Additivity and associative effects of metabolisable energy and ileal amino acid digestibility in broiler diets combining sorghum with different protein sources

Context

Poultry diets consist of several ingredients contributing specific amounts of nutrients and it is assumed that the supply from each ingredient is additive when diets are formulated. However, the additivity of apparent metabolisable energy (AME) and ileal amino acid digestibility in broiler diets combining sorghum with different protein sources has not been examined.

Aims

To determine the additivity of AME along with ileal digestibility values for protein and amino acids in diets combining sorghum with different protein sources.

Methods

The digestibility assays, based on semi-purified diets containing sorghum, sunflower meal (SFM), meat and bone meal (MBM), soybean meal (SBM), canola meal (CM), and cottonseed meal (CSM), were fed individually, or sorghum was combined with the different protein sources. Each diet was fed to three cages of 12 17-day-old broilers for 7 days. Excreta was collected for the final 3 days and at the end of the assay, digesta was collected from the terminal ileum for digestibility determination.

Key results

When sorghum was mixed with the different protein sources, all predicted values for protein digestibility were additive, but for AME only the value for sorghum + SFM was additive. All other predicted AME values for sorghum combinations were different (P < 0.05) from the determined value. There were significant (P < 0.05) differences between predicted and determined amino acid digestibility coefficients, but amino acids showing associative effects varied among the different sorghum protein source combinations.

Conclusions

Overall, the present results indicated that caution should be exercised when predicting the AME and apparent ileal amino acid digestibility values for sorghum-based diets from values determined with individual feed ingredients.

Implications

The study indicated that positive and negative interactions are likely to occur among dietary ingredients in mixed diets, which has implications for both energy and protein utilisation.