Correlation between blood collection time and serum cortisol levels in Bos indicus cows subject to stressful stimuli

The presence of a circadian rhythm in cortisol release in cattle is not well understood. Thus, studies that seek for studying the relation between cortisol and stress could be influenced by a possible variation of this hormone release during the hours of the day. This study sought to determine whether the blood draw time was correlated with serum cortisol levels in cows submitted to a stressful situation. A total of 261 Bos Indicus, multiparous, lactating and healthy cows were used. These animals had body condition score between 2.5 and 3 (on a scale which 1 is very thin and 5 is too fat). The cows had a mean age of 6.6 ? 1.45 years and were submitted to management corral, followed by Artificial Insemination. Blood collection was performed at the time when the animals were restrained in the chute for insemination by puncturing through the coccygeal vein, with the use of vacuum tubes containing clot activator. Cortisol levels were measured by ELISA kit. The measurements were performed in duplicate, with a coefficient of inter-assay variation of less than 10%. To check whether there was a correlation between the collection time and cortisol levels, the Spearman correlation was performed, R software, version 3.2.1, was used to do this analysis. The blood collection times ranged from 7:08 to 18:39 hours. Serum cortisol levels presented above baseline (average of 51.01 ? 35.84), showing that the management of the situation in the corral was a sufficient stimulus to cause stress to the animals. The correlation between cortisol levels and the blood draw time was almost zero (rs = -0.02). Therefore, it can be concluded that the blood draw time did not affect the levels of cortisol in Bos indicus cattle subjected to stress. Thereby, studies performed in this field do not need to worry for the time that the experiment is done to ensure reliable results