Many articles have been published recently that address the impacts of stress on dairy producers as they face challenges related to increasing input costs, greater government regulations, labor issues and more. Of course, nutritionists, veterinarians and other professionals that support the dairy industry are stressed, too. Stress is known to have negative impacts on people’s productivity, health and mental well-being. Interestingly, the same stress physiology present in humans is common to all mammals, and dairy cattle are similarly impacted by stress.
Stress is caused by stressors. A stressor is simply a perceived threat. By “perceived,” we don’t mean “imagined,” but rather any threat (real or imagined) that the brain detects. Stressors can be physiological or psychological. Examples of physiological stressors in dairy cattle include calving, high temperature and humidity, and infections. Although cows certainly do not encounter the same psychological stressors humans do, they do experience stress related to social interactions (such as over-crowding) and fear. Stress is the body’s response to a stressor. Stress was designed to protect mammals from immediate physical threats, such as predators. Modern dairy cows face relatively low risk of predation, but the physiological response to stressors remains the same as originally designed. Stress involves a cascade of hormonal responses. The overall response is the same for all stressors. But the extent of the response differs depending upon the stressor, and there is significant variation between individuals in the degree of stress experienced.
When a stressor is recognized, the brain sends a distress signal to the hypothalamus. The hypothalamus serves as the starting point for the stress response. The first part of the stress response is activation of the sympathetic nervous system, which initiates the fight-or-flight response (also known as adrenaline rush), with the release of catecholamines into the blood in just seconds. Next, the hypothalamus initiates activation of the hypothalamic pituitary adrenal (HPA) axis, with the secretion of corticotropin releasing hormone (CRH) into the blood. This hormone triggers the release of adrenocorticotropic hormone (ACTH) from the pituitary gland. ACTH travels through the blood to the adrenal gland and stimulates the release of glucocorticoids from the adrenal cortex. In both humans and cows, this glucocorticoid is cortisol. The actions of cortisol occur within minutes to hours following recognition of a stressor. Unlike catecholamines, the effects of cortisol are not immediately apparent to the animal. However, the impacts of cortisol can be broad and long-lasting. Cortisol helps fuel an encounter with a threat by elevating blood glucose
concentrations and helps sustain the fight-or-flight response. However, cortisol also suppresses immune function, acting on many cells and proteins that comprise the immune system. Broader effects of cortisol include altered growth and reproduction, which become evident during long-term exposure to stressors.
Inflammation is a complex immune response intended to help protect the body from infection and promote healing of damaged tissue. Typical symptoms of inflammation are redness, swelling, heat, pain and loss of function, as seen when a cow has mastitis. Inflammation occurs both at the site of the infection (local) and throughout the entire body (systemic). A normal immune response will typically involve some inflammation. This is necessary and helps the animal overcome infections and diseases more rapidly.
However, inflammation that is either excessive or prolonged is harmful to the animal, partly due to the high demand for energy and other nutrients. A stressor is often present with an inflammatory response, whether inflammation is related to damage from an injury or release of proteins from immune cells combatting a bacterial infection in the udder. Immune suppression resulting from high blood cortisol is designed to lessen the inflammatory
response, likely to conserve energy and other nutrients.
Acute inflammation is beneficial and needed to overcome infections. Chronic inflammation, however, can be harmful. In humans, chronic inflammation may lead to heart disease, Alzheimer’s disease, arthritis, obesity, depression and other disorders. In dairy cows, chronic inflammation increases the risk of infectious and metabolic diseases, impaired reproduction and lower milk production. Short-term exposure to a stressor can result in acute stress. Long-term or repeated exposure to one or more stressors can result in chronic stress. Acute stress can result in excessive immune suppression, leading to increased risk of infections and diseases. Chronic stress can result in excessive or prolonged inflammation, leading to negative energy balance, metabolic disease and infertility in dairy cows.
There are many stressors that can impact dairy cow health, production, reproduction and well-being. Stressors that have been proven in controlled research to bring about a stress response in dairy cattle include:
• Exposure to molds and mycotoxins
• Improper feeding management
• Inadequate sanitation
• Negative social interactions
• Insufficient cow comfort
• High temperature-humidity index
• Aversive handling
• Calving
• Dry-off
• Transportation
• Poor air quality
It’s important to note that the impacts of multiple stressors on animals are additive and can increase the intensity and duration of the stress response. Several nutrients have been demonstrated to alter the stress response in dairy cows, as indicated by changes in cortisol secretion or blood cortisol concentrations, such as vitamin E and chromium.
Nutritional feed additives have also been demonstrated to affect the stress response in dairy cattle. For example, some nutritional specialty products improve immune function and have been shown to help lower blood cortisol concentrations and limit the impact of cortisol in immune cells.
In dairy cattle, as in humans, stress impairs performance, health and well-being. Just as in humans, steps should be taken to reduce exposure to stressors and/or reduce the stress experienced by dairy cattle facing these stressors to improve productivity and quality of life. Where or how can you educe the impacts of stress in your dairy herd?
Would you like your farm to be assessed? Please contact our dairy technical specialist.
