LAUSR

The objective was to create an in vitro model of ruminal acidosis. Ruminal acidosis is a prevalent metabolic disorder in beef feedlot cattle characterized by low rumen pH. Experiment 1 was a 3×3 factorial design testing the interaction between inoculum composition and quantity of substrate. Eighteen flasks containing 150 mL of inoculum were used to evaluate treatments in duplicate. Treatments evaluated inoculum ratios (buffer: rumen fluid) 2:1, 4:1, and 6:1 in combination with substrate levels 5 g, 7.5 g, and 10 g. Manual pH was collected every 4 h for 36 h. Experiment 2 was a completely randomized design with 5 treatments evaluated in triplicate. Treatments consisted of a 4:1 inoculum ratio (150 mL) with the substrate (7.5 g) consisting of 70% corn (PCON), 40% corn (NCON), or increasing inclusions (10, 20, and 30%) of wheat in place of corn (W10, W20, and W30). Flasks were incubated at 39°C for 36 h with continual pH measurement. In exp. 1, 7.5 g of substrate remained between a pH of 5.6 and 5.0 (P ≤ 0.01) for a longer period (880 min) than 10 g of substrate (520 min). In experiment 2, a treatment by time interaction (P ≤ 0.05) was observed for pH with NCON having the greatest pH over 8-36 h and PCON having the lowest pH over 4-8 h. A treatment by time interaction (P = 0.02) was observed for lactate at 20 h with PCON having the greatest and W30 having the lowest concentration. Treatment PCON had the greatest (P < 0.01) acetate and propionate concentration. The W20 treatment remained between a pH of 5.6 and 5.0 (P < 0.01) for a longer period than PCON. Results suggest W20 and PCON may be the most useful in vitro models of ruminal acidosis.