How does stress affect our horses? [H&H VIP]

  • Research into the way our horses think and behave has the potential to enhance welfare and performance. Gemma Pearson MRCVS explains how the latest findings about stress are relevant to how we handle, train and compete horses

    Stress ­— it’s the buzzword of the modern generation and something most of us aim to beat. But how does it affect our horses?

    One of the functions of stress is to motivate an animal to cope with challenges, so in this respect it can be beneficial. Persistent stress, however, can exhaust the feedback mechanisms that enable the horse to handle such pressure, with detrimental effects.

    Stress may result from external adverse stimulation, or be a consequence of the horse being denied the chance to satisfy an internal drive to carry out certain innate behaviours, such as trickle feeding, socialising with other horses or exercising freely.

    The subject was one of the main themes of the International Society for Equitation Science (ISES) conference in Denmark this summer, where the implications of important new studies into stress were discussed.

    Measuring stress levels
    Stress can be quantified in a variety of ways by measuring aspects of the HPA (hypothalamic -pituitary-adrenocortical axis.

    While heart rate monitoring is a common, cheap and non-invasive method of measuring stress levels, its accuracy is limited as it is influenced by other factors such as exercise. Heart rate variability — whether the heart rhythm remains steady or is constantly altering — more accurately reflects stress levels.

    In one study, this variability was combined with observation of behavioural signs of pain to compare different analgesic (pain relief) protocols in stallions following castration.

    Heart rate variability may become commonly used, as sensors can now be incorporated into “smart” textiles and used in equipment such as saddlecloths.

    Another quantifying method is to measure cortisol, one of the hormones or glucocorticoids released in response to the activation of the HPA axis.

    Traditionally, cortisol levels were measured from blood — yet taking a blood sample may in itself be stressful and could affect results. Scientists are now able to measure cortisol in saliva, urine and faeces samples, which are more easily collected.

    Methods and limitations
    Faecal cortisol helps build a picture of the stress a horse has experienced in the previous 24-48 hours, as the substance takes time to accumulate in faeces.

    While it is a useful tool for comparing stress over longer periods, such as during two different stabling regimes, it cannot tell us about the rapid fluctuations that occur in response to acute stress.

    Urine cortisol more closely monitors fluctuations in blood levels, but, again, represents a pooled (mixed) sample of stress responses since the last time the horse urinated. Cortisol can also be measured from hair, but this is not considered a good indicator of how much stress the horse is actually experiencing because the skin can generate its own local stress response.

    Saliva samples can be taken via a small swab, a procedure generally very well tolerated by horses. They mirror blood cortisol levels accurately, making saliva samples ideal for measuring the rapid responses to different stimulation. But they are less accurate over a longer period of time.

    Frequent samples would need to be taken to account for fluctuating levels, which would be very expensive.

    Core eyeball temperature can be measured with a thermography (heat-sensing) camera and has been shown to correlate to blood cortisol levels. This has the advantage that the measurement can be taken from a distance, without the risk of inducing further stress, but it can be influenced by external factors such as sunlight.

    Scientists generally agree that combining several of these physiological measurements with behavioural observations produces more reliable results than relying on a single measurement does.

    The ISES conference concluded that, although further research is needed in this area, direct observations of a horse’s behaviour may ultimately be one of the most accurate ways of deciding how much stress he is undergoing. With some basic training, monitoring how a horse behaves is a method readily available to every owner.

    Which horses learn faster?
    How horses cope with stress depends on their genetic make-up and previous experience.

    One of the research papers used salivary cortisol measurements from 52 Danish warmbloods to compare how horses react in a show environment. Unsurprisingly, cortisol levels were higher at the show than at home — and had not returned to baseline after four days. Reactivity at home was also measured.

    The more reactive horses tended to have higher cortisol levels at the show, yet cortisol levels did not influence their show performance.

    Another study looked at how stress influences learning. Researchers found that horses undergoing stress had a reduced capacity to learn, unless the source of the stress was the task to be completed — in which case the horses performed better.

    If pressure was applied to the horse, causing a degree of stress, he was more motivated to find a way to remove the pressure and successfully complete the task.

    This may explain why more reactive horses are often considered more talented or intelligent. It may be that they are more motivated to remove any pressure than more laid-back horses, and so learn faster.

    Scratching vs patting
    Scratching horse around the withers has been shown in several studies to lower his heart rate, potentially reducing stress. But patting is associated with a faster heart rate,  which may represent a small fear/flight response.

    A further study with riding school horses compared responses to patting or wither-scratching by a handler on the ground. The horses displayed more ear movement when patted, indicating increased arousal. Yet wither-scratching was more likely to induce head lowering, which is associated with relaxation, and also induced lip twitching and mutual grooming.

    Wither-scratching appears to be a more effective reward than patting, and may boost bonding between horses and their owners.

    This article was first published in Horse & Hound magazine (23 October 2014)