Tannin/protein interactions : effects on digestibility coefficients and endogenous losses in broilers
Tannic acid was shown to increase weight loss, and total excretion of endogenous dry matter, nitrogen, uric acid, amino acids, energy and minerals (calcium, magnesium and phosphorus) in broilers. There was a positive linear correlation between the amount of tannic acid dosed, and the weight and endogenous losses. A variation in the excretion of individual amino acids and minerals in response to dosed tannic acid was also noted. The mechanisms of increased endogenous losses of the birds as influenced by tannic acid were investigated and discussed. Feeding gelatin and casein significantly reduced the adverse effects of tannic acid on weight and endogenous losses. There were also significant improvements in digestibility coefficients and metabolisable energy values of protein in tannic acid dosed birds when the amount of the protein increased. Although in birds not dosed with tannic acid, casein had a higher digestibility and metabolisable energy compared to gelatin, tannic acid dosed birds showed greater improvements in digestibility coefficients when receiving gelatin. The above contrast was possible because of the higher affinity of gelatin for tannic acid compared to the casein. Polyethylene glycol (PEG) was shown to alleviate the weight and endogenous losses of tannic acid dosed birds not receiving protein. Polyethlene glycol also reduce the endogenous losses as well as improved the digestibility coefficients and metabolisable energy values of tannic acid dosed birds when receiving the lower amounts of gelatin (6g) or casein (6 and 12g). However, the effect of PEG was less apparent in tannic acid dosed birds receiving the higher amounts of gelatin (12 and 18g) and casein (18g). In conclusion, the presence of tannins in poultry diets are likely to increase the endogenous losses of organic materials as well as minerals, but this is likely to depend on the type and amount of diet ingredients such as proteins and additives (eg. PEG).