Effect of cation–anion balance in feed on urine pH in rabbits in comparison with other species

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Paulus (2010) tried to acidify the urine of rabbits in order to acidify litter and thus reduce ammonia emission. She added methionine, calcium chloride or benzoic acid in concentration of 1% of air‐DM to typical rabbit feed. These additions failed to acidify urine in rabbits. Rückert et al. (2016) added ammonium chloride to rabbit feed and reported no effect on urine pH. Brewer and Cruise (1994) described a low renal carbo‐anhydrase activity in rabbits (the enzyme catalysing H+‐excretion), a finding which would explain the failure to acidify rabbit urine. By contrast, Kiwull‐Schöne et al. (2005, 2008) succeeded in reducing urine pH in rabbits below 7 with a low alkali diet. Cruz‐Soto et al. (1982) demonstrated distal acidification in the kidney in rabbits with ammonium chloride and Tsuruoka and Schwartz (1997) reported H+‐secretion in acidotic rabbits' outer medullary collecting duct of the kidney after feeding of high doses of ammonium chloride. This study aimed to compare the effect of diet composition on urine pH in rabbits with that of other monogastric species such as cats, dogs, pigs and horses. As an estimate to predict and compare the effect of diet on acid–base balance, the CAB of the diet based on the content of the cations Ca, Mg, Na and K and the anions P, Cl and S in DM was used (Langendorf and Lang, 1963; Harrington and Lemann, 1970; Oh, 2000; Remer, 2000; Riond, 2001; Poupin et al., 2012). In cats, dogs, pigs and horses, the relationship between CAB and urine pH has been investigated (Kienzle, 1991; Kienzle et al., 1991, 2006; Behnsen, 1992; Kienzle and Wilms‐Eilers, 1994; Beker et al., 1999; DeRouchey et al., 2003; Stevenson et al., 2003; Röcker, 2006; Yamka and Mickelsen, 2006; Carciofi, 2007; Berchtold, 2009; Bartges et al., 2013; Jeremias et al., 2013; Pires et al., 2013; Chen and Shan, 2014; Goren et al., 2014), which allows a direct comparison with this study with rabbits. A special point was the comparison of rabbits with horses because in contrast to dogs, cats and pigs, rabbits and horses both excrete large amounts of calcium by urine (Schryver et al., 1974; Kamphues et al., 1986; Kamphues, 1991; Redrobe, 2002; Clauss and Hummel, 2008; Kienzle and Burger, 2011). In addition, in horses the intake of green plant material such as grass or alfalfa hay, grass cobs, or grass silage as well as fresh grass has a strong impact on the relationship between CAB and urine pH (Goren et al., 2014). Green plant material, including hay and grass cobs, prevents acidification of urine in horses even if CAB is negative. In this study, we wanted to see whether there would be a similar effect of green plant material (hay and grass cobs) in rabbits. The study was approved by the animal welfare officer of the Veterinary faculty as well as the government of Upper Bavaria, the proper authorities according to German laws on animal welfare (Tierschutzgesetz; AZ 55.2‐1‐54‐2532‐124‐14).
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