Four continuous culture fermenters were used to determine the effect of varying VFA concentrations and pH on VFA production. The treatments were applied in the fermenters over 4 peri­ods in a 4 × 4 Latin square design. The 4 treatments were: control, 20 mmol/d acetate infusion (INFAC), 7 mmol/d propionate infusion (INFPR), and low pH (LOWPH). For the LOWPH, buffer composition was adjusted to lower pH by 0.5 units compared with control (ranging from 6.62 to 6.97). The fermenters were fed 40 g of a pelleted 50:50 alfalfa: concentrate diet once daily. After 7 d of adjustment, filtered liquid effluent (4 mL) was sampled at 0, 2, 4, 6, 8, 12, 16, and 22 h after feeding for VFA concentration analysis. Acetate and butyrate production were not affected by treatments. Production of propionate was higher in LOWPH (P < 0.05) compared with control and INFPR (Table 1). The hourly production of propionate was not different between treatments (P > 0.1) except at 2 h when it was higher in LOWPH compared with INFPR (P > 0.05). The acetate to propionate ratio (AP ratio) in INFPR was higher than other treatments (P < 0.05). The AP ratio of LOWPH was lower than control­. These results reveal that propionate production was increased by low pH, and higher concentration of propionate increased the AP ratio in continuous culture, thus affecting VFA recycling. Table 1. Effect of acetate infusion, propionate infusion, and low pH on VFA production¹

VFA (mmol/d)	INFAC	INFPR	LOWPH	Control	SE	P-value
Acetate	88	93	89	91	5.2	0.56
Propionate	34.7ab	30.8b	38.4a	33.4b	1.8	0.01
Butyrate	15.3	15.4	14.1	14.7	0.9	0.12
Acetate:Propionate	2.55ac	3.03a	2.32c	2.73b	0.1	<0.01

¹Production was calculated after subtracting infused VFA from entry rate.