Steam-pelleting temperatures, grain variety, feed form and protease supplementation of mediumly ground, sorghum-based broiler diets: influences on growth performance, relative gizzard weights, nutrient utilisation, starch and nitrogen digestibility

Abstract

A red sorghum was mediumly ground (3.2 mm) and incorporated into broiler diets that were steam-pelleted at conditioning temperatures of 65, 80 and 95°C and offered to male Ross 308 chicks from 7 to 28 days post hatch. This diet was also fed as unprocessed mash, reground mash following steam-pelleting at 95°C and diets steam-pelleted at 80°C were fed without and with exogenous protease. A nutritionally equivalent diet based on a mediumly ground white sorghum steam-pelleted at 95°C completed the seven dietary treatments each of which were offered to seven replicates of six birds per cage. The effects of treatment on growth performance, relative gizzard weights, nutrient utilisation [apparent metabolisable energy (AME), nitrogen (N) retention, and N-corrected AME] and apparent digestibility coefficients of starch and N in four segments of the small intestine (proximal and distal jejunum, proximal and distal ileum) were determined. Elevating conditioning temperatures at which the red sorghum-based diets were steam-pelleted reduced their protein solubilities and linearly reduced concentrations of free sulfydryl groups (P < 0.02) and the two parameters were positively correlated (P < 0.01). Elevating temperatures also linearly increased relative gizzard weights (P < 0.02). However, increasing conditioning temperatures linearly depressed feed conversion efficiency (P < 0.02), tended to depress weight gain (P < 0.08) but did not influence nutrient utilisation parameters (P > 0.50). Increasing conditioning temperatures influenced N digestibility coefficients in the distal jejunum (P < 0.01) and distal ileum (P < 0.05) in a quadratic manner. N coefficients varied from 0.565 to 0.538 and 0.638 in the distal jejunum with increasing conditioning temperatures; however, starch digestibility coefficients were not significantly influenced by conditioning temperatures. Birds offered the white sorghum-based pelleted diet conditioned at 95°C significantly outperformed (P < 0.05) their red sorghum counterparts in terms of weight gain, feed per gain, AME, N retention, starch and N digestibility coefficients in the distal ileum. The differences in starch digestibility between the two sorghum varieties along the small intestine were pronounced and the possible contributing factors are considered. Diets fed as intact pellets generated greater feed intakes and weight gains than the average of the two mash diets. In red sorghum-based diets, distal ileal starch digestibility coefficients of intact pellets were inferior to the unprocessed and reground mash. However, the distal ileal N digestibility coefficient of the unprocessed mash diet was 3.5% higher than the reground mash diet and 8.3% higher than the intact pelleted diet. The inclusion of a Bacillus lichenformis-derived protease in red sorghum-based steam-pelleted at 80°C significantly (P < 0.05) increased N digestibility coefficients in the distal jejunum, proximal ileum and distal ileum. Also, this protease significantly increased starch digestibility coefficients in the distal jejunum and proximal ileum. The implications of these findings in respect of feeding broiler chickens sorghum-based diets are discussed.