The hulls (seed coat) of tannin-free, white-flowered and tannin-containing, coloured-flowered faba beans (Viciafaba L.) were used in three experiments (1989–90) to investigate the effect of tannins on the apparent metabolizable energy corrected to zero nitrogen retention (AMEN) and apparent metabolizable nitrogen (AMN) of broiler chick diets.
In Experiment 1, the effects of four rates of hull inclusion from one white- and one colouredflowered cultivar (0, 28, 56 and 84 g/kg) on dietary AMEN and AMN were studied. There was a significant response of AMEN to hull inclusion rate (P <0·001) but no inclusion rate × cultivar interaction. For dietary AMN there was a significant effect of hull inclusion rate (P <0· 001) but no inclusion rate x cultivar interaction.
Experiment 2 consisted of two parts. In both, the total concentration of hulls was 227 g/kg but the ratio of white- to coloured-flowered hulls was varied in five increments; initially over the range 0·42–16·58 g/kg dry matter (DM) and subsequently, with another cultivar, 0·44–7·85 g/kg DM. Synthetic lysine and methionine were added to ensure that the basal diet was not limiting in these amino acids. A significant effect of tannin intake on dietary AMEN and AMN was detected across the greater range of tannin concentrations (P <0·001) but not across the smaller one.
Experiment 3 evaluated three pairs of near-isogenic faba bean lines differing only in the gene governing tannin content and flower colour. Diets were formulated using three bean inclusion rates; 200, 400 and 600 g/kg. At each rate the white- and coloured-flowered lines were mixed in the ratios 100:0, 75:25, 50:50, 25:75 and 0:100. The response in carcass nitrogen retention (CNR) to tannin intake was assessed at each bean inclusion rate and for each pair of lines.
There was a significant effect of tannin intake on CNR in only one of the nine comparisons and in that case CNR increased as dietary content increased.
It was concluded that faba bean tannins can have a negative effect on the dietary AMEN and AMN for chicks when present at a sufficiently high dietary concentration. However, the highest dietary concentration of faba bean tannins in Expt 3 was 10·52 g/kg DM, giving a maximum daily intake of 1·03 g tannins for the two-bird experimental unit. At concentrations and intakes up to this there was no evidence to suggest that faba bean tannins had a significant adverse effect.