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NUTRIENT REQUIREMENTS AND SIGNS OF DEFICIENCY AND TOXICITY ENERGY The energy requirements for various productive func- tions (growth, lactation, gestation) have received little attention. Assuming that rabbits, like most animals, vol- untarily adjust their feed intake to meet their energy needs, the lack of precise data on energy requirements is perhaps of less concern in rabbit diet formulation than the lack of data on requirements of most other nutrients. Lebas (1975a) has studied the performance of growing rabbits fed diets differing in energy content. Approxi- mately 9.5 kcal of digestible energy (DE) was required per g of body weight gain, regardless of energy content of the diet. The data suggest that a level of 2,500 kcal of DE per kg of diet will satisfy the energy needs for rapid grown, but, at energy levels lower than this, the rabbit may not be able to consume sufficient feed to meet its energy re- quirements for maximum growth. Energy requirements for maintenance, gestation, and lactation of does and bucks have not beers reported in detail. Lebas (1975b) has noted good reproductive per- formance of does fed diets of 2,500 to 2,900 kcal of DE per kg of diet. According to the results of Axelson and Erikson (1953), for an adult rabbit of 3 kg the daily metabolizable energy requirement is 200 kcal, a quantity easily pro- vided with diets of 2,100 to 2,200 kcal of DE per kg. Limited data on the DE content of feedstuffs for rabbits have been determined experimentally. Voris et al. (1940) reported digestibility coefficients for most common feedstuffs. These coefficients have been used, in conjunc- tion with gross energy values (NRC Atlas of Nutritional Data on United States and Canadian Feeds), to obtain estimated DE contents of feeds. TDN values, based on the work of Voris et al. (1940), are also presented. UTILIZATION OF ENERGY-PROVIDING NUTRI E N TS Carbohydrates The fiber fraction of feeds corresponds to the structural carbohydrates of plant material. In the past, this was measured as crude fiber. In recent years, the terms "cell wall constituents" (cwc) and "acid-detergent fiber" (ADF) have become widely used. cwc consists of hemicellulose, cellulose, lignin, and silica, while ADF consists of cel- lulose, lignin, and silica. Few digestibility coefficients for ADF and cwc in rabbits are available. Cheeke (1974a) reported a value of 26.6 percent digestibility of barley ADF; there was a high degree of variability, with indi- vidual values ranging from 8.1 to 51.8 percent. Schurg et al. (1976) found digestibility coefficients of 25.0 and 36.7 percent for ADF and cwc, respectively, of whole- plant corn pellets. Because rabbits are herbivorous, it is widely assumed that they utilize plant fiber efficiently. Available data refute this assumption. Slade and Hintz (1969) report values of 18.1 percent digestibility for alfalfa crude fiber in the rabbit, whereas in the horse, pony, and guinea pig values of 34.7, 38.1, and 38.2 percent crude fiber digesti- bility were obtained. Fonnesbeck et al. (1974) reported values of 16.1 percent digestibility of cellulose and 24.7 percent digestibility of hemicellulose in the rabbit; com- parable values for the rat were 20.7 and 25.9 percent. Maynard and Loosli (1969) cited an apparent digestion coefficient of 14 percent for crude fiber in the rabbit, as compared with values of 44, 41, 22, and 33 percent for cattle, horses, swine, and guinea pigs, respectively. It is apparent that We rabbit does not digest fiber efficiently. In spite of Me fact that crude fiber does not serve as an efficient energy source for rabbits, there is evidence that dietary fiber may have beneficial effects. As discussed later in this report (see chapter on Diets and Feeding Practices), nondigestible fiber may be necessary for nor- mal functioning of the digestive tract. Davidson and Spreadbury (1975) reported that dietary fiber levels of less than 6 percent crude fiber may promote diarrhea. Lebas (1975b) has also noted that fiber levels lower than 12 percent may promote diarrhea. Digestion of fiber in the rabbit would require the pres- ence of cellulolytic bacteria or protozoa in the cecum and/or colon, since no mammal secretes cellulase. A few studies of the digestive tract flora of rabbits have been conducted. In contrast to the situation with many animals, the gut of the postweaning rabbit is almost devoid of 2
Nutrient Requirements of Rabbits 3 Escherichia cold and Lactobacillus (Smith, 1965~. Com- pared to other animals studied, the rabbit is unique in that the flora of the large intestine is almost entirely bac- teroides (Smith, 1965~. Fuller and Moore (1971) also noted that bacteroide species were the dominant or- ganism in rabbit large intestine. Count and Fonty (1973) reported a similar finding. Many bacteroides are cellulose digesters (Hall, 1952; Hungate, 1966) so it is likely that the rabbit does have a population of cellulolytic or- ganisms. Hoover and Heitmann (1972) studied utilization of ADF by rabbits. Growth was significantly lower with a dietary level of 29.4 percent ADF than with 14.7 percent ADF. Lebas (1975a) obtained a similar growth rate with diets containing 10 and 18 percent crude fiber. Production of acetic, propionic, and butyric acids in the cecum was demonstrated. It was estimated that cecal fermentation produces an amount of volatile fatty acids equivalent to 10 to 12 percent of the daily caloric requirement. Acetate is utilized by adipose tissue and mammary gland for fatty acid synthesis (Leung and Bauman, 1976; Perret et al., 1976). Rabbits efficiently digest starch, the major carbohy- drate in cereal grains. No special problems are encoun- tered with the use of starch or sugars in the diet. There is evidence that the use of grains by rabbits may be influenced by factors other than their energy content. Cheeke (1974b) found in preference trials that when given a choice, rabbits preferred barley or wheat to corn. This difference in palatability may be the explanation for the findings of Hall and Johnston (1976) that coin-based diets gave poorer growth responses than barley- or oat- based diets. In lactation diets, oats gave the best perform- ance of four grains (wheat, oats, barley, and com) as assessed by 3-week weights of the young (Hall and Johnston, 1976~. The best performance with He lowest energy grain suggests that factors other than energy con- tent, such as palatability, are involved. Lipids Thacker (1956) fed diets containing 5, 10, 15, 20, and 25 percent fat in the form of vegetable oils and found that gains of 4- to Week-old Dutch rabbits were greater with fat levels of 10 to 25 percent than with the 5 percent level. Arrington et al. (1974) also observed better performance with fat levels of 11 and 14 percent than with 2.4 and 3.6 percent. It appears that there are no special problems associated with feeding of fat to rabbits; level used in feeds is thus dictated by the prevailing economic rela- tionship between fat sources and grains. Arrington et al. (1974) observed digestibility coefficients of 83.6 and 90.7 percent for the ether-extract fraction, largely consisting of corn oil. Inclusion of fat in the diet tends to improve palatability; Cheeke (1974) observed a preference by rabbits of a diet with 5 percent corn oil over one with no added fat; there was a distinct preference for a diet win 10 percent added corn oil over one with 20 percent oil added. Essential fatty acid deficiency in rabbits has been dem- onstrated (Ahluwalia et al., 1967~. Signs include reduced growth, loss of hair, and changes in the male reproductive system including degenerative changes in the seminifer- ous tubules, impaired sperm development, and decreased accessory gland weights. PROTE IN AN D AM I ~ O ACI D S The importance of protein quality in rabbit nutrition is well recognized. For rapid growth, rabbits are dependent upon adequate quantities of dietary essential amino acids. Bacterial protein synthesis in the cecum has been demonstrated, but this protein, obtained by means of coprophagy, apparently does not make a large con~ibu- tion to the essential amino acid needs of the young rabbit. The inability of poor-quality proteins, such as zein and gelatin, to support a normal growth rate has been demon- strated by Cheeke (1971) and Kennedy and Hershberger (1974~. Dependence on dietary essential amino acids implies that nonprotein nitrogen sources would not be useful to rabbits. Numerous studies have indicated this to be true. Olcese and Pearson (1948) found that supplementation of a low-protein diet with urea did not allow for grown. King (1971) reported that substitution of part of the plant protein in a grower diet with urea resulted in decreased grown. Cheeke (1972) observed that neither urea, biuret, nor diammonium citrate improved grown when added to a low-protein diet. Lebas and Colin (1973) obtained no response by supplementing a low-protein diet with urea. These reports provide abundant evidence that nonprotein nitrogen sources cannot be employed usefully in grower diets. Preliminary estimates of amino acid requirements of the rabbit for grown have been made. The first amino acid shown to be a dietary essential was argi- nine (McWard et al., 1967~. Essentiality of arginine, methionine, and lysine was reported by Gaman and Fisher (1970) and Cheeke (19711. Adamson and Fisher (1973) reported the essentiality of numerous other amino acids. Quantitative estimates of requirements of He es- sential amino acids have been made, but these estimates will be subject to modification as more intensive studies are reported. For example, Adamson and Fisher (1976) have reported that the arginine requirement was overes- timated in the initial studies (Cheeke, 1971; Adamson and Fisher, 1973) due to excesses of other amino acids, thus increasing the demand for arginine for urea cycle reac- tions. Adamson and Fisher (1973) and Davidson and Spreadbury (1975) have published estimates of require- ments of all of the essential amino acids. In some of the experiments of Adamson and Fisher (1973), the grown rate of the animals was very low. Davidson and Spread- bury (1975) estimated requirements by measuring amino acid composition of diets that supported a rapid growth rate. This method may lead to considerable overestima- tion. In the case of lysine, methionine, and arginine
4 Nutrient Requirements of Rabbits requirements, more extensive studies have been con- ducted. These include those of Gaman and Fisher (1970), Cheeke (1971), Lebas (1973), Colin et al. (1973), Colin (1974, 1975a, 1975b), and Adamson and Fisher (19761. Based on these studies, there is general agreement for the following: arginine, 0.6 percent; lysine, 0.65 percent; and sulfur amino acids (methionine plus cystine), 0.6 percent of the diet on an as-fed basis. These levels will support a rapid rate of growth (35-40 g/day). Further refinements of these estimates, as well as those for other amino acids, will need to be concerned with breed and strain differ- ences. The value of various protein supplements for rabbits has been studied. Cheeke and Amberg (1972) found that, at equal protein levels, soybean meal or fish meal pro- moted growth rates of 34 g per day, while growth with cottonseed meal was 25 g per day. Supplementation of cottonseed meal with lysine and methionine increased growth rate to the level obtained with the other two supplements. Lebas (1973) found soybean meal sup- ported a higher growth rate than obtained with sesame meal. Colin and Lebas (1976) have found rapeseed meal, horsebeans, and peas to be acceptable protein supple- ments after supplementation with methionine. Davidson and Spreadbury (1975) reported that fish meal, casein, and soybean meal supported greater growth than peanut meal, gelatin, and gluten when used as protein supple- ments. The various responses to different protein sup- plements are largely a consequence of their amino acid composition. In contrast to other simple-stomached animals, such as swine and poultry, the rabbit is able to utilize efficiently the protein in forage plants. For example, in the pig the digestibility of the protein in alfalfa meal is less than on percent, while in the rabbit it is about 75 percent (Slade and Hintz, 1969~. Thus, it is feasible to utilize consider- able amounts of alfalfa in rabbit diets. Cheeke and Am- berg (1972) found that a growth rate of 34 g per day was maintained with dietary alfalfa levels between 10 and 60 percent, while with 90 percent alfalfa a growth rate of 23 g per day was observed. An isolated alfalfa protein concen- trate, when substituted for soybean meal, maintained adequate growth (Cheeke, 1974); digestibility of the pro- tein was 79 percent. This protein source had a digestibil- ity of only 65 percent in rats. The ability of the rabbit to extract protein from fibrous forages, along with its will- ingness to consume these materials, suggests that greater use could be made of this animal in many protein- deficient countries of the world. The contribution of bacterial protein synthesized in He cecum and colon of the rabbit has not been evaluated quantitatively. As discussed previously, it does not ap- pear to contribute significantly to the protein needs of growing rabbits. On the other hand, it may help to main- tain nitrogen equilibrium in mature animals fed poor- quality proteins. Kennedy et al. (1970) demonstrated that amino acids can be absorbed rapidly from the rabbit cecum. Houpt (1963) hypothesized the secretion of urea from He blood into the cecum and colon. Mature rabbits allowed to engage in coprophagy maintained a positive nitrogen balance when fed gelatin but were in negative balance when coprophagy was prevented (Kennedy and Hershberger, 19741. Mature rabbits fed a low-protein (7 percent) diet showed a significant increase in nitrogen re- tention when urea was added, suggesting incorporation of urea nitrogen into protein (Slade and Robinson, 19701. Hoover and Heitmann (1975) have observed slight utili- zation of urea by rabbits nearing maturity. Quantitative protein requirements depend in part on protein quality. Sufficient data have not been developed since the last revision of this report (1966) to recommend any changes in the protein requirements. Crude protein levels of 16, 12, 15, and 17 percent are recommended for growth, maintenance, pregnancy, and lactation, respec- tively. These values assume the use of protein of adequate quality to meet essential amino acid require- ments. MINERAL ELECTS Calcium anc! Phosphorus Calcium and phosphorus are major constituents of bone; in addition, calcium has metabolic roles in blood clotting in controlling excitability of nerve and muscle tissue, and in the maintenance of acid-base equilibrium, while phosphors is a component of such vital cellular con- stituents as ATE, DNA, RNA, and phospholipids. The absorption of calcium is influenced by its level in He diet and the dietary levels of phosphorus and vitamin D. The relative importance of these factors has not been studied critically in the rabbit. In other species, calcium absorption has been shown to be dependent on a carrier protein that transports calcium through the intestinal lin- ing to the blood. The formation of this calcium- transporting protein is under the control of vitamin D. The rabbit is unusual in that the serum calcium level reflects the dietary calcium level (Chapin and Smith, 1967a) rather than being homeostatically regulated to a narrow range as in other species. Another unusual con- cept of calcium metabolism in the rabbit is that the urine is a major route for calcium excretion (Kennedy, 1965; 13esancon and Lebas, 1969; Cheeke and Amberg, 1973), while in most other animals biliary excretion is the major route. Because urinary excretion of calcium varies di- rectly with the serum calcium level, the high urinary calcium excretion by the rabbit, especially when high- calcium diets are fed (Cheeke and Amberg, 1973), is probably a reflection of the correlation between serum and dietary calcium levels in this species. Since a rise in serum calcium in most animals triggers secretion of cal- citonin from He thyroid, the less efficient serum calcium homeostasis in the rabbit suggests Hat its calcitonin se- cretion rate may be low. Rabbits do respond to injected calcitonin, which will induce hypocalcemia (Lupulescu,
Nutrient Requirements of Rabbits 5 1974), although Salako et al. (1971) found hypocalcemia following calcitonin injection in the rat and mouse but not in the rabbit. Kennedy (1965) also suggests a possible role of vitamin D in stimulating the high urinary calcium excretion. Dietary requirements for calcium and phosphorus for rabbits have been estimated. Mathieu and Smith (1961) estimated the phosphorus requirement for growth to be 0.22 percent of the diet. In an extensive study of calcium requirements, Chapin and Smith (1967a) determined that, with a dietary phosphorus level of 0.37 percent, maximum growth was achieved with 0.22 percent calcium in the diet, while 0.34 to 0.40 pecent calcium was needed for maximum bone calcification. A predominantly grain diet with 15 percent good-quality alfalfa supplies enough cal- cium and phosphorus to allow maximum rate and effi- ciency of gain, while increasing the alfalfa level to 20 percent allows for maximum bone calcification as well. Chapin and Smith (1967b) reported Mat a diet with 0.37 percent phosphorus and 0.45 percent calcium was adequate for gestation and lactation. In view of the secre- tion of these elements into milk (Lebas et al., 1971), tentative requirements of 0.75 percent calcium and 0.5 percent phosphorus are recommended for lactation. In nonruminant animals, phosphorus availability of plant sources is poor because of the presence of phytate phosphorus. In ruminants, bacterial phytase degrades Me phytate complex, freeing the phosphorus for use by the animal. Although extensive phosphorus availability studies have not been conducted with rabbits, a reason- able assumption is that the bacterial action in the cecum and colon renders plant phosphorus available, and this has been confirmed in one report (Blanco and Gueguen, 19741. Rabbits are tolerant of high dietary calcium levels. Chapin and Smith (1967b) found that diets containing as much as 4.5 percent calcium and a calcium: phosphorus ratio of 12:1 did not depress growth and resulted in normal bone ash. High (1 percent) levels of phosphorus are unpalatable, causing feed rejection (Chapin and Smith, 1967c). Potassium Hove and Herndon (1955) found that potassium defi- ciency in rabbits resulted in a severe and rapidly progres- sing muscular dystrophy. They estimated the potassium requirement for growth to be at least 0.6 percent of the diet. A deficiency is unlikely, except perhaps with pro- longed feeding of a high-grain diet. Alfalfa and other forages are rich in potassium. It has been reported that high levels of potassium (0.8-1.0 percent) may induce nephritis in rabbits (Surdeau et al., 1976~. Sodium and Chiorine Specific experimental data on salt requirements of rabbits are not available. Addition of 0.5 percent salt to the diet, or provision of salt blocks for free-choice consumption, are adequate means of providing these elements. Disad- vantage of salt blocks include greater cost, greater labor requirements, and cage corrosion in moist climates. Magnesium Kunkel and Pearson (1948) characterized magnesium de- ficiency as causing poor growth and hyperexcitability with resulting convulsions. They estimated the mag- nesium requirement for growth as 30~0 mg per 100 g of diet. There is evidence that inadequate magnesium may result in fur chewing (Gaman et al., 19701. Woodward and Reed (1969) noted alopecia, blanching of the ears, and alteration of fur texture and luster in rabbits fed a diet containing 5.6 mg of magnesium per kg of diet. Cheeke and Amberg (1973) found that the major route for mag- nesium excretion in rabbits is the urine, a pattern which is similar to the unusually high urinary calcium excretion. Iron Iron deficiency in rabbits produces microcytic, hypo- chromic anemia (Smith et al., 19441. Specific data on iron requirements are not available. At bird, rabbits have a very large iron reserve (Tar- vydas et al., 1968), so the newborn are not dependent on a supply of iron in the milk. Rabbit liver has a high iron storage capacity. Iron from transfernns in the blood is incorporated into ferritin in the liver, which in the rabbit is the immediate precursor of hemosiderin (Underwood, 19711. In view of the generous distribution of iron in feedstuffs, iron deficiency in rabbits is unlikely to be encountered under practical conditions. Because of Me iron reserve at bird, the rabbit is not susceptible to iron-deficiency anemia in the preweaning phase. Copper A deficiency of copper results in anemia and graying of He hair (Figure 1) (Smith and Ellis, 1947~. Bone abnor- malities associated win copper deficiency have been reported (Hunt et al., 1970~; the deficiency signs were accentuated by supplementation of the low copper diet win 1 percent ascorbic acid. A dietary level of 3 mg of copper per kg of diet has been suggested as approximately the requirement (Hunt and Carlton, 1965~. King (1975) reported Hat 200 ppm added copper stimulated grown rate of young rabbits. Selenium The nutritional essentiality of selenium has been demon- strated for numerous species of animals. This element has been shown to be a constituent of the enzyme glutathione peroxidase, which is involved in the disposal of peroxides in tissues. The metabolism of selenium is inextricably
6 Nutrient Requirements of Rabbits FIGURE 1 A copper-deficient rabbit, showing a graying of genetically black hair. There later develops a severe anemia (S. E. Smith, Cornell University). involved win that of vitamin E, which functions in pre- venting peroxide formation. The rabbit appears to be unusual in its metabolism of these nutrients. Rabbits fed a Torula yeast diet develop muscular dystrophy; this is preventable only by vitamin E (Draper, 1957, Hove et al., 19581. In these studies selenium had neither protective nor "sparing effect." This is in marked contrast to the situation win rats, in which either selenium or vitamin E will prevent deficiency signs when a Torula yeast diet is fed. In rats, and most other species studied, selenium and vitamin E exert a sparing effect on each other. Jenkins et al. (1970) found that a low-selenium hay that induced nutritional muscular dystrophy in lambs and calves from dams fed the hay did not produce any deficiency signs when fed to young rabbits. The addition of 1 percent linoleic acid to the hay resulted in severe muscle degen- eration that was not responsive to selenium. The results of these studies indicate that, in rabbits, selenium does not have a sparing effect on vitamin E and apparently does not have a role in disposal of peroxides. However, Cheeke and Whanger (1976) have found Mat rabbit tis- sues do have glutathione peroxidase activity with levels comparable to or higher than those of rat tissues. A1- though selenium has not been demonstrated to be a nutritional essential for the rabbit, further studies are needed to determine the significance of selenium in rabbit nutrition. On the basis of data presently available, protection against peroxide damage appears to be more dependent on vitamin E than on selenium in the rabbit. Molybdenum An excess of molybdenum induces copper deficiency, anemia, and other signs of toxicity (Arrington and Davis, 1953~. Neither an excess nor deficiency of molybdenum in rabbits under practical conditions is likely. Z. zinc In young female rabbits fed a diet containing 0.2 ppm zinc, Shaw et al. (1972, 1974) observed the following deficiency signs: reduced feed consumption, lowered hematocrit, weight loss, graying of the dark hair, elevated zinc levels in the remaining dark hair, alopecia, der- matitis, and reproductive failure. Unreceptiveness to the male, apparent failure of ovulation, and a pale, inactive endometrium were factors in the lack of fertility. Since loss of appetite was pronounced, all of the above signs may be at least partially a result of reduced intake of other nutrients. In a similar study, Apgar (1971) noted sparce hair, dermatitis, weight loss, appetite depression, sores around the mouth, and wet matted hair on the lower jaw and ruff when female rabbits were fed a diet containing less than 3 ppm zinc. Quantitative data on zinc require- ments for growth and reproduction have not been re- ported. Cobalt Cobalt is required for the synthesis of vitamin B'2 by microorganisms in the digestive tract. Utilization of cobalt by the bacterial flora is much more efficient in the rabbit than in ruminants (Simnett and Spray, 1965a). After 51 weeks on a diet containing less than 0.03 ppm cobalt, no deficiency signs in rabbits were observed. Absorption of vitamin B,2 is more efficient in the rabbit than ire man, the rat, or sheep (Simnett and Spray, 1965b). In view of these results, cobalt deficiency in rabbits under natural condi- tions is extremely unlikely. Manganese Smith and Ellis ( 1947) have described We clinical signs of manganese deficiency in rabbits, which include mal- development of the skeletal system crooked legs, brittle bones, and decreased weight, density, length, and ash content of the bones (Figure 2~. Estimates of minimum manganese levels needed to prevent obvious deficiency signs are 2.5 and 8.5 mg of manganese per kg of diet for adults and growing animals, respectively. These esti- mates are derived from the studies of Smith and Ellis (1947~. Iodine Iodine requirements of rabbits have not been studied. A sound management practice is the use of iodized salt, especially in low-iodine areas. Although the exact re- quirements are not known, diets should probably contain at least 0.2 mg of iodine per kg of diet. Excessive intakes have been observed to cause high mortality of the new- born, but amounts required to cause the toxic effects were much greater than would be present in diets without added iodine (Arrington et al., 1965).
Nutrient Requirements of Rabbits 7 ... .............. ...~.,~. .~ ,.:~..~.~. . . ... ..., ..~. .~:~ ,: ;~ ~d . -.- ~-~ ~ ~ ~ ~ ~i. i- . . ~ ~ ~ ~ . ~ - ~ ~ . ~ ~ ~ ~ :.: ~ __,_..:': ~--~: _ FIGURE 2 A manganese-deficient rabbit showing crooked front legs, which reflect the generally abnormal bone develop- ment (S. E. Smith, Cornel I U Diversity). FAT-SOLUBLE VITAMINS The qualitative need of rabbits for many of the known vitamins has been determined, but quantitative needs have been estimated for only a very few. Vitamin A Payne et al. (1972) found that 8 ,ug of vitamin A per kg of body weight per day was adequate for grown of female and breeding male rabbits. This corresponded to a level of 580 IU of vitamin A per kg of diet. However, reproduc- ing females required somewhat more tears the highest level fed, which was 14 ,ug per kg of weight, or 1,160 flu per kg of diet. They suggested Hat this requirement may approximate 20 ,ug. Normally rabbits obtain their vitamin A as pro-vitamin A, principally carotene. Phillips and Bobstedt (1938) showed that 50 ,ug of carotene per kg of body weight prevented symptoms of vitamin A deficiency and permitted normal growth and reproduction. Rela- tively large doses of carotene have been used by some (Mellanby, 1935; Mann et al., 1946) to effect a rapid recovery from the deficiency state. Unfortunately, the efficiency of the rabbit in converting carotene to vitamin A is unknown. Signs of vitamin A deficiency in rabbits are similar to those described for other animals and include retarded growth, neural lesions, ataxia, spastic paralysis, xerophthalmia, and impaired reproduction (Nelson and Lamb, 1920; Mellanby, 1935; Phillips and Bohstedt, 1938; Lamming et al., 1954a, 1954b). In addition, Lam- ming et al. (1954b) demonstrated a high incidence of hydrocephalus with stenosis of the cerebral aqueduct among rabbits reared by females maintained on low- carotene diets for a period of 14 weeks prior to mating. Lack of appetite is characteristic of a vitamin A deficiency in rabbits (Saksena et al., 1971~; these authors also noted abnormal elasticity of the lungs and aorta associated with a decreased elastin content. Vitamin D Although the quantitative requirement of vitamin D for the rabbit has not been determined, symptoms of rickets have been produced with diets that were deficient in this vitamin (Goldblatt and Moritz, 1925; Mellanby and Kil- lick, 19261. Jarl (1948) fed rabbits a diet low in vitamin D with a narrow calcium: phosphorus ratio and demon- strated that interference with bone calcification was only temporary since~by 8 to 12 weeks of age bone growth was equivalent to that of control rabbits fed supplemental vitamin D. Ringler and Abram s (1970, 1971) observed probable vitamin D toxicity in rabbits fed a diet containing 23,000 flu of vitamin D per kg. The observed signs were high blood levels of both calcium and phosphorus and calcifi- cation of soft body tissues. Similar observations were noted in another case of hypervitaminosis D in a commer- cial rabbitry (Stevenson et al., 1976~. Vitamin E Laboratory studies using semipurified diets are in good agreement that the daily vitamin E (cr-tocopherol) re- quirement is about 1 mg per kg of body weight (Macken- zie and McCollum, 1940; Eppstein and Morgulis, 1941; Hove and Harris, 1947; Hove et al., 1957~. However, Ringler and Abrams (1970, 1971) encountered wide- spread signs of vitamin E deficiency in a commercial herd of rabbits fed a locally formulated natural diet Mat pro- vided approximately 1 mg of c'-tocopherol per kg of body weight (16.7 mg/kg of diet). In other species of animals it is known that the dietary vitamin E requirement is in- creased in diets containing autooxidizable substances, as polyunsaturated fatty acids, and very low levels of selenium. Thus, it appears wise to recommend a level of vitamin E for natural, commercial diets somewhat higher than 16.7 mg per kg of diet. There are insufficient data in the literature to permit a sound recommendation, and, in the meantime, a level of 40 mg per kg of diet is suggested.
8 N utrient Requirements of Rabbits Diehl (1960) and Diehl and Kistler (1961) observed a one-third decrease in body tocopherol levels in rabbits showing liver lesions due to coccidiosis infections when compared to noninfected controls. They suggested ~at, due to the prevalence of this infection, many rabbits used in laboratory studies may be in a state of hypovitaminosis E. Muscular dystrophy in rabbits, or cod-liver oil injury, as it is referred to in the early literature, is today recognized as being primarily caused by a vitamin E deficiency. Signs of this syndrome include degeneration of the skeletal and cardiac muscles, paralysis, and fatty liver (Bragdon and Levine, 1949~. That muscular dystrophy may not be due to a simple deficiency of vitamin E is indicated by studies in which deficiencies of choline (Hove and Copeland, 1954) or potassium (Hove and Herndon, 1955) have been implicated. Proctor et al. (1961) produced this dystrophy in young rabbits fed a diet high in Torula yeast, and it was not prevented by supple- ments of either vitamin E or selenium. Still unexplained is He observation of Kaminura and Sasaki (1965) that the topical application of c'-tocopherol significantly acceler- ates the growth of hair in rabbits. Ringler and Abrams (1971) reported on a vitamin E deficiency problem in a rabbit colony in which muscular dystrophy and death of neonates and infertility in breeding does was observed. The diet contained 16.7 mg of a-tocopherol per kg. Vitamin K Limited studies indicate that intestinal synthesis of vita- min K is sufficient for normal grown but that supplemen- tal amounts may be needed for reproduction (Hogan and Hamilton, 1942; Moore et al.? 1942~. Moore et al. (1942) fed a diet deficient in vitamin K to pregnant rabbits and observed placental hemorrhage and abortion of the young. A level of 2 ppm vitamin K in a purified diet is adequate to prevent hemorrhage and abortion (F. Lebas, unpublished data). WATER-SOLUBLE VITAMINS There is now good evidence that requirements for various members of the vitamin B complex are partially or even completely satisfied through the routine practice of co- prophagy. In this manner ~ major portion of vitamins synthesized in the intestinal tract are recycled by co- prophagy and made available to the animal. Riboflavin and Pantothenic Acid Olcese et al. (1948) found that rabbits grew normally when fed pantothenic acid-deficient and riboflavin- deficient diets, and, furthermore, these rabbits excreted amounts of these vitamins greatly in excess of dietary intakes. Owen et al. (1970) has reported contrary evi- dence insofar as riboflavin is concerned. In this study, rabbits were fed a riboflavin-deficient purified diet, and no riboflavin was detected in the cecum. The difference may have been due to the different diets used, i.e., natural vs. purified ingredients. Niacin Substantial synthesis of niacin occurs in the intestinal tract of rabbits fed niacin-deficient diets (Olcese et al., 1949; Kulwich et al., 1953), but such rabbits respond significantly in growth when fed additional niacin up to 11 mg per kg of body weight (Wooley and Sebrell, 1945; Wooley, 1947~. As is true for other animals, niacin can be synthesized from tryptophan, and niacin-deficient rabbits have been shown to respond when fed this amino acid (Wooley, 1947; Kulwich et al., 1953~. Dietary deficiencies of niacin have resulted in pronounced loss of appetite followed by emaciation and diarrhea (Wooley and Seb- rell, 1945~. Thiamine Reid et al. (1963) found that rabbits fed a thiamin-free diet averaged about 3 ,ug of Jimmie per g of dry matter in the cecal contents and thus confirmed the intestinal synthesis of this vitamin. However, intestinal synthesis supplied inadequate amounts of this vitamin in that some rabbits developed a mild ataxia after prolonged feeding of the deficient diet. Reid et al. (~1963) reported that rabbits fed a thiamin-free diet along win a Niacin antagonist (neopyrithamin) developed ataxia, flaccid paralysis, con- vulsions, coma, and death. Pyridoxine Pyridoxine deficiency, as reported by Hove and Herndon (1957a>, is characterized by a decreased rate of grown and dermal and neurological signs that are similar to Hose observed in pyridoxine-deficient rats and pigs. These workers prevented the appearance of pyridoxine deficiency signs by feeding 39 ,ug of the vitamin per g of diet. Choline Choline deficiency signs were prevented when a level of 0.12 percent choline chloride was added to the diet (Hove et al., 1954, 1957~. The choline deficiency syndrome in rabbits has been described as retarded grown, fatty and cirrhotic liver, and a necrosis of He kidney tubules (Hove et al., 1954, 1957~. A progressive muscular dystrophy has been reported in rabbits fed a choline-deficient diet for more than 70 days (Hove and Copeland, 1954~. Vitamin Bus Although common rabbit diets are practically devoid of vitamin B,2, a large urinary and fecal excretion of this vitamin has been demonstrated (Kulwich et al., 1953; Rosenthal and Cravitz, 1958~. The amounts are such that
Nutrient Requirements of Rabbits 9 the rabbit should not be dependent on a dietary source. Simnett and Spray (1961) have shown that vitamin Be serum, fecal, and urinary levels are influenced by the nature of Me diet, particularly its content of cobalt, which, of course, is required for the synthesis of the vitamin. Ascorbic Acid (Vitamin C) Win respect to ascorbic acid requirements, the evidence indicates that the rabbit does not require a dietary source of vitamin C. Harris et al. (1956) demonstrated that young rabbits kept for periods up to 25 weeks on an ascorbic acicl-free diet gained weight normally and continued to excrete considerable amounts of the vitamin in their urine. This confirms the earlier observations of Nelson et al. (1922) and Hogan and Ritchie (1934~. Biotin A deficiency of biotin, characterized by loss of hair and dennatitis, occurs in those cases where raw egg white was fed over a period of time (Lease et al., 19371.