Branching Out: Rabbit Models Provide Evidence for the Use of a Willow Cultivar (<i>Salix miyabeana</i>) as a Suitable Forage Material for Monogastric Herbivores in Zoos

We assessed the suitability of a willow cultivar for monogastric herbivores in zoos using a rabbit model. Thirty-six male New Zealand White × Californian rabbits at 5 weeks of age were provided with bark and leaf material from a willow cultivar, Salix miyabeana (SX61), in one of four treatments for 6 weeks: UC (unrestricted control of pelleted diet); RC (restricted control of pelleted diet); TL (restricted pellets + fresh willow leaf material); and TB (restricted pellets + fresh willow bark material). Daily feed intake and weekly weights were recorded. Fasted blood samples were taken between 8:00 and 11:00 h, and blood biochemistry and acute phase protein (APP) were quantified at the start and end of the study. After the 6-week study period, rabbits were euthanized, and liver and kidney weights were obtained to assess hepatocellular damage. Blood biochemistry results were mainly within reference ranges and suggest a lack of toxicity or cellular damage due to feeding different amounts of willow ingredients. Greater weight gain was observed in the UC group when compared to the RC (p < 0.01), TB, and TL (p < 0.05) groups at the end of the treatment period. Total dry matter (DM) intake was greater in both TB and TL treatment groups when compared to the RC group but lower than the UC group (p < 0.05) across the treatment period. At the end of the study, phosphorus, magnesium, glucose, and AST were lower in the RC group when compared to all other treatment groups (p < 0.05) with a greater blood urea nitrogen than the TL and TB groups (p < 0.01). The TL group had greater blood potassium concentrations and a lower Na:K ratio than the UC and TB groups (p < 0.01). No differences were observed in the APP response over time and among groups; only fibrinogen was greater in the RC compared to the TB group (p < 0.05). The relative liver weight was higher in the UC group when compared to TB (p < 0.05). Given these results, willow bark and leaves at a high dietary inclusion can be considered safe and effective forage materials for monogastric herbivores in zoos.