An essential consideration in the welfare of zoo animals is to provide a good diet that meets the natural feeding ecology as close as possible. Nutrition takes a big role in longevity, disease prevention, growth and reproduction.
Hay: Rhinos are large herbivores that are adapted for gaining energy from the fermentation of fibrous plant material. Black rhinos can be fed only grass hay. If this is done, and the protein content of the grass hay is not being monitored by laboratory analyses, the addition of legume hay (also known as alfalfa hay) to the grass portion of the diet (20% of the grass hay offered) is recommended in order to ensure adequate protein levels. However, generally a mixture of 1:1 grass hay and legume hay is recommended for the Black rhino to mimic the nutrient composition of the natural diet. There is speculation that a high proportion of grass hay may lead to excessive tooth wear in Black rhinos (Taylor et al., submitted). There is no published evidence, but the exclusive use of lucerne hay for Black rhinos is discouraged. When freshly cut grass is available this could be fed as well but, when the grass is cut too short it can cause constipation of the hindgut (Clauss and Hatt, 2006). In studies of intake, digestion and passage in zoo herbivores, dry matter (DM) intakes of approximately 1% of body mass when Black rhinos were fed grass hays, and slightly higher levels (1.2 to 1.6% of body mass) when fed legume hay (Dierenfeld, 1996). According to Clauss and Hatt (2006) the maintenance requirements of hindgut fermenters should be 0.6MJ digestible energy per kg0.75 metabolic body mass.
Browse: For browsing rhino species, the addition of fresh and / or frozen browse may be essential to dietary health. Browse may contribute required nutrients that have not yet been quantified and may also be of benefit to dilute a captive diet that is too digestible (Dierenfeld, 1996). For feeding browse in the winter browse can be preserved by silaging. Browse should be fed 7 days per week. Browse that could be fed to black rhinos are:
Willow (Salix spp), Beech (Fagus spp), Hazel (Corylus spp), Ash (Fraxinus), Birch (Betula spp), Oak (Quercus spp), Hawthorn (Crataegus spp), Robinia (Robinia spp), Poplar (Populus spp), Apple (Malus spp), Cherry (Prunus spp), Prune (Prunus spp), Pear (Pyrus spp), Wild rose (Rosa spp), Blackberry (Rubus spp)
Concentrates: When feeding concentrates the pellets should be smaller than 1 cm in diameter for a proper intake of the pellets (Dierenfeld, 1996). The portion of pelleted compound feeds (or other forms of concentrates) in the diet should not exceed one-third of the overall calorific value. It should be possible to deliver sufficient amounts of energy and protein while providing a substantially lower proportion of pelleted compound feeds or concentrates in the diet. Pelleted compound feed may be used to balance mineral, vitamin and in some cases protein requirements. Pelleted compound feeds should only be used to satisfy energy needs when adequate roughage is not available. A pelleted compound feed based on lucerne meal, with a high concentration of vitamins and minerals (except iron) is recommended so that only small amounts need to be fed. When pelleted compound feeds are used it is recommended that it has high-fibre content (crude fibre 20% and acid-detergent fibre (ADF) of 25% of DM) (Clauss and Hatt, 2006). The proportion of concentrates in the diet should be between 1 and 10%.
Supplements: A possible vitamin-E deficiency has been suggested but not confirmed in zoo rhinos; current recommendations based on natural browse composition suggest that diets should contain 150 to 200 IU vitamin E/kg dry matter (a value usually surpassed in pelleted compound feed). If grown in an area prone to soil selenium (Se) deficiency, forage should be tested routinely for determination of Se content to provide data needed for balancing rations (Dierenfeld, 1996).
Iron: Over supplementation of iron is of particular concern in Black rhinos because this can cause several uncommon diseases. The recommended amount of 50 mg iron/kg DM for horses will probably be exceeded by the hay mixes described, and also by most pelleted feeds used. The use of tannin might reduce the excessive iron absorption. But there is no quantitative proof regarding supplementation of tannin in captive rhinos. According to Clauss and Hatt (2006), who assessed the results of studies regarding the effect of tannin supplementation in other species, the iron absorption will probably reduce by increased dietary tannin content. Extra supplementation with iron is not recommended (Clauss and Hatt, 2006). One Black rhino collection uses supplementation of tannin.
Fatty acids: The supplementation of linolenic acid (n-3) could be necessary to balance the amount of linoleic acid (n-6) and linolenic acid (n-3). This could be done by feeding fresh forage like freshly cut grass and browse, by increasing the proportion of grass or lucerne hay in the overall diet, by using concentrates that are based on lucerne meal rather than grain or soy products or by including linseed or linseed oil in the concentrates (Clauss and Hatt, 2006).
Salt lick: Black rhinos have been found to have higher endogenous faecal sodium losses. To counter sodium deficiency salt licks (suitable for horses) should be available ad libitum (Clauss and Hatt, 2006).