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Retention time of food in the digestive tract is a major aspect describing the digestive physiology of herbivores. Differences in feed retention times have been described for different ruminant feeding types. In this study, a dominantly grazing desert ruminant, the addax (Addax nasomaculatus), was investigated in this respect. Eight animals with a body weight (BW) of 87 +/- 5.3 kg on an ad libitum grass hay (Chloris gayana) diet were available. Co-EDTA and Cr-mordanted fibers (<2 mm) were used as pulse-dose markers. Mean retention time (MRT) in the digestive tract was calculated from faecal marker excretion. Average daily intake of the addax was found to be 1.7 kg dry matter (DM) or 60+/-8.3 g DM/kg BW0.75. The MRT of fluid and particles in the reticulo-rumen (MRTfluidRR and MRTparticleRR) were quantified to be 20+/-5.8 and 42+/-7.0 h respectively. When compared to literature data, MRTfluidRR was significantly longer than in cattle species, and MRTparticleRR was significantly longer than in 11 taxa of all feeding types. The ratio of MRTparticleRR/MRTfluidRR (2.3+/-0.5) was found to be within the range described for grazing ruminants. The long retention times found in the addax can be interpreted as an adaptation to a diet including a high proportion of slow fermenting. grasses, while the long retention time of the fluid phase can be interpreted as a consequence of water saving mechanisms of the desert-adapted addax with a potentially low water turnover and capacious water storing rumen. (C) 2007 Elsevier Inc. All rights reserved.