PSXII-17 Hydrolyzed yeast supplementation and withdrawal effect on rumen fermentation dynamics

In vitro rumen fermentation studies have shown that hydrolyzed yeast (HY; LivaltaCell HY40, Peterborough, UK) can modify the rumen microbial composition and activity (Schulze et al 2024). The aim of this study is to investigate the effects of prolonged (10 d) feeding and withdrawal of HY (15 gּ animal-1ּ d-1) on in vitro rumen fermentation dynamics. Dry rumen-fistulated Holstein cows (n = 10) were used in this study as individual experimental units. The cows had free access to haylage and drinking water and were restricted fed a corn silage-based TMR with HY for 10 d and without HY for the following 10 d. This provided 3 treatment groups, at measure points d 0 (CTRL), 10 (HY) and 20 (-HY; withdrawal) at which duplicate samples of rumen fluid were collected at 1500 h from each cow. An automated in vitro rumen fermentation model based on infrared sensors (Fermentrics), with triplicate vessels per cow, was used to analyze gas production dynamics of total gas, CO2, CH4, H2 and NH3 during a 12 and 24 h period, and 24 h pH, aOMD and aMBP. An aliquote 20 mL sample of rumen fluid mixed with 80 mL buffer and 12 h in vitro fermentation samples were analysed for volatile fatty acids (VFA) to calculate Net vFA production per cow and treatment. Statistical analysis was performed using PROC GLIMMIX procedure of SAS (9.4, SAS Institute Inc., Cary, NC) with day as fixed effect (CTRL, HY, and -HY). LSMEANS were separated and denoted different (P ≤ 0.05) using the pairwise comparison PDIFF option of SAS. HY supplementation did not impact any of the gas parameters measured at either 12 or 24 h compared with CTRL. However, 10 d after removal of HY (-HY), 12 h total gas and CO2 as well as 12 and 24 h CH4 production increased (P < 0.05), whereas 24 h H2 was reduced (P < 0.05). The CH4 content in the 24 h total gas production was greater (P < 0.05) at -HY (15.0%), whereas no differences were observed between CTRL (13.0%) and HY (12.1%). aOMD was similar for CTRL (51.4%) and -HY (52.5%) but higher (P < 0.05) for HY (55.9%). aMBP was significantly different between all three groups, with HY showing greatest (172.8mg/g), CTRL medium (154.2mg/g) and -HY least (122.1mg/ g) microbial biomass production. Net 12 h in vitro VFA production was significantly increased with HY (29.2 mmol) compared with CTRL (16.2 mmol) and -HY (18.9mmol), mainly driven by an increase of acetic and butyric acid production. The proportion of propionic acid declined from 30% for CTRL to 20% with HY and 21% wit