Bull selection and use for improved performance in pastoral herds of Tanzania
Livestock Research for Rural Development
Department of Veterinary Surgery and Theriogenology, Faculty of Veterinary Medicine, Sokoine University of Agriculture, P.O.Box 3020, Morogoro, Tanzania
Tanzania shorthorn zebu (TSHZ) cattle are extensively reared in pastoral herds under natural multiple-breeding system in Tanzania. However, no study has been carried out to describe the reproductive performance of TSHZ bulls. The objective of this study was to measure scrotal circumference (SC) and correlate it with age, heart girth (HG), spermatozoa quality and body condition (BC). 303 healthy TSHZ breeding bulls were evaluated. SC measured by using standard scrotal metal. Age was determined by dentition. HG circumference was measured by tape. BC was estimated by visual assessment. Sperm cells were collected by epididymal aspiration procedure and stained with Carbol-fuchsin. One hundred spermatozoa were counted per slide and each classified into normal or abnormal. Three parenchyma samples were taken after slaughter and processed for histological examination by staining with hemotoxylin and eosin. Forty sections of seminiferous tubules per section were evaluated and classified into either normal or abnormal. Bulls were retrospectively classified into two groups; normal (55.8%) and abnormal (44.2%). The mean SC in normal bulls was 28.9±1.8 cm and positively correlated with age, years (r=0.67, P< 0.01) and HG, cm (r=0.82, P< 0.01). The mean percentages of spermatozoa abnormalities in normal and abnormal bulls were 15.6 ± 2.5 and 39.0 ± 9.1, respectively. Bulls with large testicular lesions had poorer seminal characteristics than bulls with normal testicles and the spermatozoa abnormalities increased with severity of testicular abnormalities (P < 0.01). This study revealed that large numbers of inferior bulls are used for breeding in pastoral system that partly contributes to poor performance and low genetic improvement that currently exist in pastoral system. Also the study established the average values of SC for normal TSHZ bulls, however further studies should be done to establish the origin and effects of stump-tail spermatozoa abnormalities that had high prevalence in this study.