Lesson 3: The Concept of Stress in Dairy Cattle

The general concept of low-stress handling is being widely discussed in the dairy industry today. The National Dairy FARM Program: Farmers Assuring Responsible Management, created by the National Milk Producers Federation (NMPF) with support from Dairy Management Inc. (DMI), specifically states as a best management practice “Employees should be properly trained to handle animals with a minimum of stress to the animal, and the consequences of inhumane han¬dling should be known and enforced.” The National Dairy FARM Program is designed to demonstrate that U.S. milk producers are committed to providing the highest standards of animal care and quality assurance. This voluntary program, available to all producers, provides a consistent on-farm animal well-being program that includes education, on-farm evaluations and third-party verification. Whether it be dairy stockmanship training or a program like FARM, the increased usage of the term “low-stress cattle handling techniques” has raised the questions of what exactly is stress, and how do we determine if it is “low” or “high”?

If you ask twelve people to define “stress” you would likely get 12 different answers. This creates an interesting challenge for us if we are going to attempt to determine the level of animal stress on a particular farm and whether the stress level is “low” or “high”. If we struggle to define stress, how can we measure it? One of the goals of this lesson is to introduce the reader to the scientific study of stress biology and to suggest that farm managers and advisors can utilize this understanding to assist in the evaluation whether cow handling stress is “low” or “high” on a dairy operation. A brief history of stress research pioneers will be helpful to understand how the term came into such widespread use. Hans Seyle (1907-1982) is generally recognized for being the first researcher to demonstrate the existence of biological stress. In 1936 Seyle defined stress as "the non-specific response of the body to any demand for change." Seyle demonstrated in his research that a wide variety of noxious stimuli caused a very consistent set of pathologic changes in laboratory rats. Seyle’s work created much interest and discussion in the scientific community. The work of Robert Sapolsky is also useful in understanding the concept of biological stress. Sapolsky suggests a very useful approach by differentiating a “stressor” from the body’s “stress response”. Sapolsky defined a stressor as anything that disrupts physiological balance. A stress response is defined as the body’s adaptations designed to re-establish the balance.

Discussions at the 2011 Trends in Stress Biology course taught at Aarhus University suggested some slight refinements to the definitions.
• Stressor = event threatening or potentially threatening the homeostatic balance

• Stress Response = the bodies attempt to re-establish the homeostasis after encountering a stressor

Stressor’s can be described by their characteristics such as: duration, frequency, intensity, predictability, and ability to be controlled. It is important to note that while stressors can be physical things (heat, cold, starvation, etc.) psychological factors can also trigger the stress response in an animal in the absence of anything physically threatening to an animal. Sapolsky in his writings proposes that the stress response evolved as adaptive survival mechanism for animals. It is now increasingly recognized that the consequences of the stress response can be maladaptive and that there is a “biological cost” to the animal for mounting a stress response. It is actually incorrect to state that stress makes an animal sick. To be correct, one should state that the stress response makes you more likely to get diseases that make you sick.

There is no single litmus test for stress because of the multiple ways the body responds to stressors. Since stressors will result in both behavioral responses and physiological responses on the part of the animal proper assessment of an animal’s stress response requires one look both. One cannot interpret physiological test results without knowing the behavior. An understanding of stockmanship principles will help one to be aware of behavior responses in animals. The physiological components of the stress response are significantly influenced by the endocrine system. Broadly speaking, all stressors provoke some degree of cortisol secretion as well as a multitude of other physiologic responses. The exact orchestration the many hormones involved will vary depending on the stressor. In this way, different stressors have a different “stress signature” that describes the overall stress response. Work in this area is very interesting and in the future will most certainly allow us to improve and refine our evaluation of the physiological response to stress.

It is still our present understanding that glucocorticoids (cortisol) and catecholamine’s (adrenalin) together mediate most of the changes that form the stress response. Today, measuring cortisol remains the gold standard to evaluate the physiologic response to stressors. Researchers are actively engaged in searching for additional physiologic measures, but it is clear that cortisol does play an important role. Understanding the Hypothalamic-Pituitary-Adrenal axis (HPA axis) is critical to understanding the physiology of the stress response.

Blood sampling has been the traditional measure used to evaluate the cortisol level in an animal. However, plasma cortisol evaluation is not without issues. For example, obtaining a blood sample in itself can be stressful, especially in wildlife or zoo animals. Dr. Rupert Palme (Dept. Biomed. Sciences/Biochemistry,University of Veterinary Medicine, Vienna) and other researchers have been actively looking into alternatives to blood sampling. Cortisol is metabolized in the liver and cortisol metabolites are excreted in the urine and feces. Measuring cortisol metabolites in the feces (FCM’s) has received a significant amount of attention. Since 1997, over 130 publications have used the measurement of FCM’s on a wide variety of animal species, including dairy cattle.

In 2011, the University of Minnesota Veterinary Diagnostic Lab (VDL) completed a validation study using a commercially available Radio-Immuno-Assay (MP Biomedicals, Diagnostic Division, 13 Mountain View Avenue, Orangeburg, NY 109062) to measure FCM’s in bovine feces at our VDL. We anticipate the ability to measure FCM’s will be an important additional tool complementing behavior analysis study of dairy cattle. Measuring FCM’s hopefully will assist dairy research projects that are designed to evaluate whether a particular handling technique can be considered low-stress animal handling.