No ‘bypass’ in adult ruminants: Passage of fluid ingested vs. fluid inserted into the rumen in fistulated muskoxen (Ovibos moschatus), reindeer (Rangifer tarandus) and moose (Alces alces)

In young ruminants, the reticular groove ensures that ingested milk is channelled past the forestomach to avoid malfermentation. It has been speculated that some adult wild ruminants, in particular browsing species, maintain a functional oesophageal (reticular) groove, that soluble nutrients can thus bypass the rumen, and that thus the energetic gain from the diet can be increased. We inserted a fluid marker (Co–EDTA) via cannula into the rumen and simultaneously fed a diet that contained a second fluid marker (Sm–EDTA), and analysed the faecal marker excretion patterns, in muskoxen (Ovibos moschatus, n = 4 in two experiments each), reindeer (Rangifer tarandus, n = 4 in a total of six experiments) and moose (Alces alces, n = 1 in one experiment). In no case was the orally fed marker excreted distinctively earlier than the marker inserted into the rumen, which indicates that substantial bypass did not occur in these animals. However, differences between the three species in the excretion of the two markers from the rumen are consistent with hypothetical differences in the stratification of rumen contents. We suggest that effects previously ascribed to a “rumen bypass” in wild ruminants most likely reflect differences in the passage from the rumen.