Effect of acidosis in the late-finishing phase on rumen fermentation in feedlot steers.

The objective was to determine the effects of induced acidosis in the late-finishing phase on rumen fermentation in feedlot steers. Eleven ruminally cannulated steers (body weight [BW] = 795 kg ± 54) were blocked into two groups based on initial BW. For 195 d prior to the start of the study, cattle were consuming a basal finishing diet (60% dry-rolled corn, 15% modified distillers grains, 15% corn silage, and 10% ground corn-based supplement). Steers were randomly assigned to one of the two treatments: control ( CON ), or induced acidosis ( ACD ). Both treatments were fasted for 24 h then fed the basal finishing diet. Steers on the ACD treatment received 0.05% of BW of wheat starch via rumen cannula at 0800 and 2000 hours on day 1 and ad libitum refeeding following the fast. On days 1 and 2, CON steers were provided 25% of allotted feed every 6 h. Rumen fluid was collected every 4 h during the challenge period (hours 0 to 48), and 0, 6, and 12 h after feeding during the recovery period (hours 54 to 96). Rumen fluid was analyzed for pH, ammonia, volatile fatty acids ( VFA ), and lactate. Fecal grab samples were collected every 8 h to determine fecal pH. A treatment × day interaction ( P  = 0.03) was observed for dry matter intake during the challenge period with steers on the ACD treatments consuming more on day 1 than CON steers. Intake was not different on day 2 ( P  = 0.88). A treatment × hour effect ( P  < 0.01) was observed for ruminal pH during the challenge period with the ACD steers having a lesser pH than CON from hours 12 to 32. Duration of time below a pH of 5.6 during the challenge period was greater ( P  < 0.01) for ACD steers than CON. During the challenge period, a treatment × time interaction ( P  = 0.04) was observed for total VFA concentration with ACD steers having greater total VFA concentration from hours 12 to 36. Acetate to propionate ratio (A:P) was affected by treatment × hour ( P  = 0.04) with CON steers having greater A:P from hours 28 to 48. Rumen ammonia and lactate concentrations did not differ ( P  ≥ 0.25) between treatments or the interaction with time. Challenge and recovery period fecal pH were not affected ( P  ≥ 0.13) by treatment, time, or their interaction. Recovery period ruminal pH was not different ( P  = 0.99) between treatments. For the recovery period, total VFA and ammonia concentration were not affected by treatment, time, or their interaction ( P  ≥ 0.07). Ruminal pH and VFA were affected in the initial 48 h of induced acidosis in the late-finishing phase.