Cattle are members of the Ruminantia, which represent the most successful group of extant large herbivores in terms of species diversity. Ruminants have a multi-chambered forestomach, similar to other foregut fermenters like kangaroo, hippos, peccaries or sloths (Langer 1988); this means that plant material is partly digested by symbiotic microbes before the whole digesta – which includes the partly digested diet and the microbes – is passed on to the lower digestive tract where the herbivore’s own enzymes further digest this mixture. Because microbial protein is a major component of this mixture, foregut fermenters produce a set of specific enzymes in their glandular stomach and small intestine that help break down microbial cells, so that their protein can be used (Pacheco et al. 2007). Functional ruminants – the phylogenetic ruminants as well as the camelids – combine simple foregut fermentation with peculiar sorting mechanisms that assure that larger digesta particles are regurgitated and re-masticated (ruminated). This process of rumination is an obligatory physiological feature, facilitates a more efficient particle size reduction (Fritz et al. 2009), higher digestive efficiencies (Foose 1982) and potentially also higher food intake levels than observed in non-ruminant foregut fermenters (Clauss et al. 2010a).
Ruminant digestive anatomy and physiology
The ruminant stomach consists of four compartments – three representing the forestomach complex, and the last representing the glandular stomach (‘abomasum’), the equivalent of the stomach of monogastric animals (Hofmann & Schnorr 1982). The three forestomach compartments are, in the sequence of the digestive process, the rumen, the reticulum and the omasum (Figure 6.1). From the outside, the rumen and the reticulum form a unit – a large fermentation chamber with several sub-compartments, including the dorsal and the ventral rumen, the dorsal and ventral rumen blindsacs, the atrium ruminis and the reticulum. The whole complex is often referred to as the reticulorumen (RR). The reticulum is the most cranial part of the RR. On the right side of the RR, the omasum is a distinct structure. In contrast to the RR, which has a consistency of the digesta it contains, the omasum is more solid to the touch, and ball- or bean-shaped. The omasum leads to the abomasum, which in turn leads to the small and then the large intestine.