Open access publications in the SLU publication database

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 (PSperm 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 (PThe 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.