Performance response of lactating and growing rabbits to dietary lignin content

This study investigated the lignin requirements of rabbits. Four isofibrous diets with similar particle size, containing from 59 to 33 g ADL/kg DM, were formulated by substituting a 50 : 50 mixture of wheat straw and alfalfa hay with soya bean hulls. Forty rabbits were used to determine nutrient digestibility and caecal traits. A feeding trial was carried out using 160 weanling rabbits. Milk production was measured in 48 lactations. Rabbits fed the diet with the lowest ADL content (33 g/kg DM) led to an increase NDF (p = 0.004) and energy (p = 0.06) digestibilities and feed efficiency in fattening (p = 0.03) period. The differences between extreme diets were 28%, 4%, and 6%, respectively. Weights of caecum and caecal contents were higher and caecal pH lower (by 18%, 15% and 3%; p = 0.009, 0.02 and 0.009, respectively) for rabbits fed the diet with the lowest ADL content (33 g/kg DM) than for the average of the other three diets. A decrease of dietary ADL content from 59 to 33 g/kg DM increased linearly (p < 0.05) caecal VFA concentration (by 22%) and molar proportion of propionic acid (by 30%) and decreased that of butyric acid (by 21%). Type of diet did not affect either molar proportion of acetic acid or caecal ammonia concentration. Feed intake and performance of growing and lactating rabbits were maximal for an ADL concentration of 41 and 59 g/kg DM, respectively. Average daily gain, milk production and feed intake were impaired by 5 (p = 0.04), 12 (p < 0.001) and 10% (p < 0.001), respectively, in animals fed diets containing suboptimal ADL contents. It is concluded that a minimal ADL concentration was needed to maximise feed intake and performance, and that this level was higher in lactating than in growing rabbits. This result enhances the interest of increasing the inclusion of inexpensive highly lignified by-products (as wheat straw) in commercial feeds for adult animals with respect to the levels used at present. However, a decrease of feed efficiency when increasing dietary lignin content should also be expected according to our results.