title: Serum testosterone concentration in response to exogenous GnRH does not predict service rates or pregnancy rates by yearling, performance tested, crossbred bulls.
abstract: Ten bulls with a scrotal circumference of less than 30 cm at the end of growth performance testing, and 10 cohorts of the same age, size and breed type with a scrotal circumference greater than 30 cm were used to evaluate if testosterone response following GnRH administration could be used to test for fertility, for semen quality, and for specific pathologic testicular parenchymal changes. Serum testosterone concentrations were determined immediately before and 2 to 3 hours following intramuscular injection of 250 ug GnRH. Bulls were examined for breeding soundness, then fertility was tested in a breeding trial; testicular histology was assessed by determining the percentage of cross-sections of seminiferous tubules with no spermatocytes. The mean (+/-SEM) post-GnRH serum testosterone concentration for all bulls was 11.71 (+/-0.64) ng/ml. In order to examine for an association, the GnRH response was classified as above or below the mean for resultant serum testosterone concentration. The GnRH response classification was not related to the scrotal circumference, percentage of tubules devoid of spermatocytes, or percentage of progressively motile spermatozoa (P>0.10). The percentage of morphologically normal spermatozoa was significantly higher (P<0.05) in the bulls with a higher than mean testosterone secretion in response to GnRH injection. In the breeding trial, the percentage of heifers bred and the percentage of heifers pregnant (60 days post breeding) were not significantly different (P>0.10) between the 2 classifications of GnRH response. The GnRH response test was related to the percentage of morphologically normal spermatozoa but did not predict fertility of yearling bulls in this study.
authors.affiliation: Department of Population Medicine, University of Guelph, Guelph, Ontario, Canada N1G 2W1.
authors.forename: J A
authors.initials: JA
authors.lastname: Thompson
authors.forename: R M
authors.initials: RM
authors.lastname: Liptrap
authors.forename: W H
authors.initials: WH
authors.lastname: Johnson