The measurement of daylength in the Ile-de-France ram

Abstract

Two groups of 12 adult Ile-de-France rams were exposed to artificial 6-month light cycles in which daily illumination was provided in one or two photofractions. In Group I, daylength increased linearly from 8 to 16 h in 3 months and decreased similarly from 16 to 8 h. The daily increment or decrement (5.33 min/day) was constant. In Group II, 8 h of light were given in two parts: the main one, 7 h, began at the time of dawn in Group I and an additional 1 h light pulse was coincident with the last hour of the former group. The onset of the pulse changed therefore each day and the interval between dawn of the first block and dusk of the second block of light increased from 8 to 16 h in 3 months and declined from 16 to 8 h the next 3 months. Testicular weight was estimated by an orchidometer every 2 weeks for 2 (N = 12/group) or 3 consecutive light cycles (N = 6/group). The testicular weight variations were identical in both groups. In the 6 rams of each group studied during 3 light cycles, variations of testicular weight were submitted to an harmonic regression analysis according to time and the computed values for the mean, amplitude, period and phase were, respectively, 260 g, 66 g, 185 days and 120 days in Group I and 262 g, 65 g, 181 days and 111 days in Group II. Analysis performed for each ram gave very similar values in all individuals. It is concluded (1) that daylength is not measured by the total duration of light exposure but between two limits represented here by dawn and the pulse of light and (2) that the measurement is not limited to a particular photosensitive phase but is effective throughout the entire light cycle.