Testosterone response to synthetic gonadotropin releasing hormone injection in Sahiwal-Friesian crossbred bulls

P. Ramakrishnan, M. O. Ariff, M. Murugaiyah and M. A. Dollah

Abstract

A study was conducted to determine the effects of seven dose levels of a single intramuscular injection of a synthetic gonadotropin releasing hormone (GnRH) on plasma testosterone response in five Sahiwal-Friesian crossbred bulls aged between 23 and 25 months. The GnRH doses were 0, 0.005, 0.01, 0.02, 0.04, 0.08 and 0.16μ g/kg body weight. Blood samples were collected at 0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0 and 7.0 h post-injection of GnRH from 0900 to 1600 h. A small GnRH dosage of only 0.005 μg/kg body weight was observed to be adequate in inducing a maximum peak response in testosterone level. Increasing the dose of GnRH by a 32-fold range did not influence significantly (p >0.05) the height of testosterone response peak or testosterone area under the response curves. The optimal time to measure the height of the testosterone response peak was found to be between 2 h and 3 h post-injection of GnRH. The high correlation (r = 0.83, p <0.01) between the testosterone response peak and area under the response curve showed that a single blood sample collected at an appropriate time might be a suitable alternative to the measurement of area under the response curve which needs a series of blood samplings.

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