Seasonal variations in plasma testosterone levels in Verata and Malagueña bucks

Abstract

The influence of season and photoperiod on plasma testosterone levels and characteristics of the testosterone secretion pattern was studied in ten Verata and nine Malagueña bucks. Beginning at 4 months of age, all 19 bucks were bled by jugular venipuncture once weekly, until 23 months of age. Serial blood samples at 30-min intervals for 5.5-6 h were collected every season. Plasma testosterone level was measured by radioimmunoassay. Bucks of both breeds showed an increase (P < 0.01) in plasma testosterone levels during summer and fall, i.e.;during decreasing photoperiod. Secretion patterns had increases (P < 0.01) in mean level, basal level, peak frequency and height of these peaks during summer and fall. A certain synchronism in testosterone peak appearance throughout the blood sample collection period was observed, with greater density of first peaks between 1 and 3 h (78%) and second peaks between 4 and 6 h (71%) after dawn. Testosterone levels from July to October of Verata bucks were higher than of Malagueña bucks (P < 0.01). The results showed that both breeds had clear seasonality in plasma testosterone secretion, but more marked in Verata than in the Malagueña breed. © 1995.