Severity and prevalence of ruminal acidosis during the diet transition for commercial feedlot cattle

The objective of this study was to determine the severity and prevalence of ruminal acidosis in commercial feedlot cattle during the transition to a finishing diet. Previously backgrounded steers (n = 907) and heifers (n = 998) were chosen as a source population and housed separately in 8 pens with an average of 227 ± 13 and 249± 6 hd/pen, respectively. Within the source population, 16 steers (mean BW ±SD = 435.1 ± 32.8 kg) and 16 heifers (mean BW ± SD = 382.7 ± 49.4 kg) were used to measure reticulo-ruminal pH using an orally administered pH measurement system; however, 3 systems were not recovered on slaughter. Cattle were fed 3 times daily and were transitioned from a diet containing (forage:concentrate; F:C) 62:38 to 20:80 (DM basis) over 40 d. Dry matter intake was assessed at the pen level. The effect of diet and day within diet were analyzed using the MIXED procedure in SAS, with diet and sex as fixed effects. Dry matter intake was greater for steers than heifers (10.2 vs. 9.3 kg/d; P < 0.01) and increased from diet 1 to diet 3, reaching a peak of 10.2 kg/d, before declining to 9.4 kg/d in diet 6. Mean reticulo-ruminal pH (P < 0.01) declined from pH 6.45 in diet 1 to pH 6.10 in diet 6. Heifers had greater mean pH than steers (6.38 vs. 6.33) when averaged over the diet transition (P = 0.04). Area (pH min)/d) and duration (min/d) that pH was < 5.6 increased with decreasing F:C (P < 0.01), as did the standard deviation from mean pH (P < 0.01). Over the entire transition period 24/29 study animals experienced at least one bout indicative of acidosis (pH < 5.6 for > 180 min) but the average daily prevalence was 5.4%. The prevalence of reticulo-ruminal acidosis increased with decreasing F:C (peak of 13.5%). When days within a diet were evaluated, lowest mean pH was observed 2-d post diet change (P < 0.01). Results indicate that daily prevalence for ruminal acidosis ranges between 1 and 13% with the risk increasing with decreasing F:C. The susceptibility to ruminal acidosis may differ between steers and heifers and the second day relative to a diet change appears to be the day with the greatest risk.