Serum Cortisol Concentration in Dairy Cattle from the Amazonian Region

The study was carried out in the parish of El Triunfo, province of Pastaza, Ecuador, with the objective of evaluating variations in serum cortisol concentrations in dairy cattle. Fourteen Brown Swiss and Holstein Frisian cows were selected, which met the criteria of being healthy, free of ectoparasites, weighing between 350 and 450 kg, in or during their second lactation cycle, and without post-vaccination treatment at the time of sampling. Each cattle was sampled twice at 7:00 a.m. and 4:00 p.m., following biosecurity protocols. Samples were centrifuged at 3000 rpm for 10 minutes to obtain serum, which was analysed by Fluorescence Immunoassay (FIA) to quantify cortisol (ng/mL). Statistical analysis included the Shapiro-Wilks normality test, Student's t-test to compare cortisol concentrations between sampling times, and Pearson's correlation to assess the relationship between values obtained in the morning and afternoon. The results showed significantly higher cortisol levels in the morning (22.11 ng/mL) compared to the afternoon (16.94 ng/mL) (P < 0.050), indicating a circadian rhythm in cortisol secretion. This afternoon decrease reflected a characteristic pattern in dairy cattle from the Amazon region, suggesting that management and environmental factors influence cortisol variability, which is associated with a physiological response to moderate stress, common in management systems in the region.