While it's natural to project our own feelings onto dairy cattle, it's better to get the information straight from the source. Studies of stress in dairy cattle, conducted in the last decade, have revealed some surprising results about how your cows feel.
Researchers measure stress by monitoring cow behavior and, most often, measuring the levels of cortisol, a stress hormone. Common management procedures often create short-term stress, which can be measured by elevated serum cortisol levels.
Which procedures do cows care about? Research suggests that the following practices cause the most stress:
- Heat stress. Cortisol levels can be 10 times higher than normal during periods of elevated environmental temperature and humidity.
- Animal restraint. The longer cows are confined in lock-ups, the higher cortisol levels rise. Of course, this is compounded if it occurs under the beating sun. Individual animals can be categorized as high stress or low stress, based on cortisol levels and their restraint responses. For example, animals that were stressed by restraint as calves continue to be reactive as adult cows.
Studies show that BST administration does not change cortisol levels, even under heat-stress conditions. Rectal palpation, artificial insemination, and natural breeding do not elevate cortisol during the estrous period, but can cause stress when cows are not in heat.
- New environments. Introduction to a novel environment and separation from herdmates can cause changes in heart rate, behavior, and cortisol response for up to one week. Isolation from others is the greatest social stress for cattle. The reaction decreased quickly after reunion with other cattle and even faster when placed with familiar pen mates.
- Calving and diseases. Giving birth can be stressful for the cow. Animals needing assistance during delivery have a longer duration of abnormal cortisol levels.
Removing a newborn calf after bonding does not appear to induce stress for the mother. Heart rate is typically elevated for 10 minutes, yet cortisol levels remain normal.
Following a difficult birth or dystocia, cortisol remains elevated for at least three weeks and can be associated with failure of uterine involution and bacterial infections in the uterus. Displaced abomasum surgery and various disorders, such as milk fever, retained placenta, metritis and uterine prolapse, also cause cortisol levels to rise.
Stress response can actually be good in a way, because cortisol helps reduce pain and inflammation and regulates blood sugar. Prolonged and high cortisol levels, however, put an animal at risk for disease by suppressing immune function.
- Transportation. The classic way to elevate cortisol levels is by transporting cattle for at least 30 minutes.
Cortisol limits white blood cell function and replication and prevents these cells from moving to sites of infection. That's why new mastitis infections can occur, and chronic infections flare up after transporting.
Environmental mastitis infections increase during the summer - not only because more pathogens exist, but also because of heat stress increasing the cortisol levels and suppressing the immune function. Retained placenta may also be caused, in part, by a failing immune system.
- Reproduction. Cortisol levels also can cause failure of ovulation in anestrus cows and delayed ovulation in cycling cows by dampening luteinizing hormone (LH) pulses. This can decrease fertility during the summer.
- Milk letdown. And, finally, cortisol decreases milk protein synthesis and inhibits the release of oxytocin. Cows milked in new or stressful situations have poor milk letdown and more residual milk - a risk factor for mastitis.
To avoid these adverse effects, we know we have to reduce disease, dystocia and heat stress. But, producers and their veterinarians also should consider reducing lock-up time, minimizing group moves, gently acclimating heifers to the parlor, and handling cows in a calm and quiet manner, especially when exposed to a situation or procedure for the first time.
Marguerita B. Cattell is a consulting veterinarian in Fort Collins, Colo.