INTRODUCTION

A Google Search for the term “stress” will bring up a variety of articles. If these publications are true, a lot of people are feeling stressed, including: couples planning weddings, couples getting divorced, school teachers, retirees, poor people, wealthy people, employed and unemployed people, and many others! When it comes to people, stress is a hot topic and everyone is looking for ways to manage stress they experience. Even though articles on stress are commonplace, most, surprisingly, do not define stress. Scientists who study stress tell us it is “an organism’s response to a perceived threat” and they refer to the threat as a stressor. For example, if you hit your thumb with a hammer, the pain you feel is the stressor. The complex response to that pain (including elevated heart rate, swelling, and later, increased susceptibility to infections) is the stress. So if we want to manage the stress in our lives we need to consider both the stressor (the threat) and the stress (the body’s response to the stressor). The stress response is similar for humans and animals, including dairy cattle. Stressors can be physical (for example: a back injury in a farmer; high temperature-humidity index in dairy cows) or psychological (learning of a new, lower milk price by a farmer; moving to a new pen for a dairy cow).

The stress response is the same for all stressors; specific portions of the brain are stimulated leading to secretion of hormones: catecholamines (a familiar example is epinephrine and is partly responsible for the “fight or flight” response) and cortisol (the “stress hormone”) from the adrenal glands. Cortisol acts to help protect the animal from the stressor. For example, cortisol increases blood glucose to be used for energy and suppresses the immune system to spare nutrients and prevent excessive immune activity. The extent of the stress response differs depending upon the stressor, and there can be significant variation between individuals in the degree of stress experienced.

Short-term exposure to a stressor causes acute stress, which leads to a sharp increase in cortisol secretion. Excess cortisol during acute stress results in immunosuppression (reduction in the activity of the immune system), which can allow infections and diseases to develop or worsen. The longterm presence of a stressor causes chronic stress, which is often accompanied by reduced cortisol levels in the blood and/ or reduced responsiveness of cells to cortisol, which may increase the risk of chronic inflammation. Inflammation is a complex immune response intended to protect the body from infection and/or tissue damage. Typical symptoms of inflammation are redness, swelling, heat and pain, as seen when a human has a splinter, or a cow has mastitis. Inflammation occurs both at the site of the infection (local) and throughout the entire body (systemic). Acute inflammation is beneficial, and needed to overcome infections. Chronic, systemic inflammation is often harmful. In humans, it may lead to heart disease, Alzheimer’s disease, arthritis, obesity, depression, and many other disorders. In dairy cows, chronic, systemic inflammation increases the risk of infectious and metabolic diseases (mastitis, metritis, and others), impaired reproduction, and lower milk production.

COMMON STRESSORS IN DAIRY COWS

Calving and Dry Off The acts of giving birth (calving) and stopping milk production (dry-off) cause stress in the dairy cow. However, these events are a normal part of the lactation cycle and must occur for cows to reach their milk production potential. They are examples of acute stress.

Around the time of calving, cows experience a cascade of hormonal, metabolic, nutritional, and environmental changes that challenge the immediate health and performance of dairy cows. At calving, an increase in cortisol concentration, a decrease in serum calcium levels and a decrease in feed intake contribute to immunosuppression. At dry off, cows experience an engorgement of the udder with milk that can cause teat end leakage and increase the cow’s risk of mastitis. At the same time, cows are placed in new social groups and are subjected to dietary changes. At dry off, cows increase their production of cortisol and experience increased somatic cell counts (SCC) in their milk. Both dry off and calving result in reduced immune function and a greater risk of diseases.

Molds and Mycotoxins

Molds can affect cattle in two ways. The first way is called mycosis. Mycosis is a condition where mold invades and colonizes tissues causing disease. The second way molds can affect health is through the production of mycotoxins. Mycotoxins can negatively affect the immune system and other body processes of dairy cows. This condition is called mycotoxicosis. Rations fed to cattle generally consist of high moisture feeds with nutrients that can support the growth of molds. In addition, some crops may contain high levels of molds and/or mycotoxins at harvest. The impacts of molds and mycotoxins on the cow include impaired immunity, digestive upsets, reduced milk production, and poor reproduction.

Feeding Management

Feeding management encompasses all the feed and feeding-related tasks necessary to promote health, performance, and well-being in the dairy cow. These include the selection of feed ingredients, mixing and delivery of rations, and maintenance of feed bunks and water troughs. A lack of attention to feeding management practices can result in sorting, digestive upsets, cyclic eating patterns, and over- or under-conditioned cows, all potentially leading to or causing subacute rumen acidosis (SARA). The impacts of poor feeding management include compromised immunity, reduced milk fat and protein yield, and poor hoof health.

Sanitation

Sanitation is the cleanliness or hygiene of cattle and their environment. When poor sanitation is present, mastitis is likely. Mastitis is inflammation of the mammary gland that develops in response to pathogens entering through the teat canal and multiplying inside the gland. Infection of the mammary gland causes an increase in milk SCC. Poor hygiene increases the risk of mastitis and elevated SCC.

Social Stress

Social stress stems from a cow’s relationship with her herd mates and encompasses all the social factors that impact cow behavior, health, performance and overall well-being. Common social stressors faced by dairy cattle include pen moves, overstocking, co-mingling of young and mature cows, and limited feeding space. The negative impacts of social stress include impaired immunity, increased SCC, and lower milk production.

Comfort

Poor cow comfort is a leading welfare concern in the dairy industry and can have significant negative impacts on milk production, cow health, and overall well-being. Many factors influence cow comfort, including management, housing, flooring, and bedding. Physical signs that cows are experiencing poor cow comfort include lameness and increased hock lesions. Poor cow comfort leads to compromised immunity, lower production and increased SCC.

High Temperature-Humidity Index (THI)

Cows start to experience heat stress when THI reaches 68. At this threshold, uncooled dairy cows will have elevated body temperatures and respiration rates, as well as begin to produce less milk and have poorer reproduction. Recent research suggests acute exposure to high THI damages cells of the cow’s digestive tract, allowing pathogens and toxins to enter the blood stream. Cows exposed to high THI have impaired immune function, lower milk production, lower milk fat and protein percentages, reduced dry matter intake, poorer reproductive efficiency, shorter pregnancy lengths, and lower calf birth weights.

Ventilation

All animals require a constant supply of clean, fresh air. Adequate ventilation removes excess heat, moisture, dust, microbes, and gases that accompany housed animals. Poor ventilation is identified by observing condensation on curtains, ceilings and support structures, and increased moisture and ammonia concentrations. Poor air quality can result in immune suppression and an increased incidence of pneumonia.

HANDLING

Proper handling is often called good stockmanship. Improper (or rough) handling of dairy cattle is a psychological stressor which is characterized by fear resulting from new or unfamiliar events, pain, excessive noise, touch, and visual experiences. Improper handling has been shown to increase cows’ fear of humans, while gentle handling can reduce fear. Improper handling can result in both acute and chronic stress which may result in reduced milk production.

TRANSPORTATION

The majority of research on the impact of transportation on cattle has been conducted with beef cattle. These studies have shown that hauling cattle long distances can reduce dry matter intake and average daily gains. Some studies have also shown that cattle hauled long distances have suppressed immune function that may increase their risk of disease. We can speculate that hauling dairy cows, calves, or springing heifers long distances may also result in reduced immune function.

CONCLUSIONS

Managing stress in dairy cows results in healthier and more productive animals. Stress in dairy cows is caused by stressors that can’t be avoided (calving and dry off) and also by stressors that can be managed (feeding management, molds and mycotoxins, sanitation, social stress, cow comfort, high THI, ventilation, handling, and transportation.) These stressors affect immune function, health, and production, and their economic effects can be significant. For this reason, assessing the level of stress on your farm and developing a plan to manage stress can have a significant pay-off. Which stressors are your cows facing? What level of stress are your cows experiencing? What plan do you have in place to manage your cows’ stress?