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Formulating Diets or Feet] Mixtures Diets adequate for mink and foxes should supply sufficient nutrients in the correct proportions to meet the physiological requirements of maintenance, or maintenance plus other bio- logical functions such as reproduction, lactation, growth, and fur production. The range of nutrient levels that can be used to provide satisfactory production is quite wide and permits some flexibility of feeding practice by the fur farmer. If ingre- dients are cheap and abundant, diets may be fed that provide nutrients in excess of those actually required, and such prac- tice may be beneficial in terms of increased productivity. At times, however, it may be economically advisable to feed fur animals to grow at submaximal rates. Similar advantage has been demonstrated in studies of the protein requirement of foxes (Harris et al., 1951a; Rimeslatten, 1976~. Foxes fed a fresh meat diet gained faster than those fed meatmeal halt there was no difference in the ultimate size of the animals or in the size or quality of the pelts (Bassett, 1951~. The values of certain feed ingredients for mink are im- proved by cooking. In some cases, as with cereal grains, cook- ing improves digestibility of the carbohydrates. In others, such as fish known to contain the enzyme thiaminase or eggs that contain avidin (see sections on Biotin and Thiamin. ~D. 13 and 15), adequate cooking overcomes the effects of substances that interfere with normal metabolic availability or action of essential nutrients. In some cases, steam treatment or popping or flaking processes applied to cereal grains involve sufficient heat to produce a higher digestibility coefficient. A number of ingredients such as liver, yeast, skim milk, whey, distillers dried solubles, fish solubles, and fish meal are useful as specific nutrient sources. The use of appropriate com- binations of such feeds may reduce or eliminate the need for further supplementation with trace mineral and vitamin preparations. Conventionally, diet ingredients fed mink and foxes contain varying amounts of moisture, and, because of such variation, it is desirable to calculate diets on a "dry matter" (moisture- free) basis and then convert the amounts to an "as fed" basis or relate the nutrient values to energy content. This may be done by use of the formulas given in Tables 7, 8 and 9. Some sample formulas are given for mink diets in Table 10 and for fox diets in Table 11. Diets or feed mixtures may be calculated by using the data for nutrient requirements presented in Tables 1 to 6 and the data for composition of feeds given in Table 12. It must be remembered that these have been calculated as average data from varying numbers of samples analyzed. Under some con- ditions where variation from normal or expected values might occur, it would be advisable to have specific analyses con- ducted on a feed ingredient. Alternatives exist in the types of feed ingredients that may be used for mink and foxes. Conventional diets contain both fresh and dried materials, but interest has been shown in, and research devoted to, the formulation of diets from dried ingre- dients only. These can be mixed with water, or fed dry as pellets. To date the use of dry diets has not been widely ap- plied in practice. FEED SANITATION Feeding practices are important considerations for optimal growth and performance of mink and foxes. Feed should be kept as fresh and wholesome as possible. In the Scandinavian countries where mink feed is prepared in large central "kitch- ens," the quality of the feed ingredients, as well as the finished feed, is closely monitored according to standards described by Poulsen (1978~. Fresh (wet) feed ingredients require rapid freezing to assure good quality. Freezing fresh products in thin (3"- 4") blocks reduces bacterial growth, as the products can be thoroughly frozen and thawed rapidly. A minimum storage time is desir- able for all feedstuffs, as prolonged or improper storage may result in oxidative rancidity, vitamin destruction, and loss of taste appeal. The use of feed or feed ingredients of ques- tionable quality is inadvisable. When animals are fed conven- tional (wet) diets, any unconsumed feed from the previous feeding should be removed before additional feed is provided. Dietary alterations should be initiated gradually, as sudden 5
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6 Nutrient Requirements of Mink and Foxes drastic changes in the diet may cause the animals to go "off feed." WATER QUALITY Clean fresh water should be available at all times. In addition to avoiding microbial contamination and excessive ammonia, nitrite, and nitrate levels, the fur farmer should seek a water source that does not contain excessive quantities of such minerals as lead, arsenic, cadmium, etc., which might inter- fere with essential mineral balances or general animal health. Availability of water to animals can constitute a problem in extended periods of freezing temperatures. Use of granular or chipped ice or snow may be helpful in such situations.