Effect of Climate on the Seasonal Endocrine Profile of Native and Crossbred Sheep under Semi-arid Conditions

Thirty-four 2-year-old ewes, of which 17 were native (Chokla) and 17 were Avivastra (Chokla 67.5%×32.5% Rambouillet) breeds of similar body weights, were selected. Both groups were maintained under the same management conditions with 8 hours daily grazing. In addition to grazing, each ewe received 300 g/day of a concentrate mixture comprising crude protein 20% and 70% total digestible nutrients. The ewes of both breeds were not mated during the study. The daily environmental parameters were recorded throughout the year, consisting of four main seasons, namely rainy, winter, spring and summer. Blood samples were collected into heparinized tubes during the morning hours for seasonal studies via indwelling jugular vein catheters to avoid stress to the animals, but for the diurnal studies the sampling was done twice daily at 07:00 and 17:00 continuously for three days in each season. Cortisol, thyroxine (T4) and triiodothyronine (T3) were estimated in the samples. The T4 and T3 profiles were maximal in the summer, followed by the rainy season in both groups, with no significant differences. However, the cortisol in both the groups was maximal during the spring rather than the summer. No significant differences were found in T3 concentration between morning and afternoon in any season in the native ewes but these animals showed significantly higher levels of T4 in the morning. The opposite was true for the crossbred animals with regard to both the T3 and T4 profiles. Significantly more cortisol was found in the evening than in the morning in both genetic groups in all seasons.