Preservation Efficiency of Formic Acid and Benzoic Acid in the Ensiling of Slaughterhouse By-Products and their Subsequent Metabolism in Farmed Fur Animals

The efficiency and feasibility of organic acids, especially formic and benzoic acids in the feeding of fur animals was investigated in five experiments, where the main emphasis was focused on the metabolism and rate of elimination of these preservatives, in order to assess practical dietary recommendations. Experiments were carried out with mink (Mustela vison), blue foxes (Alopex lagopus) and raccoon dogs (Nyctereutes procyonoides). Formic acid (85% w w) alone (6 g kg, pH 4.0) stabilized slaughterhouse by-products, but only in the short term. The effect against yeasts was marginal and a regrowth of aerobic bacteria attributable to yeast growth and subsequent increase in pH resulted in organoleptic spoilage of the silages within 35 d irrespective of storage temperatures (4 and 20 °C). Additional propionic acid (2 g kg) enhanced preservation efficiency, but benzoic acid (2 g kg) was more effective against yeasts. In practical applications a combination of 10 g kg formic acid and 3 g kg sodium benzoate stabilized boneless by-products for several months. Ensiling improved apparent digestibility of fat of the boneless byproducts in mink, but for high-ash by-products it resulted in the hydrolysis of fat, indicative of the formation of insoluble calcium salts. Oxidation tests in mink showed that the mink oxidizes formate at a rather low rate. Based on the average oxidation rate of 30-35 mg kg BW h, the maximum daily formic acid (85% w w) intake was approximated to be 1 g kg BW. In blue foxes, based on the half-life measurements in blood, a maximum intake was estimated at 2.2 g kg BW. Regardless of the different methods of investigation, the results suggest that blue foxes metabolize formic acid more efficiently than mink. However, both in mink and blue foxes metabolism of formate appeared to be less efficient than in previously studied nonprimates which warrants a close monitoring of dietary concentrations in practice, particularly during periods of high feed intake. For optimal metabolism of formate, both mink and blue foxes should receive supplemental folic acid, more than is conventionally recommended for the prevention of folic acid deficiency. The results concerning the practical production trial are consistent with observed formate oxidation rates and earlier research. Dietary formic acid concentration of 5 g kg appeared to be a threshold level for preweaned and weaned mink kits. At the threshold concentration during the intensive growth period performance is temporarily deteriorated by formate irrespective to defects in production parameters at pelting. Based on the number of born and weaned kits per female and the histologically evaluated developmental stage of spermatogenesis, the level of formic acid ensiled silage did not have negative effect on reproductive performance of mink. Furthermore, reproductive performance was not affected by dietary folic acid level (1.5 and 10 mg kg DM). Weaning weights of male kits were negatively affected by the highest silage level that contributed approximately 4.5 g kg formic acid. With respect to the elimination of benzoate, none of the studied mink, blue foxes or raccoon dogs showed signs of sensitivity, but there were inter-species differences in benzoate elimination. Blue foxes eliminated large amounts of benzoate as hippuric acid (benzoylglycine) across all dietary levels whereas its proportion decreased in mink and raccoon dogs. In raccoon dogs, the proportion of hippuric acid was also initially low. In blue foxes 10% of dietary benzoate appeared unbound in urine, a value double that observed in mink and raccoon dogs. Furthermore, in this species, faecal percentage of free benzoic acid increased with dietary level and accounted for 15% with the highest dietary level of 4.15 mmol kg. Dietary glycine contents encountered in practice did not limit urinary hippuric acid excretion.