. A BSE consists of three steps, as follows:
A generalized physical examination and thorough examination of both internal and external portions of the reproductive system;
A scrotal circumference measurement
Collection and evaluation of a semen sample. The SFT format for bulls established minimum acceptable thresholds for scrotal circumference, sperm motility and sperm morphology (Table 1).

To be classified a satisfactory potential breeder, requires a satisfactory physical examination and minimum values for scrotal circumference, motility and morphology. Any bull not meeting minimums is classified either as an unsatisfactory potential breeder or classification may be deferred at the discretion of the evaluator.

Interpretation of Results

Satisfactory Potential Breeder

Bulls achieving the minimum threshold values for scrotal circumference, sperm motility and sperm morphology. These bulls must also be free of genetic, skeletal, infectious or other problems or injuries which might compromise fertility in natural mating situations.

Unsatisfactory Potential Breeder

Bulls which fail to meet minimum threshold values in any category and which have a poor prognosis for improvement. Any bull having a significant physical defect which would interfere with successfully impregnating cows should be classified as unsatisfactory. Any bull having a significant problem involving the reproductive system which is unlikely to improve should also be classified as unsatisfactory, as well as any bull of less than the minimum scrotal circumference (per age category). Any bull providing an ejaculate containing sperm less than 30% motile or less than 70% normal morphology should be classified as either unsatisfactory or deferred.

Classification Deferred

Bulls which cannot currently be rated satisfactory but are likely to improve with time or therapy. Provision is made on the SFT form for scheduling of a retest. This category includes young bulls with immature semen profiles as well as any bull which has poor semen quality considered to be capable of improvement. Also in this category are bulls failing to yield an acceptable ejaculate for reasons which are not obvious despite careful physical examination, as well as bulls with problems which are considered to be treatable. If there is doubt that a bull may be confidently assigned to either the satisfactory or unsatisfactory potential breeder category, a retest is advised and the bull should be assigned to the classification deferred category. Both false positives and false negatives are unacceptable as it is equally bad to condemn a satisfactory bull as to approve one that should be classified as unsatisfactory.

Rational for Significance of Breeding Soundness Evaluation

As recently reviewed by Chenoweth, the reproductive capabilities of breeding bulls are essential to any consideration of cow/calf economics [4]. In the 1994 NAHMS survey [5] physical unsoundness and infertility were identified as the most important culling criteria for beef bulls. Clients must be convinced of the real economic significance of identifying unsound and/or subfertile bulls prior to purchase and/or use.

While few bulls are completely sterile, even subfertile bulls adversely affect pregnancy rates. One of the earliest studies (using BSE standards in effect at that time) employed 48 bulls classified as satisfactory, questionable, or unsatisfactory potential breeders with natural mating achieving pregnancy rates of 75, 52, and 12%, respectively [6]. While fertility prediction for individual bulls was imperfect, use of bulls which failed to meet satisfactory potential breeder criteria resulted in considerably less pregnant females than did use of satisfactory bulls.

Two studies conducted by Wiltbank and Parish are germane to any discussion of the relationship between economics and the BSE [7]. Both trials utilized large multi-sire herds with cow to bull ratios of approximately 25:1. In these studies, all available bulls were subject to a BSE and bulls failing to pass the physical examination or having inadequate scrotal circumference were eliminated. In trial 1, cows were either mated with bulls of at least 80% normal sperm morphology, or with a group of control bulls. In trial 2, virgin heifers were mated with bulls of either at least 70% or 80% normal sperm morphology, or with a group of control bulls. In both trials, the control bulls had the array of percent normal sperm found in the original, larger population of available bulls.

In trial 1, using bulls with at least 80% normal sperm resulted in a 93% pregnancy rate compared with an 87% pregnancy rate for control bulls. In trial 2, pregnancy rates were 90% for heifers mated to bulls with at least 80% normal sperm, 91% for heifers mated to bulls with at least 70% normal sperm and 85% for heifers mated to control bulls. On average, bulls with acceptable physical traits and meeting minimum scrotal circumference guidelines and preselected to have at least 70% normal sperm resulted in 5 to 6% more cows pregnant than bulls unselected for normal sperm. Such differences could be expected to be even greater in single sire herds.

In another study, the most important factors influencing pregnancy rates were large scrotal circumference and relative lack of primary sperm defects [8]. These studies as well as others support the 1993 SFT guidelines which place increased emphasis on a minimum scrotal circumference based on bull age, as well as a minimum acceptable threshold of 70% normal sperm.

Outcomes of Breeding Soundness Evaluation

A recent analysis of BSE for 2898 yearling (10 to 20 months) beef bulls indicated that 72% were satisfactory potential breeders [9]. These data were from five University sponsored bull testing programs in 2 states, with the percentage of bulls classified as satisfactory varying from 65 to 79% among locations. Apparently there was considerable genetic variation in reproductive capabilities among bulls consigned to various test stations. However, in general, 28% of these young bulls did not achieve satisfactory potential breeder status within the time constraints of these testing programs. Interestingly, of the 28% of bulls classified as unsatisfactory potential breeder; 4% failed to pass the physical examination, 9% were below minimum scrotal circumference based on age, 9% had less than 70% normal sperm, 4% had less than 30% progressive motility, and 4% had both less than 70% normal sperm and less than 30% progressive motility (numbers do not sum due to rounding). Age affected the classification outcome (Fig. 1) as 10 month-old bulls were less likely to be classified as satisfactory than bulls greater than 11 months of age. There was also a tendency for fewer 11 month-old bulls to be classified as satisfactory than bulls greater than 12 months of age.

Figure 1. Percentage of bulls classified satisfactory potential breeder based on bull age (total n= 2272). Bars with different superscripts differ (P<0.05).

A retrospective analysis of BSE as classified by the 1993 SFT guidelines [1] on 3648 bulls of 10 breeds has recently been completed [10]. Of these bulls, 76% were classified as satisfactory potential breeders. Both breed (Fig. 2) and age (Fig. 3) were significant sources of variation for classification outcome.

Figure 2. Overall percentage of bull classified as satisfactory potential breeders among breeds of bulls evaluated. Bars with different superscripts differ (P<0.05).

Figure 3. Overall percentages of bulls classified as satisfactory potential breeders among ages of bulls evaluated. Bars with different superscripts differ (P<0.05). [10]

Bulls classified as unsatisfactory potential breeders were most likely to be classified as such due to inadequate normal sperm morphology or inadequate scrotal circumference. Most bulls receiving unsatisfactory classifications due to inadequate scrotal circumference were 10 and 11-month-old bulls which may have been evaluated prematurely. Santa Gertrudis bulls were the breed most likely to classify unsatisfactory due to inadequate scrotal circumference as well as a combination of spermatozoal parameters. The proportion of bulls failing due to abnormal sperm morphology was influenced most by breed; here Simbrah and Brangus bulls classified as unsatisfactory potential breeders more often than bulls of the other breeds represented. Interpretation of these results is difficult due to inconsistencies across testing stations. Bos indicus influenced composite breeds such as Santa Gertrudis mature more slowly than British or Continental breeds. This increases the chances that yearling bulls of such breeds might be relatively deficient in scrotal circumference and spermatozoal parameters. However, bulls of Bos indicus derivation at some stations were able to classify as satisfactory potential breeders at rates comparable with the other, non zebu breeds. It is possible that inconsistencies observed might be due to inadequate selection pressure for reproductive traits among some Bos indicus influenced populations, and that such breed-types may achieve satisfactory BSE classification at rates equivalent to those in bulls of the Continental and British breeds if they are exposed to adequate selection pressure for reproductive traits.

These reports confirm that the BSE performed on yearling performance-tested bulls, despite enormous variations in ages, breeds, and populations of bulls is feasible, with approximately 70 to 80% of all bulls being classified as satisfactory potential breeders.