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Foxes : Recommenclec! Dietary Allowances ENERGY The general principles discussed previously for mink apply equally well to foxes. The provision of adequate energy is a primary requirement, and it is logical and necessary to re- late the diet content of nutrients to the energy density of the diet. For explanation of energy-related terms see pages 3 through 4. Published information on energy requirements is quite limited. The use, by different authors, of different methods of expressing recommendations and intakes creates difficulties in the comparisons of even these limited data. Early estimates of the daily gross energy (E) requirements of mature foxes in North America were 111 to 121 kcal per kilo- gram of body weight (Palmer, 1928) and 121 kcal per kilo- gram of body weight (Hodson and Smith, 1942~; the latter authors also expressed their estimate of energy requirement as 2,102 kcal per square meter of body surface. The previous edi- tion of this publication (NRC, 1968), on the basis of the figure of 121 kcal per kilogram of body weight and estimates of aver- age animal weight and daily feed intake, recommended that a maintenance diet for foxes should contain 3,227 kcal of E per kilogram of dry matter. Insufficient data were available to war- rant recommendations for growth, pregnancy, or lactation. Perel'dik et al. (1972), using data from various Russian ex- perimental measurements of energy conversion by adult foxes, calculated the requirements of metabolizable energy (ME) for maintenance and claimed that these requirements were in agreement with those determined by Hodson and Smith (1945) and others, although expressed in different terms. The figures reported by Perel'dik et al. (1972) for adult mainte- nance were as follows: June to August September to October November December - 93 kcal of ME per kilogram live weight - 81 kcal of ME per kilogram live weight - 72 kcal of ME per kilogram live weight - 65 kcal of ME per kilogram live weight More data are available on the requirements of female than of male adult foxes. Perel'dik et al. (1972) listed recommended intakes of ME for female silver foxes and for male and female blue foxes of typical weight under practical conditions throughout the year. These recommendations, summarized in Table 16, include an additional variability allowance of 10 percent for "farming conditions." Although there are a few in- consistencies, the recommendations reflect the preference for reduced energy intake in the winter months with the intent of producing thinner animals for the breeding periods, as well as the fact that the requirements of blue foxes are slightly higher than those for silver foxes, at least at certain times of the year. Although a requirement for additional energy intake during pregnancy is commonly recognized, there is little agreement on the additional levels required. Perel'dik et al. (1972j reported a variety of recommendations from different investi- gators. Some investigators recommended different energy in- takes in the first and second halves of pregnancy, e. g., Abramov (1950) recommended 495 and 630 kcal of ME daily for these respective periods. However, Firstov and Kharitonov (1957) favor 560 kcal of ME per day for the whole period. During the lactation period the energy requirement of the female to provide adequate milk for the suckling pups is pro- portional to the number of pups and increases as the age of the pups increases. Various workers have, therefore, recommended that lactating females receive incremental energy intakes per pup suckled, with the increments increasing successively per week or per 10-day interval of the lactation period. Perel'dik et al. (1972), combining data from Rimeslatten in Norway and from practice on Russian fur farms, recommended that the energy intake of lactating females consist of an allowance of 450 kcal ME per day for maintenance plus the following variable amounts for successive 10-day period of lactation: 10-Day Periods Additional Energy of Lactation per Pup Daily (kcal ME)a 1st period 2nd period 3rd period 4th period 5th period 6th period 52 123 195 292 392 450 a Over the whole lactation period, the above recommendations represent an average of 250 kcal ME per pup per day. 24
Nutrient Requirements of Mink and Foxes 25 Diets supplied to fox pups after weaning must supply the re- quirements of energy for both maintenance and continuing growth. As the body weight increases, the maintenance re- quirement steadily increases up to about the seventh month of age. In addition, sufficient energy must be added to provide for growth that will occur at a gradually declining rate from the second to the eighth month of age. Perel'dik et al. (1972), working from Russian calorimetry data on maintenance re- quirements and calorific values of the weight increases ob- tained by Perel'dik (1950), calculated the total energy allow- ances for growing silver fox pups. These allowances, as shown in the following table, include an extra practical allowance of 15 percent for "farm conditions" and apply to both sexes. In practice, these allowances would be obtained by the pups from varied intakes of a diet of appropriate energy density. Somewhat higher allowances, largely because of a 30 percent higher maintenance requirement and a faster rate of growth, were recommended for blue foxes. Age of Pups (months) Live Weight at Beginning of Month (kg) Total ME Allowance (kcal/day) 2-3 3-4 4-5 s-6 6-7 7-8 1.80 3.00 4.10 s.oo s.7s 6.00 4so see 630 660 s60 490 Studies by Mamaeva (1958) indicated that the body weights of foxes late in the growing period were similar under two dif- ferent patterns of energy feeding, moderate early levels versus high early levels. The latter pattern, however, produced faster early weight increases and resulted in animals that were longer, taller, and lower in body fat than did the other pat- tern, which produced more uniform weight increases over the growing period. Moderate deficiency of energy intake causes retardation or cessation of growth; severe deficiency results in emaciation. The fur may be dull if the energy supply is inadequate. Milk yield in lactating animals may be reduced by inadequate energy intake. FATS Widely different levels of fat have been reported to give satis- factory results. Obviously, variations in the proportions of fat in the dry matter of the diet will be the major factor in adjust- ing the energy density of diets. Up to 44 percent of fresh fat was used in fox diets without any detrimental effects (Bassett, 1951~. Rancid fat should not be used, as it can create a vitamin E deficiency. Perel'dik et al. (1972) recommended that fat be used to meet particular dietary requirements, at levels supplying from 23 to 49 percent of the digestible energy (DE). Rimeslatten (1976) reported that the ME energy from fat had a higher productive value than that from either carbohydrates or protein. It has been estimated that silver foxes should receive a minimum of 2 to 3 g of essential fatty acids (linoleic and linolenic acids) daily to prevent hyperkeratosis and dandruff (Ender and Helgebostad, 1951~. CARBOHYDRATES Limited studies of the possible requirements for carbohydrates or of the possible limitations of their use in fox diets have been conducted. Earlier reports (Schaefer et al., 1947a; Tove et al., 1949) indicated that an experimental diet containing up to 66 percent sucrose was apparently adequate. Most practical diets for foxes will contain much lower levels of carbohydrate, pri- marily starch. Perel'dik et al. (1972) recommended, for a vari- ety of diets for silver foxes, carbohydrate levels supplying be- tween 14.3 and 35.0 percent of the DE; for blue foxes, the recommended proportions of DE supplied by carbohydrate ranged from 12.4 to 31.3 percent. It has been generally observed that foxes can utilize carbo- hydrate better than can mink (Rimeslatten, 1951), and fox diets usually contain higher levels of carbohydrate than do those for mink. Rimeslatten (1976) has generalized that the percentage of ME supplied by carbohydrate can be 5 to 10 per- cent higher in fox than in mink diets. PROTEIN A detailed commentary on the critical importance of protein quality in fur animal nutrition is presented in the mink section of this report. Maintenance Studies by Rimeslatten (1976a) indicate that, with proper energy balance, there are no significant differences in body weight of blue foxes raised from weaning to pelting on diets containing from 22 to 45 percent of ME from digestible pro- tein (for calculations see Table 9~. Thus it appears that for the blue fox the maintenance protein requirement, as opposed to that required for growth, would be met by diets containing 22 percent of ME from digestible protein. Gestation Rimeslatten (1976b) has indicated that estrus, breeding, and reproduction of blue foxes are not significantly influenced by alterations in digestible protein concentration ranging from 25 to 40 percent of ME. A slight reduction in litter size at birth was observed when vixens were fed protein levels below 31-32 percent of the ME; however, this difference was not judged to be significant. On the basis of these observations, Rimeslatten recommended a minimum protein level of 30 percent of ME. Lactation Rimeslatten's studies (1976b) have indicated reduced weight gains of fox pups when their dams received diets containing protein levels below 30 percent of ME.
26 Nutrient Requirements of Mink and Foxes Early Growth (7-16 Weeks) Harris et al. (1951a) found that more than 40.7 percent pro- tein (dry weight basis) was required to attain maximum nitrogen storage in fox pups between 7 and 23 weeks of age. However, growth of foxes on a diet containing only 24.5 per- cent protein was equivalent to that of foxes on higher protein levels. Rimeslatten (1976b) has observed that fox pups raised on diets containing less than 28-30 percent ME from digestible protein attained normal body weight but displayed reduced body length. Late Growth and Fur Development (16 Weeks to PeltingJ Rimeslatten (1976b) has recommended a level of 25 percent ME from digestible protein for the period from 16 weeks of age to pelting. Fur development and quality were not signifi- cantly affected by dietary protein concentrations within the range of 26 to 38 percent ME from digestible protein. Data presented in Tables 3 and 4 represent minimum pro- tein requirements of foxes during different phases of the life cycle. Fox producers and fox pellet manufacturers may wish to include higher concentrations of protein, thus providing a margin of safety above the minimum protein requirement. FAT-SOLUBLE VITAMINS Vitamin A (RetinolJ The minimum amount of vitamin A necessary to prevent ner- vous signs in young foxes lies between IS and 25 IU per kilo- gram of body weight per day (Smith, 1942~. This vitamin is not stored in the liver until 50 to 100 IU vitamin A per kilogram of body weight per day are fed. Although a fox can apparently utilize carotene as a source of vitamin A, the carotene is poorly assimilated (Coombes et al., 1940~. When carotene is being used to satisfy vitamin A re- quirements of foxes, a conversion factor of 6.0 should be ap- plied to compensate for the inefficiency of utilization of caro- tene (Bassett et al., 1946~. Until further data are available, it is recommended that growing foxes be supplied at least 100 IU of vitamin A, or 600 IU (360 ,ug) of §-carotene, per kilogram of body weight per day. During rapid growth, the requirement would be 66 IU of vitamin A per 100 kcal ME. A fox can tolerate large doses of vitamin A (Helgebostad, 1955~. A dosage of 40 IU of this vitamin per gram of body weight, administered daily over a period of 3 to 4 months, pro- duced no toxic signs; 200 IU per gram of body weight, admin- istered daily over a period of 1 to 2 months, produced signs of hypervitaminosis A in pups. Signs of excess were anorexia, bone changes with exostoses, decalcification and spontaneous fractures, loss of fur, exophthalmia, cramp, and local hyper- esthesia of the skin. Signs of Deficiency Foxes fed a diet deficient in vitamin A develop a series of nervous derangements, usually manifested in this order: first, trembling or cocking of the head; next, unsteadiness (resulting from a disturbed sense of balance); then a tendency to run in circles (Smith, 1942~. Often, in attempting to observe an object behind them, de- ficient animals, instead of turning around in the normal fashion, jerk their heads over their shoulders, lose their bal- ance, and fall over. These signs usually begin 1 to 5 months after the animals are placed on a diet deficient in vitamin A. Later, certain ani- mals, after being excited, exhibit increased nervousness until they pass into coma or tetany, which lasts S to 15 minutes. The nervous signs are not eliminated by feeding or injecting vita- min A. Typical xerophthalmia occurs after 4 to 6 months. The deciduous and permanent teeth of foxes deficient in vitamin A are present in the same socket, and in many in- stances the adult incisors are small, discolored, and chipped or broken on the corners (Bassett et al., 1946~. Another sign is a high incidence of urinary calculi. Vitamin D Harris et al. (19Slb) found that a diet of natural foodstuffs that assayed 0.82 IU of vitamin D per gram, or 22 IU per 100 kcal ME, was adequate for growing foxes; the diet was fed without a vitamin D supplement. A daily supplement of 200 IU of vita- min D per kg of body weight with this diet did not prevent ra- chitic changes when calcium or phosphorus was deficient, and it did not improve physiological responses at adequate mineral levels. Vitamin D supplementation of practical fox diets, therefore, does not appear necessary. Signs of Deficiency Rickets can be produced in foxes by feed- ing diets having low vitamin D content and abnormal cal- cium-to-phosphorus ratios (Hanson, 193S; Ott and Coombes, 1941 Smith and Barnes, 1941; Harris et al., 1945, 1951b). Signs of rickets in growing kits are generally seen between 2 and 4 months of age. Vitamin E Vitamin E requirements of foxes have not been determined. However, it appears that when good-quality foodstuffs are given, supplemental sources of vitamin E are unnecessary for growth or reproduction. Vitamin E levels in the diet are re- lated to the presence or absence of other nutrients and antioxi- dants. (See discussion in sections on vitamin E and antioxi- dants, mink section.) Signs of Deficiency Experimental production of yellow fat disease in foxes has been described (Ender and lIelgebostad, 1953~. The disease is often characterized by hemorrhagic diathesis and red, swollen, hemorrhagic gastrointestinal mu- cosa with or without ulcers. Calcium incrustations are found in the endothelium of the large vessels and in the muscles and kidneys. Vitamin K The requirements for vitamin K have not been determined for foxes. Perel'dik et al. (1972) reports that, on farms where silver
and blue foxes were born with subcutaneous hemorrhages, the enrichment of the diets of pregnant females with vitamin K was beneficial. WATER-SOLUBLE VITAMINS ASCOTbiC Acid (Vitamin C) Lack of vitamin C has no visible effect on the health of grow- ing foxes or on the quality of their fur (Mathiesen, 1939, 1942~. However, there is a relation between vitamin A and ascorbic acid in the nutrition of foxes (Bassett et al., 1948), which sug- gests that a deficiency of vitamin A in the diet will reduce tis- sue synthesis of vitamin C below physiological requirements. Biotin In a study of biotin deficiency signs of foxes, Helgebostad et al. (1959) used a diet that supplied a total of 30 percent of protein as raw egg white. This diet was fed to pregnant foxes, with the result that some of the young were born with changes in hair color (Figure 10~. Some of those born with normal hair devel- oped the changes in color after receiving the egg-white diet themselves. The condition rapidly improved in all deficient animals that were given 1 mg of biotin twice a week. At au- topsy, the liver showed extensive fat infiltration in deprived animals. There were also large amounts of sudanophilic sub- stances in the kidneys and myocardium. This confirmed earlier work (Gunn, 1948) in which 25 percent of raw egg powder was used in the diet of fox pups as the source of animal protein. Gunn found that deficiency signs developed after 12 to 15 weeks. The pups showed graying and loss of fur over the body and tail, eye infections, and gray muzzles. Finally, the legs became weak. Adding 5 percent of yeast to the diet or pressure cooking the egg powder greatly reduced the signs. Folic Acid Folic acid is required in the fox diet. It is suggested that 0.2 mg per kilogram of dry diet, or S.2 ,ug per 100 kcal ME, be ac- cepted as the tentative requirement (Schaefer et al., 1947a). Folic acid conjugates are incapable of replacing folio acid in the fox diet (Tove et al., 1949~. Signs of Deficiency Foxes fed a purified diet deficient in folio acid develop anorexia, loss of body weight, and a decrease in hemoglobin and in red and white blood cells (Schaefer et al., 1947a). This condition causes death if it is not treated. Niacin Schaefer et al. (1947b) have suggested that foxes be fed 0.39 to 2.0 mg of niacin (as calculated from dose feedings) per kilo- gram of body weight per day. It is suggested that 10 mg per kilogram of dry diet, or 0.26 mg per 100 kcal ME, be accepted as the tentative requirement. Signs of Deficiency Foxes fed a purified diet deficient in nia- cin show anorexia, loss of body weight, and typical black Nutrient Requirements of Mink and Foxes 27 FIGURE 10 Biotin deficiency. The newborn fox pup on the left is from a biotin-deficient dam that received a diet containing raw egg white. Thin, gray pelt and deformed legs are apparent. A control diet containing cooked egg white was fed to the dam of the pup on the right, which is also newborn. SOURCE: A. Helgebostad, Veterinary College of Norway, lIeggedal. tongue, which is characterized by severe inflammation of the gums and fiery redness of the lips, tongue, and gums. If the animals are not treated with niacin, they develop severe diar- rhea, pass into a coma, and die (Hodson and Loosli, 1942; Schaefer et al., 1947b). Pantothenic Acid Schaefer et al. (1947b) have suggested that foxes require be- tween 2.S and 15.0 mg of pantothenic acid per kilogram of diet. Until further information is available, it is suggested that the tentative requirement be 8.0 mg per kilogram of dry diet, or 0.21 mg per 100 kcal of metabolizable energy. Signs of Deficiency Signs of deficiency include cessation of growth, and coma (Schaefer et al., 1947b). Death is sudden. Necropsies reveal gross fatty degeneration of the liver, catar- rhal enteritis, and cloudy swelling and congestion of the kidneys.
28 Nutrient Requirements of Mink and Foxes Riboflavin Experiments designed to show the importance of some of the B-complex vitamins in nutrition of foxes indicate that the min- imum and maximum levels of riboflavin for pups are greater than 1.25 and less than 4.0 mg per kilogram of diet (Schaefer et al., 1947b). In other experiments, a basal diet calculated to contain 1.3 to 1.6 mg per kilogram produced riboflavin defi- ciency in the blue fox (Rimeslatten, 1958~. The requirement per 100 kcal of ME is at least 0.1 mg for larger pups and 0.15 mg during pregnancy and lactation. Signs of Deficiency Riboflavin deficiency in foxes results in a decreased rate of growth within 2 weeks after the animals are placed on a deficient diet (Schaefer et al., 1947b). After 3 or 4 weeks, signs of muscular weakness, chronic spasms, and coma occur. When riboflavin is administered, animals will recover within a few hours. If foxes are further maintained on the defi- cient diet, opacity of the cornea and a decrease in pigment production in the fur are noted. Rimeslatten (1958) showed Mat a riboflavin deficiency will develop in litters of blue foxes fed a diet containing a poor- quality sun-dried eviscerated cod meal. Additions of B-complex vitamins to the diet relieve the deficiency signs. When ribofla- vin is omitted, pups on the diet develop fatty dermatitis, and the fur becomes much paler (sometimes nearly white) in 5 to 7 weeks (Figure 11~. Part or all of the hair falls out, the eye lenses become opaque, and muscular weaknesses develop. When a B-complex vitamin supplement containing riboflavin is fed, both dermatitis and muscular control improve in a few days. In 1 to 2 weeks, a new pigmented pelt begins to develop. The omission of B-complex vitamins other than riboflavin from the supplement produced no harmful effects. T~ · ntamin The minimum requirement of mature foxes for thiamin hydrochloride is 800 lug per kilogram of dry feed, or 21 jug per 100 kcal of ME (Harris and Loosli, 1949~. Kringstad and Lunde (1940) found that 0.2 mg of thiamin per animal per day is sufficient to prevent deficiency signs. The minimum level of thiamin suggested for practical feed- ing of foxes is 1.0 mg per kilogram of drv feed. or 27 up Der 1 no kcal of metabolizable energy. Signs of Deficiency Thiamin deficiency in foxes results in anorexia, weakness, convulsions, and paralysis (Ender and Helgebostad, 1939; Kringstad and Lunde, 1940~. Convulsions may occur just before death, which usually comes 48 to 72 hours after the onset of neurological signs. In advanced stages of the disease, foxes moan as if in great pain. At autopsy, the liver usually exhibits severe degeneration. Bilateral vascular lesions, found principally in the paraventricular gray matter of the brain, are useful in diagnosing the disease. Chastek paralysis is a deficiency disease of foxes and mink. It occurs when certain types of raw fish (see Table 15) are in- cluded in the diet (Green et al., 1941~. These fish contain the enzyme thiaminase. When they are included in a mixed ra- tion, the enzyme destroys the thiamin in the diet. The disease ~ ~_ ~ e ~ ~ ~ FIGURE 11 Riboflavin deficiency. Right after 7 weeks on a diet deficient in riboflavin, 12-week-old blue fox showed depigmentation, shedding of fur, and dermatitis. Left: littermate was fed the same diet supplemented with riboflavin. SOURCE: H. Rimeslatten, Agricul- tural College of Norway, Vollebekk. can be prevented or cured by feeding or injecting thiamin. When whole carp is fed to foxes at a level of 20 percent of the wet diet, 5 to 10 mg of thiamin hydrochloride per animal per day are required to prevent Chastek paralysis (Green et al., 1941~. It is suggested that 0.5 to 0.75 mg of thiamin hydro- chloride in sterile distilled water be injected subcutaneously into each fox to cure thiamin deficiency and that subsequently the animals be given a diet containing the recommended amounts of thiamin (Harris and Loosli, 1949~. Cooking the fish at 83°C (181°F) for at least 5 minutes destroys the thi- aminase and renders them safe. vitamin Be A level of 2.0 mg of pyridoxine per kilogram of diet will pre- vent signs of deficiency in foxes (Schaefer et al., 1947b). In the absence of more critical information, it is suggested that 2.0 mg per kilogram of dry diet, or 50 ,ug per 100 kcal of ME, be accepted as the tentative requirement. Signs of Deficiency A pyridoxine deficiency results in ano- rexia, cessation of growth, and a decrease in hemoglobin (Schaefer et al., 1947b).
vitamin B12 No experiments to estimate vitamin BE requirements of foxes have been reported. Practical diets for foxes usually contain sufficient amounts of animal protein. Such diets and addi- tional vitamin BE from intestinal synthesis should provide an adequate supply of this vitamin. M INK RAL S Calcium and Phosphorus The calcium requirement of the growing fox 7 to 37 weeks of age is between 0.5 percent and 0.6 percent in the dry diet (Harris et al., 1945, 1951b). Additionally, the ratio between calcium and phosphorus is important in the nutrition of foxes (Harris et al., 1951b). A calcium-to-phosphorus ratio ranging between 1.0:1.0 and 1.7:1.0 appears to be satisfactory. Ratios outside this range may result in improper growth of bone, even when the diet contains large amounts of vitamin D. Signs of Deficiency Fox pups on a rachitogenic diet develop a stiffness of the rear legs and begin walking on their pasterns rather than on their toes (Smith and Barnes, 1941~. Their leg joints swell, and the leg bones become bent. The ash content of the dry, fat-free ulnas decreases from a normal of 62 to 67 per- cent to 36 to 47 percent. The long bones become soft and vas- cular. Serum calcium falls from 10.8-11.8 mg per 100 ml to 7.2-9.0 mg after 8 weeks on the rachitogenic diet. Harris et al. (1945) have supplied evidence that calcium- deficient foxes show, progressively: lameness, recurrent spasms, Nutrient Requirements of Mink and Foxes 29 crooked legs, arid enlargement of the cranial bones, especially the maxillae and palatines (Figure 12~. The muzzle becomes en- larged, the gums become swollen, and the teeth become loose. Foxes on low-phosphorus diets reveal symptoms of lame- ness, crooked legs, enlarged joints, and a mineral poverty of the bones, shown by X-ray (Harris et al., 1951b). Occasion- ally, an animal develops an undershot jaw. Sodium and Chlorine In the absence of data on the specific requirements of foxes for sodium and chlorine, it has been recommended that the diet be fortified with 0.5 percent sodium chloride (salt) calculated on a dry matter basis (NRC, 1968~. Iron The minimum iron requirement of foxes is not known; how- ever, uncooked fish of the cod family (such as Pacific hake, Atlantic whiting, and coalfish) have been shown to interfere with iron absorption in rats and mink (Costley, 1970~. An in- creased requirement for iron has been observed in silver and blue foxes fed air-dried cod (Rimeslatten, 1959~. In this case supplementation of the diet with liver was reported to be beneficial. Supplementation of the breeder diet with iron sul- fate has been reported to reduce litter size (Goncharenko, 1976~. Iron deficiency signs include anemia and depigmenta- tion of underfur (Rimeslatten, 1959~. Cobalt, Copper, iodine, Manganese, and Zinc In the absence of firm data on the minimum requirements of foxes for these minerals, results of Soviet studies with trace FIGURE 12 Calcium deficiency. Each of these bones is a right humerus taken from a female fox. The bone on the right was taken from a fox that received 0.5 percent calcium in its diet, plus sunshine. The others are from foxes kept in the shade and fed (left to right) 0.16, 0.2, 0.3, 0.4, and 0.5 percent calcium in the dry diet. SOURCE: L. E. Harris, Utah State University, and C. F. Bassett, USDA.
30 Nutrient Requirements of Mink and Foxes mineral fortification of standard fox diets are pertinent. All values are in units of weight of supplement per kilogram of animal body weight per day. Simultaneous supplementation of the fox breeder diet with cobalt (0.5 mg CoCl2), manganese (1.0 mg MnSO4), and zinc (0.2 mg ZnSO4) has been reported to decrease pup mortality (Berzins, 1957~. However, superim- posing copper (0.5 mg CuSO4) supplementation upon the pre- vious treatment has been reported to increase pup mortality (Berzins, 1967~. Supplementing the breeder diet with iodine (0.05 mg KI), cobalt (0.5 mg CoCl2), and a considerably lower level of copper (0.05 mg CuSO4) from 10 days before breeding through pregnancy and lactation has been reported to increase litter size (Zotova, 1968~. Low level cobalt (0.5 ,ug CoCl2) and iodine (0.1 ,ug KI) supplementation of grower diets was reported to increase pup size and improve fur quality (Bukovskaya, 1969~. These experiences relate to a typical diet of meat, fish, and cereal and may or may not be reproducible in diets of other composition. For a discussion of feed additives, disorders related to nutri- tion, and toxic substances in the feed supply pertaining to fox nutrition see pages 18 through 23.