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Determining Nutritional Requirements The nutrient requirements listed in this publication are recom- mendations based on experimental evidence. Results of studies on nutrient requirements are reported in the text in the units in which they were originally published. The tables on nutrient requirements present the values on the basis of comparable digestibility and energy levels. Methods of calculation or estimation are shown in the appendix of each table. In gen- eral, the investigators who provided such evidence followed standard procedures: · Conclusions were based on the performance of groups of animals. · Performance responses, such as gains in body weight or efficiency of feed conversion, were arrived at on the basis of average values from such animal groups. · Deficiency signs, when used as criteria of nutrient inade- quacy, relate to appearance of such signs in an experimental group and complete absence of signs in groups receiving the specific nutrient considered. The subcommittee believes that the values stated herein for the various nutrients will permit maintenance of normal health and productivity in animals. It must be recognized, however, that adequate nutrition is dependent on other fac- tors in addition to the diet composition per se. While the values given have not been increased by any arbitrary "safety factors," fur farmers or feed manufacturers catering to them may wish to increase the concentrations of nutrients to offset effects of stress factors such as extremes of weather or in- cidences of certain diseases or parasitism. Such increases may be particularly applicable in cases of nutrients that are known to be unstable and subject to deterioration in feed storage, pro- cessing, or handling. Recommendations on nutrient requirements are necessarily related to the size of individual animals and to their specific production activities, e.g., reproduction, lactation, growth, furring. Unfortunately, base data on the nutrient needs for specific physiological states are often lacking. In practice, although production activities throughout an animal unit may be reasonably consistent, the sizes of individual animals may vary considerably. In particular, because of the great dif- ferences in size and growth rates of males and females, specific requirements have been given separately for the two sexes. Consequently, the data presented may need to be adjusted and should, therefore, be treated as guides to adequate nutri- tion, rather than fixed standards to be rigidly followed. Requirements are expressed on three bases: percentage of dietary dry matter (DM), or amount per kilogram (kg) of DM fed (Tables 1 and 3), daily nutrient requirements per animal (Tables 2, 5, and 6), and amounts per 100 kilocalorie (kcal) metabolizable energy (ME) (Table 4~. This last method of ex- pression has been added because both mink and foxes are fed varying amounts of fat, and this has a strong influence on energy concentration of the total diet. When the energy con- centration is increased, other nutrient contents must be in- creased also, as the tendency will be for the animals to con- sume lower quantities of the diet. This matter is discussed in the following section. NATURE AND FUNCTION OF ENERGY: ITS CENTRAL ROLE IN NUTRITION Energy is not itself a nutrient but is, rather, a property con- tributed to diets by the three macronutrients: fats (lipids), car- bohydrates, and proteins; it is measured as a physical property and expressed in either kilocalories (kcal) or kilojoules Skid. One kcal is equivalent of 4.184 k]. Of these three energy sup- pliers, the most concentrated source is fat, a unit weight of which supplies more than twice as much energy as the same weight of carbohydrates or proteins. The animal requires feed energy for body heat, for body biochemical reactions, for physical activity, and, as the life- stage situation dictates, for one or more of the following: growth, fur production, reproduction, and lactation. In the absence of adequate available energy supplies, the perfor- mance of the animals in these life phases will be suboptimal. Thus, as in all other animal species, the role of feed energy is central to the metabolism and performance of mink and foxes. Failure to recognize this vital fact in much of the earlier ex 2
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perimentation on nutritional requirements of mink very seriously limits the applicability of some of the data and the confidence in the conclusions drawn from those studies. Responses to improved levels of various nutrients may be masked or biased because of insufficient energy supplies in test diets. Unfortunately, the reports of numerous mink-feeding experiments, especially those evaluating the use of practical feedstuffs, lack clear statements on the energy content of the diets involved. Subsequent calculations of approximate energy content of such diets, where published information made this possible, have suggested that many of the reported shortcom- ings in performance were due, at least in part, to inadequate dietary energy concentration. Similarly, many studies on requirements of protein or of mi- cronutrients or on responses to nonnutrient additives or con- taminants have used diets either too low or entirely uncon- trolled in energy content. Reported results from such investigations may, thus, be unreliable or misleading. Recognition of the central role of energy has led to the prac- tice of expressing nutrient requirements and recommendations on the basis of energy (i.e., nutrient-to-energy ratio). Insofar as practically possible, that principle will be followed in this publication. EXPRESSION OF ENERGY CONTENT OF DIETS AND ENERGY REQUIREMENTS Several methods of stating energy content of fur animal diets and the energy needs of the animals have been used by dif- ferent investigators; this has posed difficulties for those wishing to interpret and compare statements and has caused confusion in making practical application of the information. Figures for energy values of feeds have been stated in three different ways: (1) gross energy (E), which is the total com- bustible energy of feeds determined in a bomb calorimeter; (2) digestible energy (DE), which is that portion of the feed E that does not appear in the feces; and (3) metabolizable energy (ME), which is that portion of the feed E that is utilized by the animal for maintenance, production, and energy storage, as it is not lost in either feces or urine. ME, it is agreed, is the best of these measures. Whichever of the three measures is used, energy is most ap- propriately expressed as kilocalories per gram (or per 100 g, or per kilogram) of dry matter in the feed. Among the many forms used for stating requirements or recommended intakes of energy by the animals are the follow- ing: . 1. in relationship to composition of the ration kcal of E or DE OF ME per stated weight (1 g or 100 g or 1 kg) of diet dry matter. 2. daily energy intake in relationship to body size of the animal kcal of E or DE or ME per 100 g (or per kilogram) of body weight. 3. daily energy intake in relationship to metabolic body size (MBS) of the animal kilocalories of E or DE or ME per kilogram of metabolic body size. Nutrient Requirements of Mink and Foxes 3 Much scientific work has shown that the most suitable basis for expressing requirements of energy in an animal is the metabolic body size (MBS) of the animal, rather than the ac- tual body weight (BW). MBS, a concept of the metabolizing mass of the body, is defined as body weight (in kilograms) raised or calculated to the 0.75 power or, as commonly ex- pressed: MBSkg = BWkg0 75 (Kleiber, 1947~. This basis of MBS is one that does not lend itself readily to practical applications, largely because of the introduction of a concept and a calcula- tion with which mink producers may not be familiar. In such situations, actual body weight (BOO) is a more familiar and comprehensible basis; consequently, practical recommenda- tions may be more usefully converted to and expressed on the BW basis, even though investigational results may have been expressed originally on the basis of MBS. This practice has been followed in the present publication. Despite the biological superiority of ME as a measure of energy, especially for research purposes, its practical use is severely limited by the paucity of available data on ME values of common feedstuffs and diets for mink and foxes. (It should be emphasized that published ME values of foodstuffs for om- nivores such as poultry or swine are not valid for the naturally carnivorous mink.) Many more data on E of feedstuffs are readily available, and additional data can be much more eas- fly determined. (E values are independent of species.) In view of the fact that ME values are not strictly additive, i.e., the ME value of a mixed diet is not necessarily the sum of the ME values (even if available) for all the ingredients, it can be argued that relatively little error will be introduced by multi- plying E values for mixed diets by a factor representing a typical experimentally determined ratio between the ME and the E values for such diets. E values are additive. Evans (1967b, 1976, 1977) has found, in numerous experiments, that ME values of conventional North American mink diets usually range from 72 to 85 percent of E values, the overall average being 77 percent. ME as a proportion of E was usually greater for high- than for low-energy diets and for conventional diets than for diets with more vegetable protein. Chwalibog et al. (1979) in Denmark found ME values to be 80 percent of E for diets of low and moderate protein content (18 percent and 34 percent, respectively, of E), but this fell to 72 percent for diets in which 61 percent of the E was supplied as protein. There is obviously general agreement between the Danish and North American (Canadian) values. Considerably more data are available for DE of feedstuffs than for ME, but DE data are still much less available than E data. DE values also are not completely additive and will vary with ration composition and quality of ingredients. Literature reports of digestibility of E of mixed rations ranges from ap- proximately 65 to 88 percent, the lowest generally being for low-energy rations high in vegetable sources and the highest for high-energy rations of high-quality conventional, predom- inantly animal, ingredients. For purposes of estimating dietary E, the following E values for protein, fat, and car- bohydrate of 5.7, 9.5, and 4.0 kcal/g, respectively, may be used. Calculations for estimating DE and ME from these figures are presented in Tables 8 and 9. The expressions of energy requirement in relation to animal weight are, of course, often much more useful for strict
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4 Nutrient Requirements of Mink and Foxes research purposes than are the expressions on the basis of feed composition but, again, for practical purposes the latter are often more readily comprehensible and more useful. Thus energy recommendations in this report are presented in both forms. GROWTH RATES Growth curves for mink (Figures 1 and 2) and for foxes (Figures 3 and 4) are presented as indicators of normal, ex 2400 2200 _ 2000 _ 1 800 _ 1600 _ E 1400 _ - ~ 1 200 _ ~ 1~ _ _ 600 _ 400 _ 200 /' o 0 30 60 90 /' ~//~'' / ''' 1 1 1 1 1 1 120 150 180 210 240 AGE (days) FIGURE 1 Standard dark mink growth curve. SOURCE: N. Wehr, J. E. Oldfield, and J. Adair, Oregon State University, Corvallis. 1 1 00 1 000 900 800 700 - 600 3 500 400 300 200 100 n pected performance. The various color types of mink are not differentiated in these figures, as it is now assumed that they will grow similarly if given adequate nutrition in satisfactory environments. Early growth is extremely rapid for the young mink and constitutes a period of critical nutritional demand. This period is illustrated separately in Figure 2 to allow more detail than is possible in a comprehensive growth curve. Sepa- rate growth data are presented for silver and blue foxes in Figures 3 and 4, respectively. 6000 5000 _ N,dle 2000 _ 1000 . // // /~/ Aft' 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 AGE tweaks) FIGURE 3 Silver fox growth curve. SOURCE: FI. Rimeslatten, Agricultural College of Norway, Vollebekk. 7000: ; 1 _ 1 _ J ~I I I I 0 1 0 20 30 40 50 60 AG E (days ) 6000 _ 5000 _ / o, _ /// 3 ,7/ ~ 3000 _ / _~~ Mild FIGURE 2 Standard dark mink early growth curve. SOURCE: N. Wehr, J. E. Oldfield, and J. Adair, Oregon State University, Cor- vallis. 2000 _ 1000 o it// // // // // //' / if' // // // ~///- M`~o / 0 2 4 6 8 10 12 14 16 18 20 22 24 26 AGE (weeks) FIGURE 4 Blue fox growth curve. SOURCE: H. Rimeslatten, Agri- cultural College of Norway, Vollebekk.
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