A comparative study on the quality of semen from Zulu rams at various ages and during different seasons in KwaZulu-Natal, South Africa

Zulu sheep is a Nguni breed indigenous to the KwaZulu-Natal (KZN) province of South Africa, and is reported to be under threat of extinction. Knowledge of factors that may cause the declining numbers is required for the planning of conservation programmes. We designed a study to evaluate the effect of ram age, season and geographic location on the quality of semen. The aim was to determine which rams are best for breeding and at which time of the year. A total of 192 rams were sampled for semen and blood in 8 different locations, three times each season. Semen was collected via electro-ejaculation into semen collection tubes while venous blood was collected from the jugular vein for testosterone and white blood cell (WBC) assay. Parameters used to assess semen quality were volume, pH, sperm concentration, progressive and mass motility, percentages of live and abnormal spermatozoa. The average semen volume and spermatozoa concentration per ejaculate were similar for autumn, winter and spring while being significantly different during summer (P < 0.05). Average testosterone and WBC count were significantly higher in winter (P < 0.05; P < 0.005). The effects of age on all seminal parameters, except for pH, were significant (P < 0.05). Semen volume, sperm concentration, motility and live sperm increased linearly up to 3 years of age while scrotal circumference and live spermatozoa were greater (P < 0.05) for rams at 3 than 4 years. Rams at 3 years also showed average testosterone and WBC counts that were significantly higher (P < 0.05; P < 0.005). A positive correlation between age and semen volume, spermatozoa concentration and semen colour was observed. The geographical location of rams did not have a significant effect on most of the semen parameters. For conservation purposes it would be most efficient to select breeding sires from among the 3-year-old rams for the winter and spring breeding programme.