Steam-pelleting temperatures and grain variety of finely ground, sorghum-based broiler diets. 1. Influence on growth performance, relative gizzard weights, nutrient utilisation, starch and nitrogen digestibility

Sorghum-based diets were offered to male Ross 308 chicks from 7 to 28 days post-hatch as a 2 × 3 factorial array of treatments. The diets were based on either a white (Liberty) or red (Venture) sorghum that were finely ground through a 2.0-mm hammer-mill screen before incorporation into diets that were steam-pelleted at conditioning temperatures of 70°C, 80°C and 90°C. Protein solubilities and concentrations of disulfide bonds and free sulfydryl groups in sorghums and sorghum-based diets were determined. Concentrations of phenolic compounds and antioxidant activities in both sorghums were analysed and the presence or absence of a pigmented testa was detected. In vivo parameters assessed included growth performance, relative gizzard weights, nutrient utilisation and apparent starch and nitrogen (N) digestibility coefficients were determined at four small-intestinal sites. Increasing conditioning temperatures depressed dietary protein solubilities and induced changes in concentrations of disulfide bonds and free sulfydryl groups that were more pronounced in red sorghum-based diets. The red sorghum contained a higher concentration of phenolic compounds and had a higher antioxidant activity than did white sorghum. A pigmented testa was not present in both sorghums, which indicates that they do not contain condensed tannin. There was a significant interaction between sorghum type and conditioning temperature for weight gain; however, diets based on white or red sorghums did not generate any significant differences in weight gains, feed intakes or feed conversion ratios as main effects. It was anticipated that birds would perform better on white sorghum-based diets but the likelihood is that the fine 2.0-mm grind disadvantaged the softer-textured white sorghum. As main effects, red sorghum-based diets had significantly higher densities of N-corrected apparent metabolisable energy, a higher starch digestibility coefficient in the distal jejunum and higher N digestibility coefficients in the distal jejunum, proximal ileum and distal ileum than did white sorghum-based diets. Red sorghum-based diets generated significantly heavier relative gizzard weights, which appeared to enhance N digestibility coefficients relative to the white sorghum diets. Increasing conditioning temperatures linearly increased starch digestibility coefficients in the proximal jejunum and distal ileum and N digestibility coefficients in the proximal jejunum, distal jejunum and distal ileum to significant extents. Conditioning temperatures did not significantly influence gizzard weights or parameters of growth performance and nutrient utilisation. Several significant interactions between the main effects were observed, which suggests that the two sorghums responded somewhat differently to increasing conditioning temperatures.