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THE EFFECT OF SEXUAL STIMULATION ON SEXUAL ACTIVITY AND THE

RESPONSE OF LUTEINIZING HORMONE TO EXOGENOUS GNRH

 

M.J. Herselman, P.R. King and M van Heerden

Grootfontein ADI, Private Bag X529, Middelburg, 5900

 


A relative large proportion of adult Merino rams is sexually inactive, which might be linked to the relative low reproduction rate of Merino sheep in general. In practice, rams are normally selected on production and subjective traits, while evaluation of their reproductive potential is limited to the assessment of semen quantity and quality. The interrelationships between LH secretion, testosterone secretion, testis diameter, libido and female reproduction suggest that it might be possible to base selection for reproduction in rams on LH secretion. In the present study the influence of sexual stimulation on different aspects of LH secretion (following GnRH administration) and libido were investigated. Forty-five adult Merino rams were subjected to GnRH response tests. Two µg of  GnRH (Fertagyl; Intervet, Boxmeer, Holland) was injected into the left jugular vein of each ram.  Blood samples were collected into heparinized vacuum tubes from the right jugular vein 30 and five minutes prior to treatment and at 10, 20, 30, 40, 50 and 60 minutes following treatment. After collecting the 60 minute blood sample, each ram was again injected with 2 µg of GnRH.   This was followed with further blood sampling at 10 minute intervals for 60 minutes. Within 10 minutes after collection, the blood samples were centrifuged at 2000G for 20 minutes, after which the plasma was aspirated and stored at -20 °C for LH assays. LH in plasma was analyzed by radio-immunoassay according to the method of Kritzinger (1982). Two days after the GnRH stimulation test, each ram was subjected to four libido tests per day for three consecutive days. For each test, each ram was exposed for a ten minute period to ten ewes in which oestrus was artificially induced and libido was scored on a scale from zero to four. For statistical analysis the twelve scores of each ram were summed to represent the ram's libido score. Following the libido tests all rams were exposed to oestrual ewes for a period of 21 days. The day after concluding the exposure to oestrual ewes a second GnRH test was performed which was followed by another 12 libido tests executed over a further three day period. The GLM procedure of SAS (Littell et al., 1991) was used for statistical analysis. Repeatability of libido and other parameters were estimated by intra-class correlation coefficients. Repeatability of libido tests (four tests per ram) was high  (0.863 ± 0.011). Sexual stimulation did not influence libido and was reflected by a correlation of 0.698 ± 0.054 between libido measured before and after a 21 day exposure period to oestrual ewes. The LH response to two exogenous GnRH doses was not influenced by exposure to oestrual ewes and was also reflected by relatively high repeatability of different intervals of the dose response curve. No correlation was found between libido and LH response. It is concluded that, as sexual exposure of young rams to oestrual ewes did not influence the response of LH to GnRH, this response might be utilized as indirect selection parameter for female reproduction in rams at a young age. Further studies to validate this are, however, necessary.