Effect of an Aspergillus oryzae-derived extract additive on metabolizable amino acid availability in growing dairy heifers.

Aspergillus oryzae (AO), a filamentous fungi used as a feed additive in livestock diets, has been shown to enhance the rumen microbial community, thereby improving rumen function and performance in dairy and beef cattle. The objectives of this study were to evaluate the effects of an Aspergillus oryzae-derived extract product (AOP) on absorption of essential amino acids (EAA) from microbial crude protein (MCP) and absorption of EAA from metabolizable protein in growing dairy heifers. Sixteen cannulated Holstein heifers were used in a crossover design with 2 23-d periods and 2 treatments: control (CTL) and AOP supplementation. The CTL group received the basal TMR, the AOP group received the basal TMR with the addition of 1g/d of AOP, and both groups were fed and provided with water ad-libitum. The basal TMR consisted of corn silage (52.7%), oat hay (31.8%), and a grain mixture with soybean meal, rumen-protected amino acids, vitamins, and minerals. On d 11, heifers were moved into metabolism rooms and housed in individual pens to begin measuring individual feed intake. On d18, heifers were haltered and began a 96-h constant ruminal infusion of 15N-labeled ammonium sulfate and whole rumen content samples were collected from the anterior, medial, and dorsal sections of the rumen at 0 min, and then every 6 h relative to the start of infusion until the infusion ended. On d 19, heifers were fitted with 2 jugular catheters and on d 20, all heifers began a 48-h infusion of 1g of 13C-labeled AA and 77 mg of 13C-labeled His. Blood samples were collected at -30, -15, and 0 min and then every 3 h relative to the start of infusion until the infusion ended. Results showed that AOP tended to increase dry matter intake (DMI), crude protein (CP), acid detergent fiber (ADF), and neutral detergent fiber (NDF) intake. Average body weight (P > 0.05) was not different between CTL and AOP groups. The AOP decreased plasma His concentrations, tended to decrease Phe plasma concentrations, and Ile absorption estimated from an isotope dilution method. The AOP did not affect rumen microbial-derived EAA (MEAA) and EAA absorption from metabolizable protein, determined as EAA plasma entry rate using the isotope dilution method. In conclusion, AOP improved dry matter and nutrient intake, however, it had no influence on EAA absorption in growing dairy heifers fed a fiber-based diet.