Digestibility coefficients of crude nutrients in raw hulled acorns (Quercus pubescens Willd.) fed to growing pigs

Acorns (fruits of the Genus Quercus) are widely used in some Mediterranean countries as feed for pigs raised under extensive conditions. Despite the traditional use of acorns for fattening pigs, little is known about the digestibility coefficients of nutrients of this feed source. A feeding trial was carried out to determine the apparent total tract digestibility (ATTD) of crude nutrients in growing pigs fed with an acorn-based diet (70% raw hulled shredded acorns combined with a complete pelleted diet for pigs, as fed). The acorns were observed to contain about 510 g of digestible organic matter per 1 kg DM, a value lower than predicted from the fibre content (CFi) in raw acorns (y = 92 − 1.68 × CFi in DM). The digestible organic matter was found to be based on starch mainly, and to a lesser extent, on digestible fibre and digestible fat. On calculation, the metabolizable energy content (MJ ME/kg DM), based on the results of the digestibility of raw acorns, was observed to be 9.5 MJ ME/kg DM, making it comparable to sugar beet pulp or to about 70% of the energy density of cereals such as wheat, or to about 75% compared to barley. A negative value for crude protein ATTD (−30%) was highlighted: the high nitrogen faecal losses observed, perhaps due to the high tannin contents (51.2 g tannic acid equivalents/kg DM in raw acorns), can reasonably be considered to be due to the bacterial proteins synthesized during the increased microbial fermentation in the hindgut, then shed via faecal material. Due to the digestibility coefficients of nutrients, acorns may well be more suitable for use in combined diets for fattening pigs, rather than in diets for growers.