Fishmeal and maize starch inclusions in sorghum-soybean meal diets generate different responses in growth performance, nutrient utilisation, starch and protein digestive dynamics of broiler chickens

This study comprised a 2 × 2 factorial array of dietary treatments offered to male Ross 308 broiler chicks from 15 to 28 days post-hatch. The dietary treatments consisted of a sorghum-soybean meal diet in which either sorghum was partially substituted by maize starch (200 g/kg) or soybean meal was partially substituted by fishmeal (175 g/kg). Growth performance, nutrient utilisation, digesta retention times in four small intestinal segments, starch and protein (N) digestibility coefficients and disappearance rates (g/bird/day) and starch:protein disappearance rate ratios in four small intestinal segments were determined. The partial substitution of soybean meal by fishmeal had the more profound effects on the parameters assessed as fishmeal inclusions improved weight gain by 12.1% (1260 versus 1124 g/bird, P < 0.001) and FCR by 8.13% (1.299 versus 1.414, P < 0.001). This substitution significantly enhanced starch digestibility coefficients in four small intestinal segments culminating in an increase of 18.9% (0.937 versus 0.788; P < 0.001) in the distal ileum. The partial substitution of soybean meal by fishmeal significantly increased digesta retention time in the small intestine from 210 to 289 min. Starch digestibility was significantly correlated with growth performance (weight gain, FCR) and energy utilisation (AME, ME:GE ratios, AMEn) and these parameters were all significantly enhanced by the fishmeal substitution. Consideration is given to the possible mechanisms whereby the partial substitution of soybean meal by fishmeal promoted starch digestibility and energy utilisation and growth performance. Instructively, starch:protein disappearance rate ratios in the proximal jejunum were quadratically related to weight gain (r = 0.849; P < 0.001) and FCR (r = 0.838; P < 0.001) such that a ratio of 3.59:1 would generate the maximum weight gain of 1265 g/bird and a ratio of 3.88:1 would generate the minimum FCR of 1.287. These relationships emphasise the importance of digestive dynamics and in these examples there was an ideal balance between the bilateral bioavailabilities of starch and protein.