For many years, the rearing of healthy, vital calves for the future dairy herd has been the major focus for farmers, advisors, and researchers. Evidence of the effects of calf feeding, particularly the concept of metabolic programming (Soberon et al., 2011; reviewed by Hammon et al., 2020), on animal productivity later in life provided the impetus for a more detailed examination of the methods of calf rearing applied in current practice on farms and the potential for improvement. Furthermore, efficient rearing (referring to fast growth without becoming diseased), was seen as an option to reduce the age at first calving and thus to increase economic sustainability, although longevity is not necessarily correlated to a low age at first calving (Krpálková et al., 2014). However, ad libitum liquid feeding, thereby providing milk-fed calves with adequate nutrition, has been demonstrated to improve their development and resilience (e.g. Curtis et al., 2018).