1. Introduction
1.1. Stomach development
During the first weeks of life, calves are functionally monogastric and milk is the primary source of nutrition. Upon drinking milk, the oesophageal (reticular) groove (sulcus retículi) is activated and the milk is shunted directly past the forestomachs to the abomasum [1]. A number of factors trigger this oesophageal reflex, including sucking behaviour, warm milk, the position of the calf's head while drinking [1] and familiarity with the feeding method [2].
For newborn and very young calves, milk in the forestomachs is usually unproblematic. The rumen, along with the reticulum and omasum, is not yet developed and empties into the abomasum within hours [3]. For calves with ruminal development (2–3 weeks and older) [4], large quantities of milk in the rumen may pose a problem. The lactose is converted to lactic acid and other organic acids, or the milk proteins may rot, which may lead to a change in pH, subsequently affecting the rumen microflora causing indigestion, diarrhoea and reduced growth [1].
Milk is thought to enter the rumen in one of two ways. The first is through insufficient closure of the oesophageal groove [5]. The second results from overfeeding fluids beyond the capacity of the abomasum, causing backflow into the reticulorumen [5], [6] and [7]. While the scientific origin remains unclear, it is widely believed that the capacity of the abomasum is about 2 l, and that milk meal sizes beyond this volume will cause milk to enter the rumen. In a study from 2012, Flor et al. also calculated the abomasal volume to be less than 2 l based on computed tomography (CT) scans [8]. They did not, however, report the size of the offered milk meal or actual intake by the calf.