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Feec! Aciclitives Feed. additives are considered as nonnutritive substances added' in small amounts to the diet to improve animal per- formance. No requirements are listed for them. ANTIBIOTICS Antibiotics are substances that when fecal at Tow levels modify bacterial populations ire. the digestive tract in favor of d'esir- able microorganisms, while at higher levels they tend to sup- press disease-producing organisms. The response of mink and foxes to dietary antibiotics has been variable and may depend upon the quality of the diet and the health of the animal. Increased growth rate of: weaned kits and improved pelt. quality have been attributed to the feeding. of' aureomyc~n, zinc bacitracin (Bassett and Warner, 1962), and terramycin (Luther,. 1952; Warner et al., 195~8~. Anti- bitotics fed to female breeders have also been. reported to in crease weaning weights; and reduce kit mortality. Wood (1963) and Admix (1955:), however, observed no significant im- provement in growth or- fur quality of mink from feeding several different antibiotics at levels ranging from 0.4 mg to 110' mg per kilogram of diet:. Swedish anti Norwegian. ~;~n- vestigators were also unable to confirm any growth improve- ment: from feeding terramycin, penicillin, or bacitracin from weaning- to pelting, but did note a. growth response in kits where the feed was supplemented with bacitracin during the lactation period (Glen,-lIansen, 1911~. In studies with. foxes (Breirem et al., 1955), aureomycin and penicillin. were found to have no effect on. reproduction or pup growth where fed to vixens, nor did these antibiotics, or terra- mycin, improve growth of' young foxes from weaning to peit- ~ng. According to NorUfeldt et al. (as reported by AitLen, 19~), the- addition of 10 mg aureomycin per kilogram of dry matter of diet frotn weaning to pelting failed to enhance the growth of young foxes, although the growth of nursing pups of vixens given 10 mg penicillin per kilogram of dry matter of' diet was improved. Russian investigations, reported by Aitken (1963), showed that au:reomycin, penicillin, or l:evomycetin fed to foxes en- hanced breeding performance, pup growth, and pelt value and reduced mortality due to digestive disorders. A study by Petersen (1953) also showed improved growth and pelt quality of young foxes by the additions of penicillin (2S,000' IU per kilogram dry matter) tot the diet. There is presently some concern that the continuous feed~.n.g of' low levels of antibiotics to animals may enhance the pro- liferation of resistant strains of microorganisms and thereby reduce the effectiveness of antibiotics in disease outbreaks. Therefore, ~regulations- governing the use of antibiotics in animal feeds are subject to change. ANTIOXII)'ANTS Antioxidant- are adder! to feeds to preserve nutrients and help prevent oxidation of' fat. Horse meat and fish Improperly stored, or stored for prolonged periods, are especially prone to oxida- tion and when fed to mink may cause steat~tis (yellow fat disease). ~ Studies by teekley et al. (1962') have shown that butylated' hy- droxytoluene (BHT), 2,4,5-trihydroxy butyrophenone (THBP), or dehydroethoxy trimethylquinoline (ethoxyquin) when added at a level of 123 mg per kilogram of wet diet were effective in preventing steatitis in mink fed diets that contained high levels of' fish waste. Travis and Schaibl'e (1961) found no adverse ef- fects on reproduct~or>, growth, or fur quality from feeding 0.2 percent BHT or 0.12S percent ethoxyquin (10 times the allow- able level in reedy to mink. Di.phenyI-p-phenylene diamine (DPPD) added to mink diets at 123 ma; per kilogram of wet feed was effective in controlling steatit~s but had a detri.mer~tal effect on reproduction (Leekley and Cabell, 1959~. lg