Comparative digestibility of carbohydrates of microbial products and their metabolisable energy values in chicks and rats

The digestibility of the carbohydrate components and the metabolisable energy (ME) values of two yeasts grown on hydrocarbon (BPG and BPL yeasts) and a filamentous fungus grown on carbohydrate waste (RHM fungus) have been investigated in rats and chicks. The microbial products were included at three different levels. Mannan, and chicks, and glycogen have been shown to be the major constituent polysaccharides of these microbial products. Glycogen in the three samples tested was found to be completely digested by both animals. Mannan, β‐glucan and chitin were less digestible and their utilisation varied according to the level of inclusion in the diet. The difference in the ability of the animals to digest the cell wall carbohydrates or to metabolise the gross energy from the test ingredients was significant (P 0.001). Rats digested up to 68.1 % chitin, 47.4% β‐glucan and 41.8% mannan but the highest value obtained for chicks was 12.9% for mannan. Chitin had the lowest digestibility in chicks varying between 0.7 and 2.9%, whilst values for β‐glucan were between 3.3 and 11.7%. Uncorrected ME values for BPG yeast, BPL yeast and RHM fungus, respectively, were 13 617, 13 190 and 12 194 kJ kg−1 dry matter for chicks and 16 380, 15 903 and 14 107 kJ kg−1 DM for rats. Correction of these values to zero nitrogen retention lowered the values by about 18% in chicks and 8 % in rats. Adjusting the values to 30 and 50% nitrogen retention for rats and chicks, respectively, resulted in values which were very close to uncorrected values.