What is stress?
There are many perceptions about what ‘stress’ is and we mainly think about it in human terms and consider ‘stress’ to be bad for our health. But from a physiological and evolutionary perspective, stress has a different context and can be good and bad!
Stress is a physiological state in which the body responds (behaviourally and/or physiologically) to a challenge (a ‘stressor’) that can threaten the well-being of the animal.
The outcome of a physiological stress response largely depends on how the situation is perceived by the animal. A physiological stress response can be perceived as ‘positive’ (e.g. during mating) and as ‘negative’ (e.g. when attacked by a predator). 

Positive stress
A state of physiological stress is absolutely necessary for survival in many ‘life-or-death’ (‘acute stress’) situations: it provides the animal with the opportunity to adapt to the situation to avoid more severe consequences of the challenge.  For example, it allows it to react more quickly and to run faster, or deal with an injury. Therefore, stress in this context is good.

Negative stress
Animal welfare has historically largely focussed on what we would call negative stress, such as pain, fear or hunger, that arise in situations that farmed deer have little control over.
When a deer cannot reduce the impact of a stressor or make it go away, stress becomes a chronic state which is detrimental to the animal and will threaten its health and well-being. For example, constant nervousness due to the prolonged presence of predators can prevent deer from feeding properly and thus, through poor nutrition, affect well-being and consequently production. However, chronic stress can have more direct effects on animal welfare.  For instance, prolonged exposure to chronic stress and hormones secreted  primarily by the adrenal gland can interfere with other physiological processes in the animal. For example, adrenal corticosteroids secreted over long periods of time can interfere with ovary function and disrupt reproduction.

What does the adrenal gland do?
The adrenal gland produces a number of hormones that elicit various ‘heightened’  or ‘aroused’ states in the animal.

With acute stressors that require a rapid ‘flight or fight’ response (e.g. sudden attack by a predator), a burst of adrenaline is produced by the adrenal gland that seemingly gives the animal miraculous physical powers. This rapidly-acting hormone affects many aspects of the animal’s physiology involved in reaction speed, aggression and muscle power, to name a few. However, the effects are short-lasting as adrenaline is rapidly cleared from the body. Once the acute stressor is gone, the animal quickly returns to a normal physiological state.

Where stressors are present on a chronic basis (e.g. continual presence of predators that necessitates the animal to be in a constant state of vigilance, or as a response to an injury), the deer needs a longer-term shift in its response to them.  In such a state, the adrenal gland secretes longer-acting hormones in the corticosteroid family (e.g. cortisol). Corticosterioids regulate many aspects of physiology, such as reproduction and immune function in the absence of stressors, in a moderate, sustained way. However, when they are continually secreted at high levels as a response to chronic stress, they can shut-down or dampen those normal biological functions.

Stressors facing farmed deer
Farmed deer face several potential stressful situations due to the nature of farming. Some will be more severe than others, and some can be avoided or minimised. For example, stressful periods in a farmed deer’s life may include yarding, transport, unfamiliar humans or dogs, regrouping, weaning, and inclement weather. If animals are given the opportunity to adapt by expressing their natural behaviour, they will cope with the situation and no harm is done. However, it is in the farmer’s interest to minimise stress in deer as a stressed deer is likely to have lower production and can be a threat to humans and other deer depending on the situation where the stress occurs. If animals are able to predict or control a situation, stress levels are in general lower.

How does stress affect production?
We know that chronic stress impacts on production, but it is hard to accurately measure in most cases. The effects can include poor reproductive performance, low growth rates due to reduced feed intake, and greater incidence of disease due to suppression of the immune system. It is also often associated with an increased incidence of fatigue and impact injuries.

Becoming familiar with normal behaviour of deer is important in order to observe when deer are in a stressed state. Behaviours that indicate stress, include fence-pacing, excessive and prolonged panting, aggressions, general nervousness and frequent vocalisations. If these signs continue for a long time, the animals may be in chronic stress and physical signs can include noticeable weight loss and hair loss.

How do I mitigate these effects?
Clearly, it is in the farmer’s interest to minimise stress on the farm. One of the most common stressors encountered on farms is under-feeding. Hungry animals are stressed and exhibit various behaviours associated with chronic stress (e.g. fence pacing). Good nutrition is arguably the cornerstone to happy deer.  Preventing other types of stress is easier than dealing with the effect of stress.  See advice on managing nutritional stress risk.

Nevertheless, if animals are showing signs of chronic stress, something is wrong within their environment and their welfare is most likely impaired. It is important to identify the source of the problem and remove it. It could be something as simple as the continual close presence of a perceived predator (e.g. the farm dog) that constantly disturbs them. Consider also their social environment: are individuals failing to cope with dominant or disruptive individuals?  If so, consider mobbing disruptive individuals differently.