Measuring cortisol in blood is challenging because the process of collecting the sample itself can increase cortisol levels. Cortisol, often called the stress hormone, rises when people or animals experience stress, pain, or discomfort. At Virginia Tech, we conducted a study during the summer of 2024 in which cows were subjected to four different treatments: no handling (negative control), artificial insemination, ruminal incubation, and esophageal tubing. We chose these interventions because they have potential to stress cows, especially stomach tubing, which we expected to raise cortisol levels.
Although our hypothesis was that cortisol concentrations in blood would be significantly higher after these interventions relative to before the interventions, we encountered two unexpected results. First, cortisol levels in blood were not elevated 30 minutes after the interventions. It is possible, however, that waiting 30 minutes might have been too long to detect elevated cortisol levels, which could be a flaw in our study design. We based this timing on another study, but it may not have been appropriate for our purposes.
Second, cortisol concentrations in blood before the interventions were low to normal. This made us question the initial assumption that blood sampling itself raises cortisol levels. We collected blood samples while the cows were voluntarily eating and locked in headlocks, either after feeding or milking. The blood collection process took less than 15 seconds in most cases, suggesting that we did not stress the cows. This finding challenges the common belief that cortisol levels rise simply by inserting a needle into a blood vessel.
Upon reviewing the literature, we found an article in the Journal of Dairy Science stating that handling and restraining dairy cattle rapidly boosts cortisol concentration in the blood. However, this statement was based on studies involving wild animals, such as white-tailed deer and buffalo. In the deer study, restraining the animals took five to 10 minutes, which is much longer and more stressful than our quick blood sampling process on cows. In the buffalo study, the animals were restrained in a chute, which is also more stressful than our method of restraining cows in headlocks while they ate.
An interesting observation from our study was related to the insemination process. Some cows were inseminated in the familiar environment of the freestall barn, while others were inseminated in the palpation rail outside their pens, which is less familiar to cows. Cows inseminated in the freestall barn had lower cortisol levels compared to those in the palpation rail, both before and after insemination. This suggests that the environment plays a significant role in stress levels, and further research is needed to determine whether the palpation rail itself or the process of moving cows to the rail causes stress.
There are many misconceptions about animal stress and welfare, even within the scientific community. Our research team is committed to conducting additional studies to better understand how handling affects stress and welfare in dairy cattle. We hope to provide useful information for dairy farmers and consumers in the near future.
