Stress plays a major part in the development of respiratory disease, as well as an important role in herd health for the period of time right after calves arrive at the feedlot. While stress is an inevitable part of this early feeding period, proper handling and management will result in lower morbidity, mortality and medical costs.
Calves will start on feed quicker and gain better, which means better carcass quality and fewer days on feed. And low-stress handling means higher real-dollar returns on your investment. Here's an overview of how psychological stress affects animals, as well as a few tips on how to minimize its negative effects.
Three types of stress
Defining and measuring stress is difficult, because what's stressful to one individual may be a positive experience for another. But the broadest way to define stress is to describe the changes that take place within an animal in response to a stimulus - for example, changes in heart rate, blood pressure, cell function or hormone levels.
Stressful events occur in three general forms:
Stressful events of short duration and moderate intensity - branding or tagging, for example - elicit short-term adrenal responses that generally don't affect long-term health. In such a “sympatho-adrenal response,” the adrenal hormones epinephrine and norepinephrine are released along with signals from the nervous system that increase blood pressure and heart rate and divert blood and energy to help the animal “fight” or “flee.”
Stressors that last long enough and are large enough induce a secondary stress response in addition to the sympatho-adrenal response. The effects of such acute stress or distress are more long term.
The condition is characterized by increased release of glucocorticoid hormones from the adrenal gland. Normally, glucocorticoids circulate throughout the body at low levels and play an important role in energy metabolism. Abnormal elevation of glucocorticoids changes these dynamics.
In stressful situations, increased glucocorticoid concentrations help mobilize energy stores to sustain a fight for survival, but also negatively affect the neuro-chemical pathways that regulate the immune, digestive and reproductive systems. This complex interaction of hormones in the hypothalamic pituitary region causes a decrease in the production of reproductive and growth hormones.
The trickle-down effects are a disruption of reproductive function and metabolic changes that cause anorexia and catabolization or breakdown of protein. Heart rate and metabolism increase, and energy used for things like immune function, reproduction, digestion and growth is redirected to the muscular system to help the animal fight or flee what it sees as dangerous.
Research shows that stress suppresses the immune system by inhibiting the maturation, activation and function of most types of cells in the immune system. This negatively affects cattle two ways. They're more susceptible to disease for a period of time after a stressful event, and vaccines administered to stressed cattle might not produce a reaction large enough to secure immunity.
The result is that for about a week after a stressful event, disease resistance is lower, food intake and protein accrual are reduced and reproductive cycles may be disrupted.
Chronic stress is an extension of acute stress; the same mechanisms control both types of stress, but chronic stress is constant.
An example of chronic stress is a “buller” steer that is selected and ridden constantly by its penmates. Being constantly under inescapable physical and psychological stress can overload the adrenal and other organ systems to exhaustion. This makes the animal more susceptible to disease, and unable to carry out the metabolic processes necessary for survival.
If their environment isn't changed to alleviate the source of stress, chronically stressed cattle will lose weight and often die from sickness or exhaustion.
Shipping fever, or bovine respiratory disease (BRD), is the number-one cause of morbidity and mortality in feedlot cattle in the first 45 days on feed. Largely caused by two bacteria, Mannheimia Haemolytica and Histophilus Somnus, these bacteria are normally present in the feedlot environment, but a healthy immune system is generally strong enough to prevent infection.
In times of acute stress, such as transportation or extensive handling, however, the immune system is suppressed, giving these opportunistic bacteria an opening.
The most stressed cattle in the industry are calves weaned and transported to the auction, then transported again after marketing. Each event is stressful by itself; combined with exposure to several new diseases, they're a surefire recipe for BRD.
As many as 40% of such cattle arriving at a feedlot require subsequent treatment for BRD, making mass administration of antibiotics common upon feedlot arrival. While this aids the cattle in fighting disease during the period of immune suppression following auction and transport, such mass medication is becoming increasingly unpopular because of its cost and consumer fears of developing antibiotic-resistant bacteria. This is leading buyers to look for ways to minimize stress before cattle get to the feedlot so that mass medication isn't necessary.
Handling and transporting cattle can't be avoided, but the stressors can be managed to minimize their impacts. The goal in good cattle handling is to keep cattle's stress levels below the threshold of having negative affects. Here are four principles:
Be prepared. Get your crew organized and ensure sufficient help so the amount of time each animal spends in a stressful situation is minimized.
Be slow, quiet and deliberate. Cattle are prey animals; they equate quick movement with a personal attack. Moving slowly and quietly when sorting minimizes a stressful event. If you can get cattle to the chute without stressing them, what happens in the chute will probably not last long enough to cause long-term problems.
Facility design is important in low-stress handling. Designed to work with cattle's natural instincts, such facilities allow handlers to process cattle with minimal pressure.
Use electric cattle prods sparingly. If such prods are used more than 25% of the time, you need to reevaluate your facilites and handling procedures.
Create a safe place. Allow cattle time to quiet down with access to feed, and especially water, after a stressful event. This can short circuit any stress response that began during handling and limit the negative effects the stress might have caused.
Boone Carter, PAS, is a research assistant in animal science and behavior at Texas A&M University.