Rumination of different-sized particles in muskoxen (Ovibos moschatus) and moose (Alces alces) on grass and browse diets, and implications for rumination in different ruminant feeding types

The obligatory, periodic regurgitation of forestomach material and its subsequent re-mastication is the hallmark of the most diverse extant large herbivore group, the ruminants. Although the process of rumination is well understood in domestic species, differences between free-ranging wild ruminant species, for example of different body size or different feeding type, remain speculative to date. Here we investigate the proportion of plastic particles of varying size (1, 10 and 20 mm) and density (1.03, 1.20 and 1.44 mg/ml) that are recovered intact or ruminated-upon after insertion into the reticulorumen (RR) of domestic cattle (Bos primigenius f. taurus) on grass silage, and of muskoxen (Ovibos moschatus; n = 4) and moose (Alces alces; n = 2) both fed browse and grass diets. In the three species, the proportion of particles leaving the RR intact depended on particle size, with density showing no effect in this study. The major proportion of 1 mm particles was excreted intact, whereas intact 10–20 mm particles were only excreted sporadically, and not in all animals. Intact particles were mostly found in the initial samples after marker application, and mean retention times of intact particles were always shorter than those of ruminated particles. There were no differences between moose and muskoxen, but diet had a significant effect, with a higher proportion of 1 mm particles ruminated upon on the grass diet in both species, indicating a higher ‘filter-bed effect’ with entrapment of small particles in a fibre mat in the RR on a grass diet. Given that less particles were ruminated on the grass diet, one could either assume that free-ranging browsers ruminate less than grazers on similar food intakes (or that they have higher food intakes at similar levels of rumination). The existing data on time-budgets of free-ranging ruminants, however, does not suffice to test these hypotheses. The fact that indication of a ‘filter-bed effect’ was also detectable in moose raises the question whether adaptations described in ‘cattle-type’ ruminants really serve to re-inforce the processes of RR contents stratification and the ‘filter-bed’, or whether they are not rather directed at other aims, such as maximizing microbial yield from the RR.