When steers (n = 4) were fed increasing amounts of concentrate (0, 45,
or 90% of DM) and decreasing amounts of forage, the VFA concentration
increased (P < .001) and ruminal pH, acetate:propionate ratio, and di
ssociated ammonia declined (P < .001). Acetate:propionate ratio and di
ssociated ammonia were highly correlated (r(2) = .82 and .65, respecti
vely) with ruminal pH. In vivo acetate:propionate ratio was highly cor
related (r(2). = 78) with the capacity of the bacteria to produce meth
ane from H-2 and CO2 in vitro, and in vivo pH dissociated ammonia was
correlated (r(2) = .59) with the capacity of the bacteria to produce a
mmonia from protein hydrolysate. The role of pH in regulating methane
and ammonia production was supported by the effect of pH in vitro. Whe
n bacteria from cattle fed concentrate or forage were incubated at pH
values from 6.5 to 5.7, methane production decreased (P < .001) from 4
8 to 7 nmol mg protein-l min-l and from 14 to 2 nmol mg protein(-1) .m
in(-1) respectively. The reduction in in vitro pH (6.5 to 5.7) also de
creased (P < .001) the rates of ammonia production, but only if the ba
cteria were obtained from cattle fed forage (28 to 15 nmol mg protein(
-1). min(-1)). Bacteria from cattle fed 90% concentrate had similar (P
> .05) rates of ammonia production at pH 6.5 to 5.7 (approximately 12
nmol.mg.protein.min(-1)). These results indicated that ruminal pH aff
ected ruminal methane production, acetate:propionate ratio, deaminatio
n, and ammonia concentration.