2.1. Studied Traits

Bull fertility traits include semen volume, concentration, mass motility, individual fresh semen motility, diluted semen individual prefreezing motility, diluted semen individual postfreezing motility, total semen dose produced, and semen color. Morphological traits include body weight, scrotum circumference, right testicle length, left testicle length, right testicle width, and left testicle width of bulls. Besides these traits of bulls, other qualitative and quantitative information obtained from key informant interviews were also a part of this study.

2.2. Data Collection

Key informant interview was conducted with the five experts of the Livestock Development Institute (LDI) to collect information about bull recruitment, bull selection, semen dissemination, recording practice, bull rotation, mid-service bull evaluation based on progenies performance, and the plan to implement genomic selection of genetically superior bulls. Key informants must possess deep and relevant knowledge about the research topic; thus, five experts working on bull recruitment, selection, and breeding were selected and participated in this study. Semen production, dissemination, semen quality, and preservice andrological data for candidate bulls and bulls used for frozen semen production were obtained from LDI, which serves as a frozen semen source for a country. About 908 records of 68 bulls were used to evaluate the semen quality of bulls used for semen production. The preservice andrological and fertility data were collected from 2011 to 2022 from 193 candidate bulls. As a limitation, this study could not trace the information (performance and pedigree) about the paternal and progenies of bulls used for semen production due to the absence of sufficient records.

2.3. Semen Collection and Processing

Bulls were checked regularly for reproductive diseases, and an andrological examination was done before service and when something happens in semen quality during semen production. Semen is collected thrice weekly and 1–2 times per bull. Before collecting semen, bulls were given baths to remove manure from their prepuce. Semen was collected using a bovine artificial vagina in the early morning hours between 9:00 and 10:00 AM, according to Salisbury et al. [8]. The semen was first examined for volume, color, concentration, and motility after being collected. After that, it was processed in accordance with the laboratory’s normal operating procedure. The minimum standards are an initial volume greater than or equal to 2 ml, a color ranging from milky white to creamy, a concentration greater than or equal to 500 million/ml, and an initial motility greater than or equal to 70% [9]. Ejaculates were diluted with an OptiXcell extender to achieve a spermatozoa concentration of 142.86 million/ml, and then they were put into labeled tiny straws and sealed. After being allowed to acclimate at 4°C for around 4 h, the semen in the straws was transferred to a programed bio-freezer, where the temperature was further lowered to −140°C using liquid nitrogen vapor. Finally, liquid nitrogen canisters were used to keep these straws until they could be distributed and used in the fields [9].

2.4. Data Analysis

Following testing for outlier and normality of data, a general linear model procedure of SAS [10] was employed to analyze the fertility and morphological data of candidate bulls and bulls used for semen production. The chi-square test was employed to evaluate the association of categorical variables, such as semen color, with sources of variations. Descriptive statistics was also used to summarize the data. A Pearson correlation test using R software was used to evaluate the relationship between bulls’ andrological and fertility traits.

3.1. Bull Recruitment and Selection

The bulls used as sires for the next generation are the major determinant for genetic progress in dairy cattle genetic improvement programs. Therefore, sire selection is a vital decision that affects future production, health, and profit in the following generations of dairy cows [11]. In Ethiopia, the LDI has been used as a national frozen semen source for the genetic improvement of cattle through crossbreeding using AI since 1981. This institution plays a vital role in dairy cattle genetic improvement and saves foreign currency by producing semen within the country. Currently, about 55 improved bulls for frozen semen production at the national level. Bulls’ service length ranged from 2 to 7 years, and the semen could be stored for up to 10 years, according to key informants. The Netherlands, small-scale dairy farms, ranches, and research centers were the sources of improved bulls used for frozen semen production (Figure 1). Currently, small-scale dairy farms, followed by research centers, are the major source of bulls used for frozen semen production. Using small-scale dairy farms as a source of bulls may reduce the cost associated with the importation of improved bulls and ensure sustainability through continuous provision of bulls, but the problem is the absence of evidence about the estimated breeding value of bulls obtained from small-scale dairy farms due to the absence of complete pedigree and performance records.

Bull calves were selected from small-scale dairy farms based on the milk yield performance of their dam, conformation, growth, and health status. In addition, these bulls were checked for reproductive diseases such as brucellosis, bovine viral diarrhea (BVD), infectious bovine rhinotracheitis (IBR), and tuberculosis. The genetic correlation between milk yield and overall body conformation traits was small and positive, about 5%, according to Muller [12]. This means that the effect of selection for conformation traits on milk yield is small. Therefore, besides phenotype-based selection, the productive and reproductive traits of bulls used for semen production must be evaluated, and breeding value should be estimated based on adequate data before extensive use. Each bull used for semen production should have a detailed biography, which contains estimated breeding value, performance record of the dam/offspring of the bull, pedigree information, and disease-free certification.

For a successful genetic improvement program, first, specific selection criteria could be set, and the economic weight for traits included in the breeding goal should be quantified. Once we have this information, bulls must be selected based on breeding value estimated using different sources of information before service and evaluated based on their progeny performance (using ≥25 progenies/sire) after starting service to select the best bull and to enhance the genetic progress. Nevertheless, the selection of bulls and bull calves was not based on estimated breeding value; that is, bulls were selected based on performance records (dam milk yield), conformation traits, and information obtained from owners due to the absence of complete performance and pedigree records in the sources of bulls. Nowadays, nongovernmental organizations (African Dairy Genetic Gains and Public Private Partnership for Artificial Insemination Delivery) are being integrated with LDI to improve the data collection, analytics, and feedback systems in the dairy sector. According to Gebreyohanes et al. [13], a few bulls were selected based on genomic breeding value, purchased, and donated to LDI for semen production with the support of these projects. Besides, high-grade crossbred bulls are being selected from the research centers to use as a semen source at the national level. Indeed, these attempts are a good initiation in a country where there is no organized recording system and state-of-the-art genomic tools or technologies. Effective genetic improvement in dairy requires close collaboration at the national and international levels. Therefore, the sustainability of this initiative depends on the integration of different stakeholders or actors in dairy cattle production.

3.2. Semen Dissemination

Three experts evaluated the bull management, semen production, processing, and storage based on the predefined standard. The bull management being done at LDI seems good. The semen production, processing, and storage were also as per the recommended procedure to prove quality and at the maximum precaution to prevent the dissemination of reproductive diseases. The semen production and dissemination across years showed an undulating trend; the semen dissemination increased from 2011 to 2014 and decreased from 2014 to 2017 (Figure 2). In general, semen dissemination has increased by 28,880 straws per year, and semen production has increased by 31,445 straws per year since 2010. Nevertheless, the dissemination of semen was not based on the production system or level of production inputs; that is, Holstein Friesian (HF) crossbreds and pure breeds (HF and Jersey) were disseminated to extensive to intensive production systems based on customers’ interests. This type of dissemination, or dissemination based on the interest of producers only without prior analysis of the suitability of crossbreds for a given production environment, may not be profitable and efficient, as all improved breeds are not fit with all production systems. Besides, there is no proper mechanism for controlling indiscriminate insemination and crossbreeding due to the absence of records. The absence of strong collaboration and communication with regional and stakeholders at different levels is aggravating this problem, according to key informants. The absence of an organized recording system for disseminated semen/bull information at different stages could result in inbreeding in the herd and may have an adverse effect on the productivity and fitness of crossbreds. Therefore, developing a national database to estimate the breeding value, to track bull usage, genetic lineage, and semen distribution patterns is quite important to improve productivity and to reduce the effect of inbreeding depression.

Based on analysis done using semen dissemination records, semen from 203 bulls was disseminated to five zones of the Amhara region for the last 12 years (2010–2021). These bulls were indigenous (Begait, Fogera, and Boran), improved breeds (Jersey and HF), and crossbreds (50% HF × Boran, 75% Friesian × Boran, and Friesian × Fogera), as shown in Table 1. About 78% of the semen disseminated was from the HF breed. In addition, Jersey and 75% HF × Boran contributed about 11% and 7% of semen disseminated in the region, respectively (Figure 3). Although the aim is not clear, the Begait breed was disseminated to west Gojjam, east Gojjam, and north Shewa, while Boran was distributed to north Shewa and south Wollo areas. Semen from the indigenous Fogera breed was distributed to its origin and south Wollo. One of the main causes of endangering rare breeds of cattle is the high level of gene flow and mixing between native breeds [14, 15]. Therefore, the introduction of different breeds is recognized as having the potential to alter the genetic makeup of native breed populations depending on the level of gene flow. In addition, the dissemination of different breeds out of their suitable habitat may reduce their fitness and production potential.

Although semen from 203 bulls was disseminated to five zones of the Amhara region for the last 12 years, about 22 HF and 3 Jersey, a total of 25 bulls, contributed about 54.8% of the semen disseminated in five zones of the Amhara region. These top 25 bulls produce 11,805 to 52,870 doses per individual, with a service length varied from 1 to 6 years (Figure 4). This indicates overutilization of few bulls and this may reduce the genetic diversity and cause performance and fitness deterioration in crossbred animals. Therefore, the contribution of a bull in the herd should be at most 15%. Of 25 bulls, about 13 bulls were from Holeta bull-dam farm, 6 were imported from the Netherlands, 3 were from Debre-Zeit, 2 from Mukaturi, and 1 from Wolayita Sodo farms. These bulls were extensively utilized without knowing their genetic superiority, as they were not selected based on estimated breeding value and not evaluated based on the performance of their progenies after service (mid-service). For instance, age at first service for crossbred heifers varied from 19 to 34.7 months under the Ethiopian production system based on previous studies. This indicates that the probability of mating of sire with his female progenies could be high, as a large semen dose was disseminated for a maximum of 6 years from the same bull to the same area. Moreover, it may have a profound negative effect on the productivity, fitness, and genetic diversity of the population.

The duration of using bull for frozen semen production can be determined by the expected level of genetic diversity, estimated breeding value, health status, and semen quality. To prevent inbreeding and maintain genetic diversity within a population, the use of a single bull’s semen in the same area should not be greater than a year. Breeding bulls should be rotated to different areas to ensure a wide genetic base and reduce the risk of inbreeding-related issues. In terms of breeding value, the bull will remain in use as long as it continues to produce genetically superior offspring that contribute positively to herd improvement. However, older bulls may be replaced by younger ones with better-estimated breeding value as new and potentially better genetics become available. If a bull remains healthy, maintains good semen quality, and is genetically superior, it can be used for a longer period by rotating to different areas of the country to reduce the inbreeding level in the herd.

3.3. Preservice Bull Fertility

Evaluation of semen quantity and quality of candidate bull is required to keep the crossbreeding program economically viable [16]. The least-square means for preservice bull fertility traits is presented in Table 2. In this study, bulls selected for semen production had good semen volume, concentration, mass motility, individual motility, and live cells, which were higher than the minimum standard. Likewise, the morphological defect was low, which was 0.99% and 7.17% for major and minor cell defects, respectively. The accepted standard for clinical andrological tests is >350,000,000 (0.32 sperm cell concentration in 1 ml seminal plasma), >70% initial forward motility, 0%–20% morphological defect, >70% live cell count, and semen color (milky, yellowish, and creamy) according to Penny [17]. Therefore, the fertility of candidate bulls passed the preservice examination indicates that bull selection for semen production was as per the predefined bull selection standard.

Breed significantly influenced volume, concentration, mass motility, minor cell defect, live cell, and dead cell. HF, Jersey, and 75% of HF–Boran crossbreds had statistically similar (p > 0.05) phenotypes for most fertility traits. The semen volume varied between 3.31 and 6.38 ml; a higher volume was observed for HF, followed by Jersey and Friesian–Boran crossbreds. Similarly, a higher semen volume for Friesian than for Jersey bull has been observed by Fiaz et al. [16]. The semen volume for HF and Jersey in this study is higher than the report of Fiaz et al. [16] for HF and Jersey bulls. However, the individual motility in this study was lower than the report of Fiaz et al. [16]. The lowest and highest semen concentrations were observed for 75% Holstein Frisia–Boran crossbred and Fogera × Begait × Sheko, respectively. Despite the semen volume, the semen concentration and live cells of the Boran breed were higher, and the dead cells were lower than those of the HF breed. This suggests that Bos indicus cattle had better semen concentration and individual forward motility than Bos taurous, although it was insignificant. Likewise, Lemma and Shemsu [18] noted that Boran bulls produced the most concentrated semen with the highest mass motility than crossbreds. However, all breeds had statistically similar (p > 0.05) libido, individual forward motility, and major cell defects. Although the influence of age on most fertility traits was nonsignificant (p > 0.05), the semen volume and dead cells were lower in the early age of bulls. The increased semen volume with the age of the bull could be explained by physiological changes such as an increase in body mass [19].

The normal color of bull semen ranges from thick whitish to slightly yellowish fluid [20]. The preservice semen color of candidate bulls in this study is shown in Table 3. The selection of bull was significantly associated with the semen color. The dominant semen color for selected bulls for frozen semen production was creamy and milky, with a frequency of 40.3% and 34.7%, respectively. Nevertheless, the breed and age of the bull were not associated with semen color. On the other hand, nonselected bulls had creamy, milky, and watery semen colors. According to Lemma and Shemsu [18], a watery semen color is an indicator of semen with poor concentration of sperm.

3.4. Preservice Bull Andrological Test

A preservice andrological test of the candidate bulls is important in selecting the appropriate bulls for semen production. In this study, the breed significantly influenced all investigated morphological traits (Table 4). Despite their body weight, Friesian and Jersey bulls had a close phenotype for all morphological traits considered in this study. The scrotum circumference of a bull should not be lower than 30 cm [17], and this minimum requirement was fulfilled by most of the breeds except the Boran bull. According to Barth [21], lower scrotum circumference results in lower numbers of morphologically normal sperm and lower progressive motility. This trait had moderate to high heritability [6]; thus, the selection for larger scrotal circumference size resulted in a shorter calving interval [22], higher weight gain [23], and a better pregnancy rate [24]. The selection of young bulls with above-average SC will improve the fertility potential of their female progeny, which is clearly beneficial when sires are used to produce sires used to breed replacement heifers [17]. Body weight, scrotum, and testicle size increase with the age of bulls. However, the phenotype of these morphological traits of selected bulls was not significantly different (p  > 0.05) from nonselected bulls.

3.5. Semen Quality of Bulls Used for Breeding (Frozen Semen Production)

The quality of bulls and semen is the major determinant of the fertility of the herd. In highly fertile cow herds, the pregnancy rate of cows after the first 21 days of the breeding season should be ⩾70% (Barth, 2018). Low fertility bulls could cause substantial economic losses due to low pregnancy rates, delayed conception, reduced weaning weights a year later, and the culling of open and late-conceived cows [21]. The semen quality of bulls used as a source of semen for dairy cattle producers in Ethiopia is presented in Table 5. The semen volume ranges between 7.71 ml for Boran and 13.0 ml for HF breeds. The semen volume recorded in this study was higher than the report of Kumar et al. [25] for Jersey and Jersey crossbreds. Boran had very good semen concentration compared to other breeds considered in this study, which agrees with the observations of Lemma and Shemsu [18]. HF and Jersey had significantly higher diluted semen pre and postfreezing motility than other breeds. Kumar et al. [25] for Jersey and Jersey crossbred bulls reported a higher mass motility than the current finding.

Ejaculation number had a significant effect on semen volume, concentration, and mass motility. The volume, concentration, and mass motility of semen significantly decreased in the second ejaculation compared to the first ejaculation. This result is in line with the report of Murphy et al. [26], who noted that first ejaculates had greater semen production than second ejaculates. In general, the quality of the semen produced meets the standard of semen quality. Kumar et al. [25] noted a lower prefreezing and comparable postfreezing motility than the current finding. Cooling, freezing, and thawing procedures could be the possible reasons for postfreezing semen motility reduction. The potential of selecting for semen production and quality traits is appealing due to the favorable genetic correlations between semen quality traits and the very consistent heritability estimates for these traits across studies [6].

3.6. Phenotypic Correlation of Fertility and Morphological Traits

The phenotypic correlation between bull fertility and morphological traits is illustrated in Figure 5. Semen volume was significantly correlated (r = 0.16–0.49) with body weight, testicle width, testicle length, and scrotum circumference. In agreement with this result, a significant correlation (r = 0.72) between body weight and scrotal circumference was noted by Lemma and Shemsu [18]. Scrotum circumference was moderately correlated with volume. The correlation of scrotum circumference with motility was found to be positive and nonsignificant. However, Corbet et al. [27] noted a moderate phenotypic (r = 0.42) and genetic correlation (r = 0.56) between scrotum circumference and semen motility. These suggest that these morphological traits can be used as indirect selection criteria for semen volume.

A negative and nonsignificant (p > 0.05) correlation of semen volume was observed with concentration, mass motility, and individual forward motility. In line with the current finding, Druet et al. [28] noted a low positive phenotypic and moderate negative genetic correlation between semen volume and concentration for Holstein bull. Likewise, a negative genetic and phenotypic correlation between volume and motility was reported by Druet et al. [28]. Contrary to the current finding, a positive correlation between semen volume and concentration was noted by Mathevon [29] and Berry et al. [30]. Most morphological traits were positively and significantly correlated with the body weight of bulls. However, the relationship of morphological traits with semen concentration and motility was found to be nonsignificant (p > 0.05). Semen mass motility was moderately correlated with concentration, which is in line with the report (genetic correlation = 086) of Kealey et al. [31] for Herford bull and the report (genetic correlation = 0.35) of Druet et al. [28] for Holstein cattle.