Testosterone and luteinizing hormone response to GnRH in yearling bulls of different libido

Abstract

This study was conducted to evaluate pituitary and testicular responsiveness to gonadotropin releasing hormone (GnRH) as measured by luteinizing hormone (LH) assay and by testosterone assay, respectively, in bulls that had previously exhibited either high or low libido based on a libido test. Eighty-four purebred Angus bulls were tested for libido during 1988 and 1989. Blood samples were collected from four bulls that had the highest and lowest libido scores in each year by indwelling jugular cannula at 15 min intervals for 2 h before and for 5 h after GnRH (10 ng/kg body weight, i.v.) administration. The samples were assayed for LH, testosterone and cortisol. Low libido bulls had higher (P<0.05) LH concentrations during the first hour after GnRH treatment than high libido bulls. Testosterone concentrations were higher (P<0.05) in high libido bulls before GnRH administration but were similar afterwards in both years. Cortisol concentrations were higher (P<0.05) during the first hour of sampling in low libido bulls than in high libido bulls. It was concluded that the response to a GnRH challenge as measured by LH or testosterone concentrations was not a useful indicator of libido in bulls.