Effects of varying vicine, convicine and tannin contents of faba bean seeds (Vicia faba L.) on nutritional values for broiler chicken

Faba bean tannins are known to decrease apparent protein digestibility. The vicine and convicine contained in the seeds reduce egg size in laying hens, but their effects in broiler chickens have never been reported. Vicia faba genotypes have been identified which result in a major reduction of these compounds in seeds. Tannins are located in seed hulls whereas vicine and convicine (VC) are located in the cotyledons and therefore they can be separated and re-combined in reconstituted faba beans (RFB) by mixing hulls and cotyledons from genotypes with variable tannin and VC contents, in order to evaluate their effects alone or in interaction. Six RFB were built with high, medium or low tannin contents (means of 9.9,5.8 and 1.3 g/kg, respectively) combined with high or low VC contents (means of 10.1 and 0.7 g/kg, respectively). The six RFB were incorporated in the experimental diets and fed to broiler chickens in a balance experiment to measure apparent protein digestibility and apparent metabolisable energy (AMEn) content. The AMEn was lower for the high- than for the low-tannin RFB (12.08 MJ/kg versus 12.48 MJ/kg DM; P < 0.001). Also, the AMEn was lower for the high- than for the low-VC RFB (11.99 MJ/kg versus 12.34 MJ/kg DM; P < 0.05). The negative effects of the two anti-nutritional factors on energy were additive and a linear regression on their contents explains 80% of the variability. The total tract apparent protein digestibility was lower for the high-than for the low-tannin RFB (0.75 versus 0.86; P < 0.001) with no significant interaction with VC contents, which in contrast have no significant effect on this trait. The results confirm the nutritional advantages of faba bean varieties free of tannins and low in vicine and convicine content in diets for poultry. (C) 2008 Elsevier B.V. All rights reserved.