Last update: August 16, 2011 08:26:52 AM E-mail Print




P R King, W A Coetzer*, E Roux & P A Schlebusch

Grootfontein Agricultural Development Institute, Middelburg Cape, 5900

*Department of Human and Animal Physiology, University of Stellenbosch, Stellenbosch, 6700


It is generally accepted that testosterone is essential for the manifestation of normal sexual behavior in rams. The mean plasma testosterone concentration during a 24 hour period within the breeding season (May) is significantly higher in Merino rams than in Ronderib Afrikaner rams while the mean concentrations do not differ during October. Contrary to expectation the sexual activity of Ronderib Afrikaner rams is higher than that of Merino rams during the breeding and non-breeding season. In an attempt to explain this discrepancy, testosterone turnover rate was determined in four Ronderib Afrikaner and four Merino rams. The rams were continuously infused intravenously with a known quantity (8µg/min.) of tritium labeled testosterone (3H testosterone) per minute. By determining the ratio between the rate of infusion and the mean plasma concentration of 3H testosterone at 50, 60 and 70 minutes after the beginning of infusion (3H testosterone at equilibrium), it was possible to determine the metabolic clearance rate (1/min.) of testosterone. The testosterone secretion rate (µg/min.) was obtained from the product of the metabolic clearance rate and the mean testosterone concentration. The metabolic clearance rate was 1.2071 ± 0.171 1/min. and 1.4442 ± 0.1796 1/min. for Ronderib Afrikaner and Merino rams respectively (P > 0.05). In accordance with previous results, the testosterone concentration of the Ronderib Afrikaner rams (1.43 ± 0.18) was lower than that of the Merino rams (4.42 ± 0.33) (P < 0.01). A significant (P < 0.01) difference in testosterone secretion rate between Ronderib Afrikaner (1.7175 ± 0.3129) and Merino rams (6.35 ± 0.3253) was found. The difference in plasma concentration of testosterone between Ronderib Afrikaner and Merino rams may possibly be attributed to a higher secretion rate in the Merino ram. These results are contradictory to the accepted principle that higher testosterone levels will result in increased sexual activity and it is therefore probable that other factors are also involved.



Proceedings 32nd SASAS congress