RESUMO
The objective of the present study was to describe seasonal changes in scrotal circumference (SC), live weight (LW), sperm morphology and plasma levels of testosterone (T) and thyroxine (T4) in young Corriedale rams reared under extensive conditions typical for the southern Latin 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 beginning of the experiment (September) were kept on natural pastures. At monthly intervals LW was recorded, animals were clinically examined, and SC was measured. None of the animals were used for breeding. Fifteen animals were randomly selected and bled once a month from January to December and plasma concentrations of T and T4 were determined. In addition, one semen sample was collected by electroejaculation and morphological studies were performed. The mean individual LW increase was 18 kg (50% of the initial LW) during the experiment (p < 0.01). LW decreased in autumn, with the nadir in late autumn. SC reached mean maximum levels in late summer (31.1 +/- 0.4 cm, p < 0.01), then decreased until the 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, mid-piece and total abnormalities showed monthly variation (p < 0.05), but tail abnormalities were not affected 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 were high. Maximum T4 concentrations were registered in late autumn (p < 0.01); when SC was decreasing; in mid-spring (p < 0.01) one month after shearing and in early summer (p < 0.01). The results suggest that Corriedale rams under Uruguayan extensive management systems show a reproductive seasonality that, in general terms, coincides with photoperiodic variations.
Assuntos
Reprodução , Estações do Ano , Ovinos/fisiologia , Criação de Animais Domésticos/métodos , Animais , Peso Corporal , América Latina , Masculino , Escroto/crescimento & desenvolvimento , Escroto/fisiologia , Espermatozoides/citologia , Testosterona/sangue , Tiroxina/sangueRESUMO
The present study was undertaken to estimate the time for the attainment of spermatogenesis in springborn Corriedale ram lambs under conditions of extensive grazing systems in Uruguay. Clinical (live weight, scrotal circumference, penile/preputial separation), morphological (light and electron microscopy) and endocrinological (testosterone levels) parameters were examined. Two experiments in 2 consecutive years were carried out. In Exp. I, 40 ram lambs were clinically examined, weighed, and blood-sampled at 2 week-intervals between 78 and 216 days of age. Sixteen were castrated in 3 selected periods: 132-162 (n:2), 145-175 (n:6) and 186-216 days (n:8). In Exp. II, 40 ram lambs appertaining to the next generation of the same flock were examined as above at 180-210 days of age, and castrated for morphological studies. The time for attainment of complete spermatogenesis correlated significantly with most corporal parameters (i.e. scrotal circumference (r = 0.52); testicular weight (r = 0.61), epididymal weight (r = 0.60), penile/preputial separation (r = 0.75). The age of castrated ram lambs correlated with the degree of spermatogenesis (r = 0.83), although no significant correlations were found with live weight or with levels of circulating testosterone. The histology showed major variations in the degree of development of the seminiferous epithelium. Cells undergoing degeneration were a common finding through the initial stages of spermatogenesis, coincident to the presence of sperm abnormalities and foreign cells in semen. By day 180 and onwards, both histology and seminal picture normalized. It is concluded that, under these rearing conditions, the onset of puberty (expressed as morphologically established spermatogenesis) in Corriedale ram lambs is attained at 180-216 days of age when they reach 23 cm of scrotal circumference and 191 g of testis weight. The finding of a high correlation between these parameters (r: 0.93) confirms the usefulness of the measurement of scrotal circumference during clinical examination of ram lambs in this breed.