Last update: August 15, 2011 03:15:51 PM E-mail Print

 

LEVELS OF LH AND TESTOSTERONE IN RONDERIB AFRIKANER AND MERINO RAMS IN RESPONSE TO TWO SMALL DOSES OF LH-RH

 

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

 

Merino ewes mated to Ronderib Afrikaner rams, display a higher ovulation rate (19% on average) than Merino ewes mated to Merino rams. The only visual difference between the two breeds which may explain this phenomenon is the fact that libido tests indicate that the Ronderib Afrikaner has a higher sexual activity than the Merino ram. In an attempt to identify the cause of this difference a LH-RH response test was performed to establish possible differences in hormonal control of sexual activity. Blood was sampled at ten-minute intervals for 4 hours from six Ronderib Afrikaner and six Merino rams injected twice with one micro gram LH-RH at a one-hour interval. All samples were assayed for LH and testosterone concentration. The mean LH response after the first LH-RH dose was 6.65 ± 1.58 ng/ml and 4.81 ± 0.49 ng/ml for the Ronderib Afrikaner and Merino rams respectively (P<0.05). The LH response for the Ronderib Afrikaner rams was lower (6.83 ± 1.74) than that of the Merino rams (7.77 ± 2.14) for the 60-minute period following the second LH-RH dose (P> 0.05). The ratio of the mean LH concentration following the second LH-RH dose to the mean LH concentration after the first dose (Priming effect index) was 0.987 ± 0.142 for the Ronderib Afrikaner rams and 1.657 ± 0.465 for the Merino rams (P < 0.01). The mean testosterone response in the Ronderib Afrikaner rams after the first and second LH-RH dose was 8.98 ± 3.27 ng/ml and 12.09 t 2.69 ng/ml respectively and was lower than the 10.03 ± 3.48 ng/ml and 13.24 ± 1.60 ng/ml recorded for the Merino rams (P > 0.05). From these results it is apparent that Ronderib Afrikaner and Merino rams respond differently to small exogenous LH-RH doses. The lower priming effect index obtained for the Ronderib Afrikaner rams is in accordance with the negative relationship between sexual activity and priming effect index as reported in the literature.

 

Published

Proceedings 32nd SASAS congress