REPRODUCTIVE SEASONALITY OF CORRIEDALE RAMS UNDER EXTENSIVE REARING CONDITIONS

The objective of the present study was to describe seasonal changes in scrotal circumference (SC), live weight (LW), sperm morphology and pl asma levels of testosterone (T) and thyroxine (T-4) in young Corriedal e rams reared under extensive conditions typical for the southern Lati n American region. A total of 31 Corriedale rams, 11 months of age and with a LW of 36 +/- 1.1 kg and (SC) of 23.0 +/- 0.5 cm at the beginni ng of the experiment (September) were kept on natural pastures. At mon thly intervals LW was recorded, animals were clinically examined, and SC was measured. None of the animals were used for breeding. Fifteen a nimals were randomly selected and bled once a month from January to De cember and plasma concentrations of T and T-4 were determined. In addi tion, one semen sample was collected by electroejaculation and morphol ogical studies were performed. The mean individual LW increase was 18 kg (50% of the initial LW) during the experiment (p<0.01). LW decrease d in autumn, with the nadir in late autumn. SC reached mean maximum le vels in late summer (31.1 +/- 0.4 cm, p<0.01), then decreased until th e beginning of winter (26.3 +/- 0.4 cm, p<0.01) and remained low until early spring (27.5 +/- 0.5 cm, p>0.05) to increase again between mid- spring and the end of the experimental period the following summer (30 .7 +/- 0.5 cm, p<0.01). The mean SC in winter was 16% lower than that in late summer. Semen could be collected from the rams throughout the experiment. Frequencies of sperm head, midpiece and total abnormalitie s showed monthly variation (p<0.05), but tail abnormalities were not a ffected by month. Low abnormalities were found in autumn (9.4% +/- 2.2 %). T was high during autumn (p<0.01). Minimal T4 concentrations were observed during late summer and early autumn (p<0.01) when T levels we re high. Maximum T4 concentrations were registered in late autumn (p<0 .01), when SC was decreasing; in midspring (p<0.01) one month after sh earing and in early summer (p<0.01). The results suggest that Corrieda le rams under Uruguayan extensive management systems show a reproducti ve seasonality that, in general terms, coincides with photoperiodic va riations.