Inducing subacute ruminal acidosis in lactating dairy cows.

Data from experiments in which subacute ruminal acidosis (SARA) was induced in lactating dairy cows (days in milk = 154 +/- 118) were evaluated to investigate the effectiveness of the induction protocol and its effect on production outcomes. For 13 cows in 3 trials, ruminal pH was measured continuously and recorded each minute; dry matter intake and milk yield were recorded daily. Milk composition data were obtained from 9 cows in 2 of these trials. The SARA induction protocol included 4 separate periods: 4 d of baseline [normal total mixed ration (TMR)], 1 d of 50% restricted feeding, 1 or 2 d of challenge feeding [addition of 3.5 or 4.6 kg of wheat-barley pellet (dry matter basis) to normal TMR], and 2 d of recovery measurements when feeding normal TMR. The SARA induction protocol lowered mean ruminal pH from 6.31 during the baseline period to 5.85 during the challenge period; pH remained below baseline level during the recovery period (6.16). Mean ruminal pH was highest (6.59) during the day of restricted feeding. Nadir ruminal pH decreased from baseline to challenge period (5.76 vs. 5.13). Hours below pH 5.6 increased from 1.10 to 8.26/d from baseline to challenge period and area below 5.6 (pH x min/d) increased from 15.0 to 190.3. Dry matter intake was not affected by SARA induction. Milk yield dropped from 35.2 kg/d during baseline to 31.7 k/d during the challenge period and did not return to baseline level during the recovery period (31.3 kg/d). No depression in milk fat percentage was observed when SARA was induced. Yield of fat was highest during the restricted feeding period (1.47 kg/d) and was lower during the recovery period than during the baseline period (1.12 vs. 1.31 kg/d). The protocol successfully induced SARA (low ruminal pH without signs of acute ruminal acidosis) on the challenge day. Milk yield was substantially reduced and did not recover within 2 d after the challenge.