# Effect of an Ad Libitum Milk Supply During the First Three Weeks of Life of Dairy Calves on Heart Rate and Heart Rate Variability During Feeding and Rehousing

## TL;DR

This study found that calves given unlimited milk had different heart rate responses compared to those with restricted feeding, suggesting better stress coping.

## Contribution

The study provides new insights into how early-life milk feeding strategies affect autonomic regulation and stress responses in calves.

## Key findings

- Calves with ad libitum milk had higher resting heart rates but similar peak responses during feeding and rehousing.
- Restrictive feeding led to greater autonomic stress responses during feeding.
- Ad libitum calves showed increased vagal activity during rehousing, suggesting better physiological coping.

## Abstract

This study investigated the effect of different milk feeding strategies on the physiological stress response in young calves. Half of the calves were fed a restricted amount of milk twice daily (RES group), while the other half received an unlimited supply (ADL group) during their first three weeks of life. Heart rate (HR) and heart rate variability (HRV) were measured as indicators of stress during feeding and during the rehousing process. The findings suggest that calves on a restricted feeding schedule may exhibit a more pronounced cardiac stress response at feeding time, potentially due to hunger. While both groups showed an expected increase in HR during the stressful event of rehousing, some HRV parameters indicated that calves with an unlimited milk supply may have had a different physiological coping mechanism. These results imply that a more abundant milk diet in early life may influence how calves handle physiological challenges. Further research is needed to fully understand these effects.

Early-life feeding strategies are known to affect growth, behavior, and stress physiology in dairy calves. This study examined the effects of different milk feeding regimes on heart rate (HR) and heart rate variability (HRV) during feeding and rehousing as indicators of autonomic activity. Dairy calves were fed either a restrictive milk allowance twice per day (6 L/d; RES; n = 21) or an unlimited amount of milk (ad libitum; ADL; n = 24) during the first three weeks of life. All calves were housed in individual straw bedded hutches from d 1 to 23 of life and were moved to a group pen on d 23 ± 2 of life. Starting at least one day before rehousing until one hour after the rehousing process HR, HRV, and variables in the time and frequency domain were measured continuously using a portable recording system. To study the cardiac response to the feeding process, six time windows of 5 min each were chosen as follows: resting time at 5.00 a.m., start of personnel activity in the barn, 15 min before feeding, during feeding, 15 min after feeding, and 1 h after feeding. For the evaluation of cardiac response to an unknown stressor such as rehousing, four time windows of 5 min each were selected as follows: resting time at 5.00 a.m., during rehousing, 30 min after rehousing, and 1 h after rehousing. During resting as well as before feeding and rehousing, HR was higher in ADL calves compared with RES calves. During feeding and rehousing, HR reached peak values which were comparable in both groups. HRV variables of the time and frequency domain indicated a shift towards a sympathetic dominance in the balance of the autonomic nervous system during feeding time, particularly in RES calves. Differences between resting and feeding values were demonstrated in RES calves at low-frequency and high-frequency power, whereas no differences were observed in ADL calves which did not react to the feeding process. The cardiac response of calves to rehousing was inconsistent in both groups. An increase in RMSSD and SD1 in ADL calves indicated that the vagal component in the vegetative neurological control was increased in these calves during rehousing. In conclusion, our findings indicate that restrictive milk feeding alters autonomic regulation and may increase physiological stress responses in calves.