Our objective of this study was to evaluate changes in ruminal pH, concentrations of VFA and lactate in cattle during a transition from 60% to 80% concentrate diet. Megasphaera elsdenii culture (ME; Lactipro, MS-Biotec, Wamego, KS) was administered by oral drench to crossbred heifers (n = 240; 498 ± 18 kg initial BW) at rates of 0, 25, 50, 75, or 100 mL immediately before transition from 60 to 80% concentrate diet. Cattle were fed in pens of 8 head each, and ruminal contents of animals in one pen/treatment were obtained via rumenocentesis for estimation of ruminal pH and concentrations of VFA and lactate at 5, 10, 15, 20, 25, and 30 h after dosing and introduction of the 80% concentrate diet. Lactate concentrations were unaffected by treatment and were <1 mM (P > 0.10). Differences in pH and VFA concentrations were not apparent until 20 to 25 h after dosing, and were characterized by lower propionate production and increased A:P ratios for the 75- and 100-mL dosages compared with other treatments. Megasphaera elsdenii may help to avoid pH decline frequently associated with transition to high-concentrate diets. Table 1. Ruminal pH in heifers receiving an oral drench of Megasphaera elsdenii (ME) culture (0, 25, 50, 75, or 100 mL)

Hour	ME, mL					SEM	P-value
0	25	50	75	100
5	6.03	5.80	6.08	5.73	5.89	0.101	0.06
10	5.83	5.78	5.83	5.82	5.78	0.101	0.80
15	6.02	6.10	5.95	6.16	5.93	0.095	0.36
20	5.91ac	6.13bc	5.76a	5.93abc	6.20b	0.101	0.01
25	6.23a	6.12a	6.24ab	6.58bc	6.59c	0.101	0.01
30	5.54	5.60	5.72	5.61	5.75	0.095	0.47

^abcWithin a row, means without a common superscript differ (P < 0.05).