Insights into Modern Dairy Farming Through ACTH Stimulation Tests and Heart Rate Variability Analysis
When a cow in the milking parlor restlessly shifts from one leg to another, can hardly stand still, and refuses to give milk, she is sending clear signals of stress and discomfort. In modern dairy farming, cows are exposed to numerous stressors - from cramped housing conditions to regular human interventions and painful diseases such as lameness.
Used to verify the hormonal response to stress in dairy cows.
Provides insights into the autonomic nervous system activity.
These methods are revolutionizing our understanding of dairy cow welfare and showing that stress responses are as individual as the personality of each cow 1 6 .
A cow's endocrine system has a sophisticated mechanism for coping with stressful situations: the hypothalamic-pituitary-adrenal (HPA) axis. When perceiving a stressful situation, the body releases ACTH (adrenocorticotropic hormone), which in turn stimulates the adrenal cortex to produce cortisol - the primary stress hormone 1 4 .
The ACTH stimulation test allows researchers to directly measure the reactivity of this HPA axis. Synthetic ACTH is injected and the cortisol response is tracked over a period of several hours. Interestingly, cows show high individual variability - some animals respond with a strong cortisol increase, others only with a mild reaction 1 .
While cortisol measurements show the hormonal stress response, heart rate variability (HRV) provides insights into the activity of the autonomic nervous system. HRV measures the subtle fluctuations in the time interval between individual heartbeats. High variability signals a balanced balance between the sympathetic (activation nerve) and parasympathetic (recovery nerve), while low variability indicates chronic stress 5 .
Researchers today use special heart rate monitors such as the Polar® Equine RS800CX, originally developed for horses but also providing reliable results in cows. The measured RR intervals (distances between heartbeats) are evaluated with specialized software such as Kubios HRV Analysis 1 5 .
In a groundbreaking study, scientists examined the stress response of 14 healthy, lactating dairy cows during functional claw trimming - a routine procedure that involves various stress factors for the animals: isolation from the herd, close human-animal contact, unfamiliar environment and manipulations on the body 1 .
Validation of the Polar® heart monitor by comparison with a professional ECG system (Televet® 100) - with excellent agreement (r = 0.99, p < 0.0001) 1
At each time point, blood samples were taken to determine cortisol, glucose and non-esterified fatty acids (NEFA), and simultaneously heart rate and HRV were measured 1 .
Eight weeks later, an ACTH stimulation test was performed on the same cows to characterize the reactivity of their HPA axes 1 .
During claw trimming, cortisol, glucose and NEFA increased significantly as did heart rate. However, there was high individual variation in the decline of these parameters after the procedure - some cows calmed down quickly, others remained stressed for a long time 1 .
| Parameter from ACTH Test | Correlated Parameter During Claw Trimming | Correlation Direction |
|---|---|---|
| Cortisol response | Cortisol response to claw trimming | Positive |
| Cortisol response | NEFA concentration at time T4 | Negative |
| Cortisol response | Difference T4–T3 of RR intervals | Positive |
| Cortisol response | Difference T4–T3 of SDNN | Negative |
These results demonstrate that the responsiveness of the HPA axis and the stress response to an acute stressor vary considerably from animal to animal, but are closely related to each other. Cows with high HPA axis responsiveness had a better ability to process the acute stressor, which was particularly evident in their recovery after the stressful situation ended 1 .
| Time Point | Cortisol (ng/ml) | Glucose (mg/dl) | NEFA (mmol/l) | Heart Rate (beats/min) | SDNN (ms) |
|---|---|---|---|---|---|
| T1 (Baseline) | 5.2 | 68.5 | 0.12 | 65.4 | 142.6 |
| T2 (Tilt Table) | 8.7 | 72.1 | 0.18 | 78.9 | 118.3 |
| T3 (End CT) | 12.4 | 75.6 | 0.23 | 85.7 | 95.8 |
| T4 (Recovery) | 7.9 | 70.3 | 0.16 | 71.2 | 129.4 |
Another study examined the effects of chronic stress from lameness - one of the biggest welfare problems in dairy farming. The researchers compared 51 lame with 52 non-lame cows and found significant differences in heart rate variability:
| Parameter | Non-lame Cows | Lame Cows | Significance |
|---|---|---|---|
| Heart Rate | Higher | Lower | Less activity with lameness |
| Vagal Tone | Normal | Increased | Enhanced parasympathetic activity |
| Sympathovagal Balance | Normal | Reduced | Less sympathetic activity |
| Geometric HRV Measures | Normal | Reduced | Impaired regulatory capacity |
| Fecal Glucocorticoids | Normal | Unchanged | No acute stress response |
Interestingly, lame cows showed no elevated glucocorticoid concentrations in feces, suggesting that chronic stress manifests differently than acute stress. The altered HRV with preserved cortisol concentration underscores the complementarity of both measurement methods for comprehensive stress monitoring 5 .
Modern stress research in dairy cows relies on a range of specialized methods and reagents:
The research results have immediate practical relevance for dairy farming. They show that individual differences in stress reactivity must be considered to optimize animal welfare 6 . Cows with high HPA reactivity appear to be better able to process acute stressors - an indication that stress resilience may possibly be specifically promoted 1 .
Initiatives to reduce stress in the milking parlor are gaining increasing importance. Particularly first-calving cows often show great restlessness, which is neither pleasant for the animals nor for the farmers. Through special handling methods, such as techniques based on the TTouch method, the milking atmosphere can be improved and udder health optimized 3 .
The combined application of ACTH stimulation tests and HRV analyses opens up new perspectives for individual stress assessment and the breeding of more stress-resilient herds. Since a cow's personality is reflected in its physiological stress response 6 , these methods could help to optimize housing conditions and sustainably improve animal welfare.
The challenge for the future will be to translate these scientific findings into practical management strategies that take into account both the needs of the animals and the economic requirements of the farms. Because a relaxed cow is not only a sign of animal welfare, but also the basis for sustainable milk production.