Individual variations in FSH release after a GnRH challenge and relationship with semen output in young bulls

Blood samples collected from 54 Montbéliarde young post-pubertal bulls were assayed for FSH. In the first trial, 0.25 mg GnRH was given to each of 25 bulls at 12 months of age. In a second group of 29 bulls, each received 20 mg dexamethasone followed by 0.25 mg GnRH 5 h later. Based on measurements of semen output, the bulls were classified into three categories: good, medium and poor semen producers. The total FSH response was significantly different among individuals (P less than 0.05) and repeatable (r = 0.45) only after the combined treatment. The mean total responses did not differ significantly between the 3 categories of semen producers, but individual FSH responses were significantly and negatively correlated to quantitative and qualitative semen production characteristics (r = 0.4-0.7). It was concluded that measurement of FSH after a combined dexamethasone-GnRH challenge permits the demonstration of a significant relationship between FSH release and semen production in individual bulls.