An in-Vitro Evaluation of the Effects of a Protease and a Bacillus Spp. Direct-Fed Microbial, Alone or in Combination, on the Microstructure and Protein Solubilisation of a Variety of Feed Ingredients Fed to Grower Pigs

The addition of direct-fed microbials (DFMs) or protease to grower-finisher pig diets have not consistently led to growth performance improvements, however new research suggests that there may be greater and more consistent effects of feeding protease and DFM in combination. This in-vitro study aimed to investigate the effect of a protease (PRO) and Bacillus spp. DFM (DFM) on the structure and protein solubilisation of different feed ingredients fed to growing pigs. Eight ileal cannulated barrows (initial BW 30kg) were fed one of 8 semi-purified diets in an 8x8 Latin square design. The diets consisted mostly of corn-soy (19% CP), corn (6% CP), wheat (10% CP), corn DDGS (16% CP), SBM 46 (17% CP), SBM 48 (19% CP), canola meal (15% CP) or meat and bone meal (MBM) (14% CP) with chromic oxide. Diets were fed for 7-d with 5-d for adaptation and 2-d ileal collection. The AID of CP was calculated. The corn-soy, wheat and SBM diets had higher AID of CP than the corn diet (P=<0.001), whilst the corn DDGS and MBM diets were intermediate. The AID of CP was affected by pig (P=0.034) but was not by period. Ileal samples from corn, wheat and SBM 46 diets were selected from the pig with AID of CP closest to the population average, and used for the in-vitro study. The ileal digesta samples were incubated with no additive (control), or PRO, or DFM, or PRO+DFM, in a buffer solution at 40^oC for 2 hours. Centrifugation was used to separate the supernatant which was analysed in duplicate for total protein and data was analysed using the Fit Model platform in JMP 11. The pellet was analysed by confocal laser scanning microscopy and scanning electron microscopy to determine internal and external microstructures, respectively. The PRO+DFM treatment increased protein solubilisation of SBM compared to the control (P=0.02; 1635 vs. 730 ng/μL), but PRO and DFM alone did not. Confocal laser and scanning electron microscopy revealed that PRO+DFM was effective at degrading resistant starch in corn, aggregates of protein, lipid and fibre in wheat and reducing the size of cell wall fragments in SBM. The PRO+DFM treatment was more effective than the individual additives at fragmenting the fibrous complexes in all three ingredients. In conclusion, the enzyme and DFM combination showed the greatest structural breakdown of all ingredients tested and solubilised more protein from SBM than the individual additives alone.