High dietary ash content decreases fat digestibility in the mink

This study clarifies the effect of high dietary ash and calcium on fat and fatty acid digestibilities in the mink. Differences between various calcium sources are also evaluated. We performed three digestibility experiments by the total collection method with 30 adult male minks of standard genotype. Each experiment had two dietary fat sources, beef tallow and rapeseed oil, and three different ash levels, 4, 8, and 14% in dry matter (DM) of the diet. Ash level in the diets was increased either with limestone grist (experiment A), fish offal meal (experiment B), or bone meal (experiment C). Fat level in the diets was 20% in DM. In experiment A, the digestibility of beef tallow decreased from 76 to 67% and the digestibility of rapeseed oil from 94 to 85% with increasing ash level. In experiment B, digestibility coefficients for beef tallow and rapeseed oil varied between 67-70 and 94-95, respectively. In experiment C, the digestibility of beef tallow decreased from 87 to 66% and that of rapeseed oil from 96 to 94% with increasing dietary ash content. The amount of saponified fat in feces increased in all experiments with increasing dietary ash and calcium. In the beef tallow diets, however, the amount of saponified fat was significantly higher than in the case of rapeseed oil. There were also differences between ash sources in their activity to form soaps in the gut environment. Limestone grist and bone meal seem to be more reactive than fish offal meal. Not only is the level of ash and calcium important but also the source and fatty acid composition of the dietary fat. Saponification also seems to interfere during fatty acid analysis causing a distortion of varying magnitudes depending on the dietary ash and fat sources and the extraction methods used. The digestibility coefficients determined in this study for individual fatty acids are thus more reliable the lower the dietary ash content is and the more unsaturated the fat source in question.