Reflections on rabbit diets.

Twenty years ago, a paper was published in the Veterinary Record that described selective feeding behaviour in rabbits fed on muesli mixes and the possible relationship with dental disease (Harcourt-Brown 1996 ). This paper was written in response to the high prevalence of dental disease in pet rabbits and the poor tooth and bone quality that could be seen in the skulls of rabbits affected by dental disease compared with skulls of rabbits with healthy dentition. The hypothesis and conclusion of the paper was that selective feeding could result in the consumption of a calcium-deficient diet that would predispose the animals to dental disease. Owners were questioned about the feeding habits of their rabbits and their responses showed that muesli mixes were popular, and the most palatable portions of them were maize and peas. These ingredients have an extremely low calcium content (maize 0·01%, peas 0·07%) with a highly inverse calcium-tophosphorus ratio (maize 1:25, peas 1:6) (Ministry of Agriculture, Fisheries and Food 1992 ). After its publication, a reprint of the paper was sent to a selection of leading rabbit food manufacturers, and a Yorkshire-based company, Burgess Supafeeds, responded by formulating an extruded diet for rabbits as an alternative to pellets or muesli mixes. The diet, Burgess Suparabbit Excel, prevented selective feeding and it rapidly gained popularity. Other companies came out with similar products and there is now a range of extruded and hay-based diets for pet rabbits but sadly, muesli mixes are still available. They are cheap and often advertised with claims such as healthy teeth and bones , for healthy, happy rabbits or healthy, balanced diet that encourages natural foraging on the packaging. The paper by (Meredith & Prebble, 2017) that appears in this issue of Journal of Small Animal Practice is the fifth in a series of publications that have come out of a 17-month investigation into the effect of diet on the health and welfare of rabbits (Prebble & Meredith 2014 , Meredith et al . 2015 , Prebble et al . 2015a , b ). One of these publications examined the association of diet with dental disease (Meredith et al . 2015 ) but it did little to settle the debate about the aetiopathogenesis of dental problems in pet rabbits (Jekl & Redrobe 2013 ). Clinical signs of dental disease only occurred in three rabbits and these were in the group that were fed on muesli alone. The incidence might have been higher with a different brand of muesli mix. The brand that was selected for the investigation (Russell Rabbit Complete Muesli) is not typical of muesli mixes because it contains alfalfa stalks. Alfalfa has high calcium content and is a source of lignified fibre. Both these factors are implicated in the aetiopathogenesis of dental disease. Many theories have been proposed about the cause of acquired dental disease in pet rabbits including lack of attrition (Crossley 2003 , 2005 ) and metabolic bone disease caused by calcium deficiency exacerbated by vitamin D deficiency (Harcourt-Brown 2006 , 2007 ). The lack of attrition theory proposes that tooth height increases and applies an intrusive force on the opposing teeth that results in apical elongation. The metabolic bone disease theory proposes that loss of supporting bone results in apical elongation, increased curvature and alterations of the position of the teeth that occurs in response to chewing forces on teeth supported by weakened bone (Harcourt-Brown, 2013a ). The results in the paper by Meredith et al. ( 2015 ) reported no difference in the height of the teeth in rabbits fed on muesli alone, muesli and hay, nuggets and hay or hay alone. This finding does not support the lack of attrition theory. Instead, the results showed an increase in the overall length of the teeth due to increased curvature in the group of rabbits fed on muesli mixes with or without hay. There was also widening of the interdental spaces. These findings could be seen to lend support to the metabolic bone disease hypothesis. In the conclusion of the paper, Meredith et al. ( 2015 ) say that the low intake of forage, but not calcium and phosphorus imbalance, is implicated in the development of dental disease. This conclusion was drawn from analysis of the discarded food from the groups as a whole and the observation that dental disease did not develop in the rabbits that were fed on a hay-only diet, which had a calcium content of 0·28%. At first glance, this value looks very low as it is less than half the recommended level of 0·6% for optimal bone calcification in growing rabbits (Chapin and Smith, 1967 ). However, the figure of 0·6% as a calcium requirement is a percentage of dry matter and is based on nutritional studies in which rabbits are fed on a fixed amount of a pelleted diet of known analysis. If the amount of calcium a rabbit actually ingests each day is calculated instead of a percentage of the diet, the figures look slightly different. Rough calculations based on nutritional information from commercial rabbits show that adult rabbits require approximately 500 mg calcium per day for maintenance (Harcourt-Brown, 2013b ). As the dry matter intake of the rabbits in the investigation was measured (Prebble & Meredith, 2014 ), it is possible to calculate the rabbits’ daily calcium intake. The rabbits that were fed on hay alone ate approximately 300 g of dry matter per day and the calcium content was 0·28%, so the daily calcium intake was approximately 840 mg. The rabbits that were fed on muesli mix alone ate approximately 100 g of dry matter per day and the calcium content of the diet was 0·6%, so the daily calcium intake was approximately 600 mg. This means that the calcium intake of the hay-only group was higher than the muesli mix-only group. The hay-only group could not feed selectively, whereas it was possible for an individual rabbit in the muesli mix groups to pick out the palatable low calcium ingredients. The rabbits were kept in pairs and individual food preferences were not recorded. The findings of Meredith et al ( 2015 ) that the rate of incisor growth matched the rate of attrition and that the rates varied with the type of diet were similar to the findings of Müller et al. ( 2014 ) who carried out a detailed investigation into the growth and wear of the incisor and cheek teeth of rabbits fed on diets of different abrasiveness. In the investigation, 16 adult New Zealand White rabbits were randomly fed on one of five different diets for a two-week period. The diets consisted either of grass hay, which was fed as whole forage, or complete pelleted diets of increasing abrasiveness: lucerne pellets, grass pellets, grass with