ELSEVIER

BREEDING SOUNDNESS EVALUATION OF EXTENSIVELY MANAGED BULLS IN COSTA RICA

J. Chacon’, E. P&red, E. Miiller’, L. Sdderquist’ and H. Rodriguez-Martinez’

‘,‘Department of Animal Reproduction and ‘Herd Health SAREC Interuniversity Cooperation Program, School of Veterinary Medicine

Universidad National (UNA), P.0 Box 304-3000, Heredia, Costa Rica ‘Department of Obstetrics and Gynecology, Faculty of Veterinary Medicine

Swedish University of Agricultural Sciences (SLU), Box 7039, SE-750 07, Uppsala, Sweden

Received for publication: 28 April 1998 Accepted: 17 February 1999

ABSTRACT

This paper describes the results of single breeding soundness evaluations (BSE) in 898 Bos indicus. Bos taurus and B. indicus x B. taunts bulls, 1 to 12 yr old, extensively reared in different climatic regions of Costa Rica and representing approximately 2% of the total breeding bull population. Thirty-three percent (n= 296) of the bulls were classified as unsound for breeding owing to clinical problems (9.1%, n= 82), low scrotal circumference (SC) being the most common finding, followed by unsatisfactory sperm morphology (23.9%, n= 214). The prevalence of bulls unsound for breeding was lowest in Bos indicus (29%, P<O.Ol), intermediate in B. taurus (41%), and highest in B. indicus x B. taurus (48%). The percentages of abnormal sperm heads, acrosomes and midpieces tended to be higher in the ejaculates of bulls with softer testicular consistency (p<O.OOl), a long scrotum (P<O.Ol) or a low SC (P<O.O5), and such bulls were more often classified as being unsound for breeding (WO.05). Frequencies of sperm abnormalities were higher in bulls < 2 yr of age than in older males (P<O.Ol) and were highest in B. indicus x B. taurus bulls (PiO.001). The results confirm differences between species in their adaptability to a tropical environment and support earlier evidence of an association between SC, testicular consistency and scrotal length clinical parameters, and testicular function in bulls. (D 1999 by Elsevter Science Inc.

Key words: bulls, breeding soundness, sperm abnormalities, Bos indicus, Bos taurus

INTRODUCTION

Bulls strongly affect the reproductive efficiency of breeding herds, irrespective of whether they are used for natural breeding or artificial insemination. Thus, impairment of bull fertility results in great economic losses, particularly in extensive cattle production systems. It is, consequently, essential that bulls be examined both before and during their use in breeding programs in order to identify and remove any individuals with potentially low fertility (1 , 8, 18,2 1).

Acknowledgments The study was supported by the Swedish Agency for Research Cooperation with Developing Countries (SAREC). The authors thank Mr. Edy Games and Mr. Jose Rodriguez for their excellent technical assistance, and the farmers for submitting the study cases. Theriogenology 52221-231, 1999 0093691W99/$-see front matter 0 1999 by Eteevier Science Inc. Ptl SOO93-691X(99)00124-7

222 Theriogenology

In Costa Rica as well as in other tropical countries breeding programs for beef and for dual purpose cattle are characterized by extensive management of the breeding herd. Under these conditions, the use of natural service and multiple-sire breeding systems along with the lack of record-keeping on farms make it difficult to assess the fertility of individual bulls based on the females’ conception rates (29). Since a subfertile dominant bull can cause irreparable losses during a breeding season under extensive management conditions, the importance of conducting a BSE cannot be overemphasized. There are few studies regarding the prevalence and natnre of reproductive problems among the approximately 48,000 extensively managed breeding bulls in Costa Rica (31). Based on a BSE of 520 B. indicus bulls in the dry Pacific region (dry tropics) of Costa Rica, Alvarado (2) reported that 70% of these were satisfactory for breeding, while Arias (3) found that up to 80% of the 314 bulls assessed in the Atlantic region (wet tropics) were satisfactory breedem. However, no comparative data are available regarding the breeding soundness of & indicus. B. taunts and crossbred bulls in different regions of the country, or concerning the relationship between environmental and clinical variables on the spermiograms of these bulls.

Therefore, we undertook a clinical survey to investigate the breeding soundness of Bos indicus, Bos taurus and B. indicus x B. taunts bulls, using clinical and spermiogram evaluations, under extensive management and natural mating conditions in different regions of Costa Rica. In addition, we .also studied, the relationship between environmental variables and the clinical and spermiogram tindings.

MATERIALS AND METHODS

Clinical Survey Design

Single BSE’s of Bos indicus, Bos taurus and B. indicus x B. taunts bulls were carried out on farms from 1991 through 1996. The farms were located within each of 4 distinct climatic regions of Costa Pica. Personnel from the Andrology Section of the Faculty of Veterinary Medicine (UNA) conducted the evaluations in response to requests made by farmers. This evaluation included documentation of the breeding history and a thorough clinical examination, including spermiogram screening, of the bulls. After the clinical examination, the bulls were diagnosed as sound or unsound for breeding, and the farmer was advised either to keep, re-examine, or cull the bull.

Location

The survey was performed in each of 4 distinct climatic regions of tropical Costa Rica (8”00’- 1 l”15’ N and 82”30’- 86”OO’ W), as defined by the National Climatological Institute, based on the yearly distribution of rainfall and ambient temperature. Three of the regions were located on the west side of the country, and included the North (dry), Central and South Pacific areas, while one region was situated on the north-east side, the North-Caribbean area. The average yearly humidity in the 4 zones ranged corn 7.5 to 86% (6). The mean yearly rainfall and maximum environmental temperature during the period of the clinical survey are presented below (Table 1).

AnimalSample

A total of 898 breeding Bos indicus (n= 598), Bos taunts (n= 252), and B. indicus x B. taunts (n= 48) bulls, representing 1.8% of the total bull breeding population in the country (31), were examined. The age of the bulls ranged from 13 mo to 12 yr, and they were all raised under extensive management conditions. The bulls were either continuously or intermittantly used for

Theriogenology 223

breeding, depending on the system employed on a given farm. The bulls were allowed to graze throughout the year, and molasses were used as a supplement during the dry season on 50% of the farms. No information regarding the reproductive performance of the bulls was available prior to visiting the farms.

Table 1. Mean (k SD) yearly rainfall and maximum ambient temperature in the 4 regions of Costa Rica during 1991 to 1996.

Region Dry Pacific

Rainfall (mm) 1564 f 514

Temperature (C”) 32.9 * 0.35

Central Pacific 3650 f 556 31.4 * 0.24 South Pacific 3998 f 419 31.6 zk 0.35 North-Caribbean 4265 k 590 29.2 + 0.92

Breeding Soundness Evaluation (BSE)

At the initial farm visit a questionnaire was completed which included the date, identification of the farm/owner, location, and nutritional management of the animals. Before the andrological evaluation of the bulls, a detailed description of the bull (breed, identification and age) was recorded. A clinical history of each bull was then taken, including any previous illnesses and mating regimen (single or multiple bulls, restricted or continuous mating, length of breeding season, number of females per bull, mating behavior, and dominance status). Body condition was scored (BCS) from 1 to 5 according to the system devised by the International Livestock Center for Africa (32). A score 2 3 was considered normal BCS, while scores of c 3 were considered poor BC. After assessment of body condition and gait, each bull was restrained in a chute for a clinical examination. The general clinical inspection included the eyes and the musculo-skeletal system, with special emphasis on foot and leg conformation. The specific andrological examination included inspection of the prepuce and penis (during semen collection), palpation and inspection of the scrotum and its contents, and assessment of the internal genitalia per rectum. The length of the scrotum was classified according to the distance between its distal part and the hock joint as follows: short - scrotum clearly close to the body, >15 cm above the hock joint; normal - less than 15 cm above the hock down to the level with the hock; Zong - below the hock joint. Scrotal circumference (SC) was measured using a standard scrotal metal-tape (Trueman Mfg., Edmonton, AB, Canada) at the widest mid-scrotal point. Testicular symmetry was described at inspection, and testicular consistency (TC) was determined subjectively by palpation and scored from 1 to 5. A score of 4 was considered normal, while values below or above 4 indicated a softer or harder consistency, respectively. Palpatory findings in the epididymides as well as in the internal genitalia were also recorded.

SemenEvaluation

Semen samples were obtained using a three-electrode electroejaculator (Standard Precision Electronics Inc., Boulder, CO, USA), after at least 3 min of manual stimulation per rectum of the accessory glands and pelvic urethra. After collection, sperm motility was immediately estimated subjectively using phase wntrast microscopy (x400). An ;iliquot of the collected semen sample was thereafter fixed in buffered fomtol-saline solution (20). Sperm morphology was evaluated on-farm by the same operator who examined a total of 200 spermatozoa in a wet-smear preparation using phase-contrast microscopy (x1000). Percentages of spermatozoa with head (including acrosome

224 Theriogenology

and midpiece) and tail abnormalities as well as the presence of proximal cytoplasmic droplets were recorded.

Final Classification for Breeding Soundness

Each bull was classified, according to the standards proposed by Ball et al. (4) and Chenoweth et al. (lo), as follows: breeding-sound bulls - animals that were clinically normal with a minimum SC of 30 cm at 24 mo or older, I 15% of abnormal sperm heads (including acrosomes and midpieces), and/or a maximum of 30% total sperm abnormalities; breeding- unsound buZZs - animals that did not meet the criteria for breeding soundness. In case of unsound bulls with 5 30% abnormal sperm heads or with a maximum of 50% total sperm abnormalities but without clinical problems at the time of examination, the farmer was advised to seek are-evaluation after 3 mo.

Statistical Analysis

A preliminary descriptive analysis (frequencies and proportions) was performed for the entire bull population (n= 898). In addition, the influence of the environmental variables on the clinical parameters as well as the relation between the environmental and clinical variables, respectively, and the spenniogram were analyzed using a limited set of bulls by means of linear multiple regression (GLM procedure, Statistical Analysis System; 38). These analyses included only males free of clinical abnormalities at the time of examination and for which environmental data were available (701 bulls). The model included the main effects of the climatic region, average monthly rainfall and maximum ambient temperature (to which the bull had been exposed the month before the andrological evaluation), species, age, BCS, SC, testicular consistency, and scrotum length. Spermiogram parameters and final BSE classifications were used as the dependent variables. The effects of the interactions between breed and temperature and between breed and rainfall were tested in preliminary models using a stepwise linear multiple regression to fit the data. Least squares means comparisons were performed for categorical variables when differences were significant (PcO.05). Age, BCS, and testicular consistency were categorized as follows: Age; 5 2 yr, > 2 - < 7 yr and > 7 yr-old; BC-score, < 3 and 2 3; testicular consistency, 4 and < 4 (i.e. normal and soft consistency, respectively).

RESULTS

Distribution of Andrological Examinations by Breed and Region

Means f SD for the clinical and spermiogram variables by breed are presented in Table 2.

Gut of the total group of breeding bulls examined (n= 898), 67% (598) were Bos indicus, 28% (252) Bos taunts, and 5% (48) B. indicus x B. taunts. The proportions of the different breeds examined were about the same in all 4 regions of the country (Table 3). The Dry Pacific and North- Caribbean regions accounted for 51% (453/898) and 34% (304/898), respectively, of the bulls examined (Table 3). These 2 regions contain approximately 40 and 28%, respectively, of the national beef cattle population (3 1).

Bulls Classified as Unsound for Breeding

Thirty-three percent (296 of 898) of the bulls examined were classified as unsound for breeding (Table 3) on the basis of clinical findings (9.1%, n= 82/898; Table 4) or deviations in the

Theriogtmology 225

Sptiogram (23.9%; n= 214/898). Regarding the clinical &dings, abnormalities associated with an unacceptable low SC (~30 cm, scrotal asymmetry, hypoplasia, fibrosis or atrophy) were the most common tinding (6.8%, n= 61/898; Table 4). The prevalence of other clinical alterations such as inflammatory diseases of the reproductive organs was below 1% and, irrespective of the cause, bulls in this group were culled by the farmers after the evaluation.

Table 2. Descriptive statistics (Means f SD) for the variables studied by breed of bull (n=701)

Variable Bos indicus Bos taurus B. indicus x B. taums Age (years) 4.2 f 1.8 3.7 f 1.5 3.9f 1.6 Body condition score 3.2 f 0.5 3.2 f 0.6 3.3 k 0.5 Testicular consistency (score) 3.7k0.6 3.6 f 0.6 3.6 f 0.6 Scrotal circumference (cm) 36.2 f 4.0 35.2 k 3.8 36.0 It 3.8 Prevalence of long scrotum (%) 12.0 16.0 11.0 Sperm motility (%) 61.0 f 21.0 59.0 f 22.6 51.0f26.0 Abnormal heads and mid-pieces (“‘) sperm 9.4 f 14.0 15.1 f 20.9 19.0 f 21.0 Abnormal tails and proximal cytoplasmic sperm 14.0 It 14.4, 13.7 It 14.9 18.0 f 17.6 droplets (%)

The fate of bulls classified as unsound for breeding for which a re-examination had been recommended is not known, since the farmers did not show interest in a second evaluation.

Table 3. Disttibution of the prevalence (%) of bulls classified as unsound for breeding (n=296) among species and climatic regions (number of bulls within parenthesis).

Breed Dry Central south North- Total Pacific Pacific Pacific Caribbean

Bos indicus 3 1 (94/300) 23 (20188) 30 (10/33) 28 (49/177) 29 (173/598) Bos taurus 36 (43/121) 46 (6/13) 29 (2/7) 45 (SO/l 11) 40 (lOU252) B. indicus x 44 (14/32) --______-_ _____--__ 50 (8/16) 46 (22/48) B. taurus Total 33 (151/453) 26 (26/101) 30 (12/40) 35 (107/304) 33 (296/898)

----- No data available.

Relationship between climatic and clinical variables and BSE

Clinical muameters. No statistically significant relationship was found between average rainfall or ambient temperature and BCS or testicular consistency. However, a negative effect was observed between SC and the ambient temperature to which the bull had been exposed the month before the BSE (a= -0.28, PcO.01). In addition, bulls with a low BCS (corrected by age and breed) had a significantly smaller SC than bulls with a normal BCS (34.9 vs 36.0 cm; PcO.01).

Snermioaram. Although the accuracy with which this parameter can be measured is difficult to determine when semen is collected by electroejaculation, bulls with a low BCS showed a significantly lower sperm motility than bulls with a normal BCS (56 vs 61%; P< 0.05). Likewise, sperm motility was significantly lower for bulls with a low testicular consistency score than for

226 Thwiogef7obgy

those with a normal consistency score (57 vs 61%; PC 0.05). No statistical significant relation was found between spemt motility and breed, scrotum length and/or SC.

Table 4. Distribution (%) of clinical findings in breeding bulls classified as unsound for breeding (number of cases in parentheses).

Diagnosis History of low libido Foot and/or leg problems Penile deviation at mating Phymosis Cases with unacceptable SC (< 30 cm, testicular asymmetry, hypoplasia, atrophy, fibrosis) Drchitis Sperm granuloma Seminovesiculitis and/or epididymitis Total

Percentage 0.7 (n= 6) 0.2 (n= 2) 0.2 (n= 2) 0.2 (n= 2) 6.8 (n= 61)

0.5 (n= 4) 0.2 (n= 2) 0.3 (n= 3) 9.1 (n= 82/898)

Frequencies of abnormal tails and proximal cytoplasmic droplets showed a significant positive relationship to mean monthly temperature (R= 1.44; PcO.05) and rainfall (g= 0.015; PcO.01). However, no other sperm parameters were significantly related to climatic variables. The mean percentage of abnormal sperm heads was significantly higher (p<O.Ol) among bulls < 2 yr of age than among older bulls (22 vs 14.5%). No significant differences were observed between the 2 groups of older bulls. The mean percentage of sperm-head abnormalities was significsntly higher (P<O.OOl) in B. indicus x B. taunts bulls than in both Bos indicus and Bos taunts bulls (22, 13 and 15%, respectively). No significant difference was found between Bos indicus and Bos taurns bulls in this respect. Bulls with a long scrotum and a low testicular consistency score had a significantly higher percentage of abnormal sperm heads than bulls with a normal scrotum length (19 vs 14%, PcO.01) and normal testicular consistency (20 vs 13%, P<O.OOl). In addition, a statistically significant negative relationship (R= -1.16; PcO.05) was observed between SC and the frequency of sperm head abnormalities.

Classification in terms of breeding soundness. A statistically signiticant positive relationship (g= 0.24; PcO.05) was observed between the percentage of bulls classified as unsound for breeding due to sperm abnormalities and the envimmnental temperature. The prevalence of bulls classified as unsound was lowest for the Bos indicus group (29%), followed by Bos taunts (40%; PcO.01) and B. indicus x B. taurus (46%; P<O.Ol). This trend was independent of the region studied (Table 3). The prevalence of unsound bulls was higher in bulls with a long scrotum compared with bulls with a normal scrotum length (17 vs 11%; PcO.05) and with a low testicular consistency score compared with a normal consistency (43 vs 25%; P<O.OOl). The prevalence values increased with decreasing SC (R= -0.5; P<O.O5).

DISCUSSION

The prevalence of bulls classified as unsound for breeding in the present study is similar to that previously reported in Costa Rica (29), where most breeding bulls are selected on the basis of phenotype alone. The high number of bulls classified as unsound for breeding because of clinical

Theriogenology 227

problems contirrns the importance of a detailed clinical examination prior to their use in breeding programs and points out the high value of using minimal SC as a selection criterion for bulls raised under extensive management conditions. These results are similar to the findings of earlier studies in which low SC-associated pathologies were the main reasons for culling breeding bulls (5, 8, 11, 16).

Since no relationship was observed between the ambient variables and BSC, it is suggested that the negative effect of environmental temperature on SC is a direct consequence of reduced spermatogenesis (i.e., testicular degeneration), as seen in bulls exposed to hot environments (41). However, a decrease in the SC was not reflected in a significant increase in the frequency of abnormal sperm heads (including midpieces) in the ejaculate (as an indicator of testicular degeneration) or in a reduction in testicular consistence at palpation. Similar results have been obtained in other tropical environments (23) and together they suggest that the environmental factors adversely affecting testicular function, as expressed by deviating sperm morphology, remain the same throughout the year. Nevertheless, variation in the adaptability of individual bulls to a tropical environment should also be considered. Sperm output (a major variable reflecting testicular function) was not assessed in our present study, since only a single electro-ejaculation per bull was performed. Therefore, sperm function was only partially monitored.

The fact that temperature andor rainfall were positively correlated with the percentage of abnormal sperm tails and the presence of proximal cytoplasmic droplets suggests that the environment can affect epididymal function. Several authors (22, 24, 37, 47) have reported an initial increase in the percentage of proximal cytoplasmic droplets in the ejaculate of bulls after experimental insulation of the scrotum, probably owing to epididymal dysfunction. In addition, ambient temperature correlated with an increment in the number of bulls classified as unsound for breeding. However, since the present survey was based on single examinations, individual variation could have masked the effects of temperature on the assessed parameters. A detailed morphological classification of sperm abnormalities and a periodic andrological evaluation of the same bulls over different seasons of the year would be needed to clarify the above mentioned effects. The positive correlations found between monthly average rainfall and the mean percentages of abnormal tails and proximal cytoplasmic droplets are not fully understood and suggest that factors not considered in this study were also important.

Body condition score is indicative of body live weight in the bull (7, 13, 16), and, although it affects scrotal circumference, it was not correlated with sperm morphology. This was unexpected, since a low nutritional level may delay sexual maturity and cause testicular degeneration in bulls, probably as a consequence of endocrine alterations (15). However, Mann and Walton (26) could not find any differences in sperm quality in underfed bulls, although the secretory function of their accessory glands was decreased. None of the bulls in the present study had such a low BCS that they could be attributed to underfeeding. Furthermore, the significant differences in sperm motility found between the group with a low BCS and the group with a normal BCS were, in our opinion, of little relevance, since semen was collected by electroejaculation, which can affect motility patterns of the spermatozoa. Thus, the percentage of motile spermatozoa in a single ejaculate needs to be interpreted carefully, taking into account both management conditions and clinical findings.

The higher percentage of sperm head abnormalities and higher prevalence of bulls classified as unsound for breeding among males < 2 yr of age might have been due to sexual immaturity, since sexual development tends to occur slowly under tropical conditions (43). Periodic examinations

228 Theriogenology

would be necessary to ensure that the differences between the different age groups in the present study were due to sexual immaturity.

Regarding older bulls (> 2 yr), even though no significant differences were found between groups for the variables studied, we have serious reservations to using bulls older than 7 yr for natural breeding because they have a higher incidence of lesions such as testicular senile fibrosis and tubular calcification which are associated with decreased sperm production (8, 23, 35). In addition, older bulls are more dominant in the breeding group, and the risks for inbreeding and the transmission of venereal diseases are higher when aged bulls are used in natural mating systems, which also justifies their replacement (8,27,34).

The fact that the prevalence of animals classified as unsound for breeding was higher for & taurus than for Bos indicus bulls, regardless of the climatic region in which they were located, indicates that testicular function in Bos taurus bulls is more affected than in Bos indicus bulls by the tropical environment as a result of their lower heat tolerance (9, 15, 44, 47). Vale-F&o et al. (45) and Ohashi et al. (33) reported that under tropical conditions, the prevalence of testicular degeneration was higher in B. taunts and their crosses than in B. indicus bulls and pointed out that this pathology in Bos taunts bulls is the main sign of their ma&adaptation to a tropical climate. Similar findings were noted under semitropical conditions by Fields et al. (17), who observed a decline in semen quality and testicular volume in Hereford bulls compared with Brahman bulls. The ability of Bos indicus cattle to withstand hot tropical conditions is widely recognized (9, 23, 33, 44). This trait is due to their smooth coat, a higher proportion of primary hair follicles, hairless scrotal skin, and better developed sweat and sebaceous glands, allowing them to lose more heat and moisture through evaporation. In addition, hematological differences have also been observed, which allow Bos indicus cattle to maintain lower respiration rates during periods of high ambient temperatures (44).

The significantly higher mean percentage of abnormal sperm heads as well as the higher prevalence of unsoundness for breeding found among the B. indicus x B. tam-us group was interesting, since the introduction of Zebu-blood under tropical environmental conditions enhanced the adaptability of Bos taunts (9). However, our results need to be carefully interpreted in light of the low number of crosses evaluated and the unknown percentage of Bos indicus blood in the crosses studied. Reports concerning fertility in B. indicus x B. taunts bulls under tropical conditions and extensive rearing, show similar rates of testicular degeneration in both Holstein and Holstein x B. indicus bulls (15, 33, 45). More research under the environmental and management conditions found in the tropics is needed to clarify the potential value of B. indicus x B. taunts bulls in breeding programs.

In our present study, bulls with a long scrotum had such high percentages of abnormal sperm heads that they were classified as unsound for breeding. The higher prevalence of bulls with a long scrotum was observed in the Bos taunts group. A long scrotum, caused by a permanent relaxation of the cremasteric muscle, probably predisposes the testes to trauma caused by frequent beating which can result in testicular degeneration. In addition, Riemerschmid (36), Setchell (39), and Shatik (40) suggested that venous blood flow horn the testicle could be affected in bulls with a pendulous scrotum, leading to blood stagnation, thus interfering with the thermo-regulation system in the testes.

Testicular consistency has been reported to be positively correlated with spemr production, semen quality and fertility in Holstein bulls (7, 12, 13, 14, 19). In addition, Miiller et al. (30),

Thefbgenology 229

reported that Zebu bulls with a slight-to-moderate decrease in testicular consistency showed degenerative changes in the seminiferous epithelium. The significant correlations found in the present study between testicular consistency, sperm motility and percentages of spermatozoa with abnormal heads, and the subsequent classification of bulls as unsound for breeding continn these previous findings and underscore the usefulness of palpatory findings in andrological assessments for breeding soundness.

Scrotal circumference has been reported to be positively correlated with sperm production, semen quality and age at puberty of the offspring (7,12, 16,25,28,42). Veeramachaneni-Rao et al. (46) and Madrid et al. (25) reported significantly higher percentages of sperm abnormalities and damaged tubules in young bulls with an SC ~32 cm compared with bulls with an SC 2 32 cm. Troc6niz (43) reported similar findings in Zebu bulls under grazing conditions in the tropics. In addition, Carroll et al. (8) and Ehnore et al. (16) reported that bulls with small testes tended to be unsatisfactory breeders. Thus, the selection of bulls with a large SC might increase the reproductive efficiency of the cattle herd.

lit conclusion, Bos indicus bulls were more adaptable to tropical conditions throughout Costa Rica compared with Bos taunts and crossbred bulls. However, due to the great extent of unexplained variation, especially regarding the potential effect of environmental variables on the reproductive efficiency of the bulls, the periodic evaluation of bulls during different seasons and under uniform conditions of management and climate is needed, together with a more detailed morphological description of the sperm abnormalities. We have such a study currently under way.

REFERENCES

1. Ahnquist J, Cmmingham D. Reproductive capacity of beef bulls. J Anim Sci 1967; 26: 174 abstr.

2. Alvarado E. Desarrollo de una metodologia para la evaluation andrologica de1 toro cebuino en el Pacifico Seco de Costa Rica. Thesis, Escuela de Medicina Veterinaria, Heredia-Costa Rica, 1987.

3. Arias F. Evahtacion andrologica de toros cebuinos en la zona Atlantica de Costa Rica. Tesis, Escuela de Medicina Veterinaria, Heredia-Costa Rica, 1995.

4. Ball L, Ott R, Mortimer R, Simons J. Manual for breeding soundness examination of bulls. Sot Theriogenoll983; 12: l-44.

5. Braden S. Andrologische untersuchungen von bullen in der trockenen pazifikregion (pacific0 seco) costa ricas unter besonderer berticksichtigung der hodendegeneration. Thesis, Faculty of Veterinary Medicine, Hannover, 1992.

6. Campos M. Resumen climatico de Costa Rica. Institute Metereologico National. Boletin 1994: 1-7.

7. Carter A, Wood P, Wright P. Association between scrotal circumference, live weight and sperm output in cattle. J Reprod Sci 1980; 59: 447-451.

8. Carroll E, Ball L, Scott J. Breeding soundness in bulls. a summary of 10,940 examinations. JAVMA 1963; 142: 1105-1111.

9. Cartwright T. Responses ofbeef cattle to high ambient temperatures. J Anim Sci 1955; 14: 350- 362.

10. Chenoweth P, Spitzer J, Hopkins F. A new breeding soundness evaluation form. Sot Theriogenoll992; 38: 63-70.

230 Theriogenology

11. Chenoweth P, Chase C, Thatcher M, Wilcox C, Larsen R. Breed and other effects on reproductive traits and breeding soundness categorization in young beef bulls in Florida. Theriogenology 1996; 46: 1159-1170.

12. Coulter G, Rounsaville T, Foote R. Hereditability of testicular size and consistency in Holstein bulls. JAnimSci 1976; 43: 10-11.

13. Coulter G, Foote R. Relationship of body weight to testicular size and consistency in growing

14.

15.

16.

17.

18. 19.

20. 21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31. 32.

33.

34.

Holstein bulls. J Anim Sci 1977; 44: 1076-1079. Coulter G, Foote R. Bovine testicular measurements as indicators of reproductive performance

and their relationship to productive traits in cattle. Theriogenology 1979; 11: 297-3 11. Crab0 B. Sperm production by farm animals under tropical conditions. Proc 1 lth Int Congr Anim Reprod Artif Insem 1988; 238-245.

Ehnore R, Bierschwal C, Youngquist R. Scrotal circumference measurements in 764 beef bulls. Theriogenology, 1976; 6: 485-494.

Fields M, Burns W, Warnick A. Age, season and breed effects on testicular volume and semen traits in young beefbulls. J Anim Sci 1979; 48: 1299-1304. Glover T. Semen quality and male reproductive function. Vet Ret 1959; 71: 613. Hahn J, Foote R, Cranch E. Tonometer for measuring testicular consistency of bulls to predict semen quality. J Anim Sci 1969; 29: 483-489. Hancock L. The morphology of bull spermatozoa. Br J Exp Biol 1952; 29: 445-453. Hill H, Faulkner L, Carroll E. Theory of evaluating bulls as to breeding soundness. JAVMA 1959; 135: 318-320. Janukauskas A, Gil J, Rodriguez-Martinez H, Sijderquist L, Lundeheim N. Effects of a brief elevation of scrotal temperature on the post-thaw viability of bull semen. Reprod Dom Anim 1995; 30: 271-277. Kumi-Diaka J, Nagaratnam V, Rwuaan J. Seasonal and age-related changes in semen quality and testicular morphology ofbulls in a tropical environment. Vet Ret 1981; 3: 13-15. Lagerliif N. Morphological studies on the changes in sperm structure and in the testis of bull with decreased or abolished fertility (translated title). Acta Path01 Microbial stand 1934; 19: l- 254.

Madrid N, Ott R, Rao D, Vanderwert W, Wilhns C. Scrotal circumference, seminal characteristics and testicular lesions of yearling Angus bulls. AJVR 1988; 49: 579-584. Mann T, Walton A. The effect of under-feeding on the genital functions of a bull. J Agric Sci 1953; 43: 343-347. MC Cosker T, Turner A, MC Cool C, Post T, Bell K. Brahman bull fertility in a north

Australian rangeland herd. Theriogenology 1989; 32: 285-297. Morris D, Tyner C, Morris P, Forgason R. Correlation of scrotal circumference and age in American Brahman bulls. Theriogenology 1989; 31: 489-453. Mtiller E. Andrological evaluation of bulls in the tropics. Joint IFS-SIPAR Seminar on

Animal Reproduction, 1990; 2: 143-150. Miiller E, Rodriguez-Martinez H, Braden S, Edquist L-E. Testicular ultraestructure of Zebu bulls in Costa Rica J Vet Med A 1992; 39: 382-391. National Livestock Census. Costa Rica: Ministry of Agriculture and Livestock, 1984. Nicholson M, Butterworth M. A guide to condition scoring of Zebu cattle. International

Livestock Centre for Africa, Ethiopia, 1986. Ohashi 0, Ribeiro H, Vale-Filho W. Disturbios reprodutivos em touros Bos indicus. Bos taurus e mesticos, criados em clima amazonico. Pesa Vet Bras 1988: 8: 31-35. Perez E, Conrad P, Hird D, GrtuIlo A Chacon J, Bondursnt R Noordhuizen J. Prevalence and risk factors for Trichomonas foetus infection in cattle in northeastern Costa Rica. Prev Vet Med 1992; 14: 155-165.

Theriogenology 231

35. Rao A, Bane A. Pathological conditions of the genital organs of normal fertile bulls. Indian Vet J 1985; 62: 242-256.

36. Riemerschmid G. Further observations on the scrotal skin temperature of the bull, with some remarks on the intratesticular temperature. Gnderstepoort J Vet Sci Anim Ind 1941; 17: 123- 140.

37. Roos AD, Entwistle KW. The eff~t of scrotal insulation on spermatozoal morphology and the rates of spermatogenesis and epididymal passage of spermatozoa in the bull. Theriogenology 1979; 11: 111-114.

38. SAS. Statistical Analysis System. Cary NC: SAS Institute Inc., 1996. 39. Setchell B. Testicular fluids, In: Johnson AD, Gomes WR, Vandemark VL (eds), The testis.

New York: Academic Press, 1970; 1: 46- 147. 40. Shaflk A. The cremasteric muscle, In: Johnson AD, Gomes WR, Vandemark VL (eds), The

Testis. New-York: Academic Press, 1970; 4: 48 l-489. 41. Skinner J, Louw G. Heat stress and spermatogenesis in Bos indicus and Bos taurus cattle. J

Appl Phys 1966; 21: 1784-1790. 42. Toelle V, Robison W. Estimates of genetic correlations between testicular measurements and

female reproductive traits in cattle. J Anim Sci 1985; 60: 89-99. 43. Troconiz J, Beltran J, Bastidas H, Larrcal H, Bastidas P. Testicular development, body weight changes, puberty and semen traits of growing Guzerat and Nelore bulls. Theriogenology 1991,35: 815-827.

44. Turner J. Genetic and biological aspects of Zebu adaptability. J Anim Sci 1980; 50: 1201-1205. 45. Vale-Filho V, Pinto P, Megale F, Fonseca J, Soares, L. Fertility of the bull in Brasil. A survey

of 1088 bulls and 17945 ejaculations Tom Bos taunts. Bos indicus and crosses under tropical conditions. Proc 9th. Int Congr Anim Reprod Artif Ins 1980,2: 545-548.

46. Veeramachaneni-Rao D, GttRcHeath E, McEntee K, Bolt D, Hixon J. Pathophysiology of small testes in beef bulls: Relationship between scrotal circumference, histopathologic features of testes and epididymides, seminal characteristics, and endocrine profiles. AJVR 1986; 47: 1988-1999.

47. Wildeus S, Entwistle K. Spermiogram and sperm reserves in hybrid Bos indicus x Bos taunts bulls after scrotal insulation. J Reprod Fert 1983; 69: 711-716.