Abstract
Possible age and breed differences in circulating levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone in the ram were investigated. In exp. 1, blood was collected from a group of 28 rams of various ages and breeds by venepuncture from the jugular vein at 20-min intervals for an 8-h period (May). Comparison of mean serum hormone concentrations for postpubertal Finnish Landrace (Finn), Suffolk and Managra Synthetic (Line M) rams did not reveal significant (P > 0.10) breed differences. However, when rams of the three breeds were categorized by age, the mean (±SE) LH levels in lambs (n = 8), yearling (n = 12) and adult (n = 8) animals were 2.1 ± 0.4, 1.1 ± 0.2 and 0.5 ± 0.1 ng/mL, respectively; the significant decrease (P < 0.05) in LH levels in adult vs. yearling rams was due to fewer and smaller LH releases. Mean testosterone levels averaged 1.3 ± 0.2, 2.8 ± 0.4 and 2.4 ± 0.3 ng/mL for the lamb, yearling and adult ram groups, respectively. FSH levels were similar for all age groups. In exp. 2, a series of blood samples were collected from 3-mo-old Finn (n = 6) and Suffolk/Line-M crossbred (n = 6) rams in October. In comparison with crossbred rams, rams of the more prolific Finn breed exhibited substantially higher mean levels of LH (1.0 ± 0.2 vs. 0.4 ± 0.2 ng/mL; P < 0.10), FSH (250 ± 26 vs. 121 ± 16 ng/mL; P < 0.01) and testosterone (2.8 ± 0.9 vs. 0.4 ± 0.1 ng/mL; P < 0.05); higher LH and testosterone in Finn rams were associated with larger and/or more frequent elevations in the level of these hormones during the 8-h bleeding period. Results of this study seem to indicate (1) age-associated differences in the pattern of LH release in adult rams in the spring; and (2) that breed differences in blood reproductive hormone levels in postpubertal rams are not significant in the spring, but levels in 3-mo-old rams differ during the fall months according to genotype. Key words: Age, breed, gonadotropins, testosterone, serum, ram
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