Evidence that the photoperiod controls the annual changes in testosterone secretion, testicular and body weight in subtropical male goats.

The aim of this study was to determine whether the reproductive seasonality of local male goats from subtropical Mexico (26 degrees N) is controlled by photoperiod. The control group (n = 7) remained in an open shed under natural daylight. The two experimental groups (n = 6 each) were placed in light-proof buildings and exposed for 2 years (yr) to alternations of 3 months (mo) of long days and 3 mo of short days. One group was first exposed to long days and the other one to short days. Body and testicular weights were determined every 2 wk. Blood samples were obtained weekly to determine testosterone plasma concentrations. In the control group, the body weight exhibited variations (P < 0.0001) and it increased during the non-breeding season. In both treated groups, long days stimulated weight gain and short days inhibited it (P < 0.0001). In the control group, testicular weight displayed variations (P < 0.0001), and high values were registered in June. In the treated groups, a testicular weight reduction occurred 6-9 mo after the onset of the study. Afterwards, the changes in testicular size varied according to daylength (P < 0.01). The pattern of plasma testosterone concentration in the control group varied over the study (P < 0.0001) and the levels were higher from May-June to November. In both treated groups, the changes in testosterone secretion occurred according to photoperiod changes (P < 0.0001). Short days enhanced testosterone secretion one photoperiodic cycle after the onset of the study and long days inhibited it. Local male goats from subtropical Mexico are sensitive to photoperiodic changes and this environmental cue may control the timing of the breeding season in natural conditions.