Influence of growing season, tannin content and autoclave treatment on the nutritive value of near-isogenic lines of faba beans (Vicia faba L.) when fed to leghorn chicks

1. The objective of this study was to determine the influence of heat-treatment (autoclaving) on the utilisation by chicks of near-isogenic lines of faba beans that were either tannin-free (Winter-white or Spring-white) or had tannins in the seed coat (Winter-coloured or Spring-coloured). The effect of heat treatment on a common tannin-containing cultivar, Diana, was also studied. 2. The nitrogen-corrected apparent metabolisable energy (AME(n)) values of diets containing 600 g faba beans/kg diet were affected by tannin content and autoclave treatment of faba beans, with 59% of total variance in AME(n) being attributable to the effect of autoclaving. The AME(n) values were 9% higher for chicks fed on the autoclaved compared with those fed on diets containing the raw faba beans, 5% higher for the near-isogenic faba beans with no tannins compared with those with tannins, and 4% higher for the Winter compared with the Spring cultivars. 3. The above study was carried out on droppings obtained by total collection. Similar but not identical trends were obtained when AME(n) was calculated using the chromic oxide index method (r = 0.91). 4. Both autoclaving and the use of tannin-free faba beans improved the apparent protein digestibility (APD) of the diets by similar amounts (4% as determined by the total collection method and 6% as determined by the chromic oxide index method). 5. The digestibility of most amino acids either alone or together was affected by a cultivar X tannin X processing interaction. The cultivar by tannin interaction demonstrated that autoclaving increased the digestibility of amino acids to a greater degree when the tannin-containing (for example, 9% for lysine) than when the tannin-free faba beans (4% for lysine) were used in the diets. 6. The improvements in AME(n), APD and total amino acid digestibilities of faba beans following heat treatments ranged from 18% to 33%, 5% to 16% and 5% to 11%, respectively. The results demonstrate that the method of analysis can affect the values obtained but that they do not change the overall pattern of results. The results also suggest that, in most cases, more than half of the response to heat treatment is associated with the inactivation of tannins while the balance of the response is attributable to an effect of heat treatment on the non-tannin-containing factors. In summary, heat treatment can improve the utilisation of faba bean by chickens especially when they contain tannins. 7. Chicks, unlike rats, do not seem to adapt to tannins as the excretion of 3 amino acids that are the main constituents of a proline-rich protein were not affected by dietary tannins.