Protein and Amino Acids in the Nutrition of the Growing-Furring Blue Fox

This thesis evaluates the protein and amino acid requirements of the modern growing-furring blue fox (Alopex lagopus). The effects of dietary protein and the supplemented amino acids, methionine and lysine, on nitrogen (N) retention, nutrient digestibility, growth performance, health, and fur characteristics were estimated in digestibility and N balance trials and production experiments. As well as visual grading of the skin, objective physical determinations of the leather were conducted to provide a complete picture of both the fur and the leather properties. In addition, the ideal pattern and the limiting order of some essential amino acids were determined. An insufficient dietary content of methionine impaired the digestibility of the diet. Supplementing low-protein diets with methionine may therefore increase the metabolisable energy (ME) value of the diet. The DL-form of methionine was effectively utilised by the growing-furring blue fox. In this respect, blue foxes, like dogs, differ from mink. A level of 21–22% protein of ME with 0.40 g DigMet (digestible methionine)/MJ ME (0.50 g of digestible methionine and cystine, SAA) was found to satisfy the requirement for growth, normal hair priming and production of highquality skins with firm and elastic leather. Even 15–16% protein of ME (with 0.40 g DigMet/MJ ME) may be adequate for the blue fox during the later phases of the growing-furring period. Methionine supplementation improved guard hair quality. The effects of lysine supplementation in low-protein diets, 15% or 22% protein of ME, were inconsistent. At 15% protein of ME, and about 0.60 g digestible lysine/MJ ME, supplementation of lysine did not improve performance of the growing-furring blue fox. The dietary deletion of SAA in the N balance trial resulted in severe responses of young blue foxes, exceeding those to the deletion of any other amino acid (lysine, threonine, histidine, tryptophan). The results corroborated those of the production experiments. On typical diets containing slaughter by-products and fish, the first limiting amino acids for the growing-furring blue fox are thus SAA, then histidine followed by threonine and tryptophan (not necessarily in that order) and, finally, lysine. The ideal pattern of amino acids was: lysine = 100, SAA 77, threonine 64, histidine 55, and tryptophan 22. As this pattern is based on measurements conducted on young weaned blue foxes, the relative SAA requirement of blue foxes is likely to be higher later in the growing-furring period (from September onwards) due to the development of winter fur. Neither disturbances in clinical health nor mortality related to experimental diets were found. At the lowest level, 15% protein of ME, the relative weight of liver was increased. This could be prevented by methionine supplementation up to 0.40 g DigMet/MJ ME. The blue fox seems able to adapt to changes in dietary protein and amino acid supply to a greater extent than mink, which is a strict carnivore. Blue foxes use protein primarily for tissue synthesis, not as an energy source. Nitrogen excretion can be declined by 2–3 g per blue fox per day by reducing the dietary protein level from about 25–30% to 22% of ME. The practical application of low-protein diets, with supplemental methionine when required, is recommended. This would be beneficial in terms of reduced feed expenses and lower N emissions to the environment, without compromising the welfare and performance of the growing-furring blue fox.