Maximizing the nutritional value of unprocessed soybean meal through supplementation with complex microbial enzyme products

Abstract

This thesis presents findings of a comprehensive research project on the potential of raw soybean meal (RSBM) as partial replacement of commercial soybean meal (SBM) in diets for broiler chickens. There was an extensive review of literature on the subject, followed by one in vitro experiment and five feeding trials. The first experiment of this thesis (Chapter 3) investigated the physico-chemical properties of RSBM as a feed ingredient and the influence of the ingredient on the physical and chemical properties of broiler diets. The effects of heating (95⁰C) the soybean samples over different durations on the nutrient composition and concentrations of anti-nutritive factors (ANF) were assessed. The enzymatic in vitro digestibility of nutrients in the samples was also evaluated. The results of the in vitro study showed that heating the soybean samples at 95⁰C for up to 60 min was not sufficient to reduce the levels of ANF, particularly the trypsin inhibitors (TI). Replacing commercial SBM with RSBM (up 30%) in diets reduced the quality of the diets; for example, the urease activity (UA) and nitrogen solubility index (NSI) were increased. The in vitro digestibility of DM and CP as well as phytate were improved when the samples were incubated in a cocktail of protease and phytase compared to when these individual enzymes were used or not used. In addition to the pellet durability index (PDI), the effects of two pelleting methods on diets containing graded levels of RSBM were assessed in another experiment (Chapter 4). The concentrations of ANF, particularly TI and performance of birds were evaluated. Feed particle distribution and dietary electrolyte balance (DEB) were also investigated in this experiment. Steam-pelleting the diets containing high levels of RSBM improved the PDI, compared to cold-pelleting but the amino acid profiles were better in cold-pelleted and mash samples than the steam-pelleted diets. Increasing the level of RSBM, particularly replacing 30% of commercial SBM (9% of diet) reduced the feed intake (FI), body weight gain (BWG) and feed conversion ratio (FCR) of broilers when fed from one to 14 days of age. Birds fed on the steam-pelleted diets had reduced BWG. Increasing RSBM in diets affected the development of internal organs, particularly the weight of the pancreas and duodenum, which was increased. The response of broilers fed on cold-pelleted diets containing graded levels of RSBM (replacing 0, 10 or 20% of SBM) and supplemented with increasing levels of microbial protease is reported in Chapter 5. The gross response of the birds, in terms of FI, BWG, and FCR; development of internal organs, and meat yield were assessed. Furthermore, the activities of digestive enzymes, ileal nutrient digestibility, intestinal mucosal morphometry and concentration of DNA in the pancreas were also investigated in this experiment. Although the contents of ANF, especially TI exceeded the threshold level for poultry, the gross response (BWG, FI and FCR) of birds on the tested diets was statistically similar to that of bids fed on the RSBM-free diets. These results may be in response to microbial protease supplementation, ameliorating the adverse effects of ANF, particularly TI. The activities of some digestive enzymes at 24 d of age, for example trypsin and chymotrypsin improved in response to protease supplementation. The apparent ileal digestibility (AID) of CP and amino acids (AA) were reduced with increased levels of RSBM in diets. These same parameters were further evaluated in another trial when diets contained a relatively higher level of RSBM (replacing commercial SBM at 25%) and extra-dosing with microbial protease and phytase (Chapter 6 of this thesis). Feeding the birds on diets containing this higher level of RSBM did not statistically reduce the gross response of broilers over 1-35 d, and these results further suggest the positive effects of extra-dosing of the diets with microbial protease and phytase. The villus height and crypt depth of broilers at 10 d of age were increased when the diets were extra-dosed with phytase. A preliminary cost-benefit analysis also showed that the use of the enzyme supplements even at extra-dose levels did not substantially increase the cost of the diets. The effects of the test ingredient (RSBM) on dietary protein utilization were evaluated in Chapter 7. This involved measurements of AID of protein and AA. Endogenous nitrogen secretion at the ileum was also measured and used to calculate the standardized ileal digestibility's (SID) of protein and AA. The test ingredient reduced both the AID and SID of protein and AA but these were improved by supplemental protease. The wellbeing of broilers, in terms of mortality, footpad dermatitis, intestinal lesions, tibia bone characteristics and litter quality were assessed and reported in Chapter 8. These traits were all not significantly different in chickens on the test diets compared to those on the RSBM-free diet. The concentrations of plasma myo-inositol, Na and Cl at 24 d of age were not affected by RSBM or protease supplementation. The results confirmed what was observed in previous Chapters in terms of productivity and health of the birds on the RSBM-containing diets. A major finding of this project is that although the dietary concentration of TI was substantially increased with increase in level of RSBM, there was no drastic impact on productivity or health of the birds. This may be due to the effect of the supplemental enzymes included in the diets. It can be concluded that RSBM could replace commercial SBM at levels beyond what was previously thought possible provided the diets are supplemented with appropriate microbial enzymes. The preliminary cost-benefit analysis of using RSBM indicate good returns but further studies are required into the behaviour of proteins in RSBM, especially in the presence of the test protease.