126 The inclusion of <i>Aspergillus oryzae</i> prebiotic counteracts the decline on neutral detergent fiber digestibility associated with increased intake level, on corn silage and sorghum silage-based diets

Abstract The inclusion of Aspergillus oryzae prebiotic (AOP; Amaferm, Biozyme Inc., St Joseph, MO) in ruminant diets has been proven to stimulate cellulolytic ruminal microorganisms, although, its positive effect on nutrient digestibility seems to be diet dependent. Our objective was to evaluate how AOP influences the apparent total tract digestibility of dry matter (DM) and neutral detergent fiber (NDF) at varying levels of intake, for diets with different forage sources and inclusion levels. Data were compiled from 3 experiments containing 4 diets for backgrounding cattle, assessing the use of AOP as a feed additive. The studies were Exp. 1): corn silage-based diet (CS; n = 24 steers), Exp. 2-A): sorghum silage-based diet (SS; n = 20 heifers), Exp. 2-B): citrus pulp and cotton gin byproducts-based diet (BP; n = 20 heifers), and Exp. 3): bermudagrass hay-based diet (BH; n = 24 steers). Separate ANCOVA analyses by diet were conducted using SAS, including in the model the effect of treatment (TRT; AOP vs. No AOP) and the interaction between TRT and level of intake (dry matter intake; DMI). Level of intake was calculated as percentage of body weight (DMI/BW). Least square mean differences between treatments were conducted at different levels of intake at which both treatments were represented. For apparent total tract digestibility of DM and NDF (NDFD), the TRT × DMI/BW interaction was significant (P ≤ 0.05) in the case of the CS and SS diets, but not in the BP and BH diets (P ≥ 0.10). For both silage diets, NDF and DM digestibility decreased as the level of intake increased. Consequently, the positive effect of AOP on NDFD was greater at increased levels of intake (3.3 and 3.5% of the BW for CS and SS, respectively), resulting in 9.8% improvement on NDFD for the CS diet (P = 0.04, 37.9 vs. 41.6% for No AOP vs. AOP, respectively) and 25% improvement in the SS diet (P = 0.01, 42.5 vs. 53.2% for No AOP vs. AOP, respectively). At a moderate intake (2.5% of the BW), AOP improved NDFD by 6.0 and 14.1% for the CS and SS diets, respectively (P ≤ 0.05). While at reduced intake (1.8 and 1.7% of BW, for CS and SS), NDFD did not differ (P ≥ 0.10) between treatments for either CS (47.9%) or SS (56.5%) diets. In conclusion, the inclusion of AOP in silage-based diets attenuates the decline on apparent total tract digestibility of NDF associated with increased feed intake, helping to maintain digestibility closer to its maximum potential. Whereas, at reduced levels of intake, it is more unlikely to observe a positive effect of AOP on fiber digestibility.