Many stressors occur in the first few months of a calf's life. Just being born is a huge stress, and we have limited control over that one.
Other stressors, however, are the result of management decisions. Too many stressors can be too large and occur too close to each other. Better management has the potential to reduce these stresses and, certainly, spread them out over time.
Why is stress undesirable? "When a calf is stressed the brain signals the adrenal glands to begin making a steroid hormone, cortisol,” according to David Hale, DVM. “Five of the main effects of cortisol are: (1) blood pressure increases; (2) strength of heart muscle contraction increases; (3) blood is diverted from peripheral organs to vital ones; (4) blood sugar goes up and sugar used by body cells goes down; and (5) acute reactions of tissue cells to trauma and/or toxins is prevented or inhibited.
Usually these effects of cortisol are positive. That is, they are critical for survival in life- threatening situations, but there is a downside to cortisol. It decreases the body defense mechanisms in at least four ways:
Cortisol decreases white blood cell movement to infection sites. It also decreases destruction of foreign material by body cells (phagocytosis).
Cortisol decreases interferon production (interferon is the body's alarm system for viral infections).
Cortisol decreases production of two types of white blood cells (eosinophils and lymphocytes).
Cortisol decreases antibody production.
All this adds up to the body turning off the immune system defenses in an attempt to survive crisis situations. On one hand, this may increase short-term survival. On the other hand, the result is often an overwhelming infection and death. Reducing stresses and spreading them out are critical to success.
Environment: Overcrowded resting areas, too little bunk space, inadequate bedding and poor air quality all add to stressful conditions for prefresh cows. The stress induced cortisol released in these cows will negatively affect the calf even before birth.
Pathogen load in the calving area: The cleaner the calving area, the lower the chances of adult cow manure getting into a newborn's mouth. As soon as a calf can stand up is the time to get her into a clean place. Some dairies have facilities in which the mother can lick the calf but the calf cannot reach the dam's contaminated hair coat.
Adequate nutrition: All calves should have enough to eat to meet not only their maintenance needs but also to grow. If dry matter intake from milk, milk replacer and/or calf starter grain falls below maintenance levels, the calf is under a lot of stress. Most often, this occurs in below-freezing weather.
Transportation: Loading calves on and off a stock trailer is a stressful event. Even moves of less than ten minutes still involve the "on-off" event. Best management suggests that we plan housing to minimize the number of these events.
Weaning calves: Weaning-induced stress may result in cortisol release for as long as a week. Look for reduced rates of gain for the week post-weaning. Alternatively, we may see pneumonia in calves five to seven days after weaning.
Housing: Just moving from individual to group housing is stressful. This change may be separated from a move to a new barn by having pens that convert from individual to group size. In addition, every subsequent group change will add stress among transitioning heifers.
Management tasks: Dehorning and vaccinating both add more stresses, although for a relatively short time.
Stacking stressors: Alone, each of the listed stressors is a threat to calf health and growth. When stressful events happen at the same time, the risk is multiplied for respiratory illness and lower growth rates.
Adapted from Calving Ease, April 2006, Sam Leadley, Attica Veterinary Associates