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Doctoral thesis, 2000

Breeding soundness evaluation of zebu bulls : with special reference to variations in clinical parameters and sperm characteristics in sires extensively managed in the dry tropics of Costa Rica

Chacón, Jorge;

Abstract

In Costa Rica and other tropical regions, beef cattle production is based upon Zebu (Bos indicus) breeds extensively managed on natural pasture. The reproductive efficiency of the breeding herd is low. To what extent the sires play a major role in this problem is yet undetermined mainly due to the nature of exploitation and the lack of periodical andrological controls to determine their suitability for natural breeding.

This thesis aimed to investigate the breeding soundness of bulls extensively managed in Costa Rica, with special reference to the identification of variables associated with their final ranking as potential breeders. Particular attention was paid to the prevalence of sperm abnormalities in the ejaculate, as well as eventual seasonal variations—and their causes—in clinical parameters and the spermiogramme.

Breeding soundness evaluations (BSE) were performed under field conditions in Bos indicus (Bi) (n = 598), Bos taunts (Bt) (n = 252) and Bos indicus x Bos taurus bulls (Bi x Bt) (n = 48) in four different climatic regions of Costa Rica. The bulls, whose number represented about 2% of the total bull population in the country, were 1-12 years of age and extensively managed. Thirty-three percent of the bulls (296/898) were classified as unsound for breeding, 9.1% owing to clinical problems (82/898) and 23.9% to unsatisfactory sperm morphology (214/898). Low scrotal circumference (SC; less than 30 cm) was the most common clinical finding in bulls more than 2 years of age. The percentage of unsound bulls with sperm abnormalities in the ejaculate, especially head defects and proximal droplets, increased as SC decreased (P< 0.05), being higher in bulls having a decreased testicular consistency (TC) (P< 0.001) and long scrotum (P< 0.01). Frequencies ofsperm abnormalities were also higher in bulls less than 2 years of age than in older sires (P< 0.01) and were highest in Bi x Bt bulls (P< 0.001). The prevalence of bulls unsound for breeding was lower (P< 0.01) in Bi sires (29%), intermediate in Bt (41%) and higher in Bi x Bt (48%).

Sperm morphological examinations in 302 Zebu bulls, aged 1.5-9 years, from the dry Pacific zone of Costa Rica, confirmed the relationship between sperm morphology and clinical andrological parameters. Bulls with a SC less than 30 cm showed a higher prevalence for proximal cytoplasmic droplets than bulls whose SC was greater than 30 cm (P< 0.05). Likewise, bulls with a long scrotum and decreased TC at palpation showed higher percentages of abnormal sperm heads (including nuclear abnormalities) in the ejaculate than bulls with normal scrotal length (SL) and normal TC (P< 0.05). Zebu bulls less than 2 years of age showed a higher percentage of missing acrosomes and proximal cytoplasmic droplets than older sires (P< 0.05). The abnormality most commonly seen in ejaculates from sires sound for breeding was bent tails with entrapped cytoplasmic droplets. Phase contrast microscopy of fixed wet-smears was a convenient method to assess sperm morphology on-farm since it allowed the diagnosis (based on sperm morphology) of up to 88% of unsound bulls. However, a follow-up quantification of stained smears is advisable to confirm the primary assessment.

The fine morphology oftestes and cauda epididymides from 11 sexually mature Zebu bulls with normal (n = 3) or decreased TC (n = 8) at palpation was also studied. Bulls having a slight to moderate reduction in TC showed more, albeit slight, degenerative changes in the seminiferous epithelium, compared with control animals. Cellular debris present in the Sertoli cells, abnormal acrosomes and impaired chromatin condensation during spermiogenesis were also common in bulls with a decreased TC. Palpation for TC is an important component ofthe clinical andrological evaluation ofbreeding Zebu bulls.

Sexually mature Brahman bulls (n = 25) extensively managed, were andrologically examined once monthly for 13 months to determine whether clinical and spermatological parameters in Zebu bulls show seasonal changes under extensive rearing conditions, in the dry Pacific region ofCosta Rica. Ofthe clinical and sperm parameters examined, only SC showed a significant seasonal variation. This variation was positively related to body condition rather than to the changes in climatic variables prevailing during the study period.

The overall results confirm the higher adaptability of Zebu bulls to the tropical environment, as well as the relationship between SL, SC and TC with testicular normality and, therefore, with a normal sperm morphology in the ejaculate. The findings also suggest that nutrition, rather than ambient temperature, is the major factor contributing to variations in the reproductive function ofZebu bulls in the tropics.

Keywords

Zebu (Bos indicus) bulls; breeding soundness evaluation (BSE); extensive rearing conditions; testicular consistency; scrotum length; Testis; Cauda epididymides; utrastructure; sperm morphology

Published in

Acta Universitatis Agriculturae Sueciae. Veterinaria

2000, number: 70
ISBN: 91-576-5910-9
Publisher: Swedish University of Agricultural Sciences

Authors' information

Chacón, Jorge
Swedish University of Agricultural Sciences, Department of Obstetrics and Gynaecology

UKÄ Subject classification

Genetics and Breeding
Clinical Science

URI (permanent link to this page)

https://res.slu.se/id/publ/117465