DEFINING THE RISK, PREVALENCE, AND PATHOLOGICAL THRESOLD OF LOW RUMINAL pH IN FEELOT CATTLE

The diet transition phase is thought to be the highest risk period for development of low ruminal pH, while pathology associated with low reticulo-ruminal pH (RRpH) induced ruminal acidosis (RA) is often found at slaughter, months after the diet transition. Two experiments were conducted to 1) determine the risk of low RRpH during the transition phase and 2) explore the association of rumen fermentation and acute phase protein response during finishing with pathology identified post mortem. In experiment 1, RRpH was measured in 32 mixed breed steers (n = 16) and heifers (n = 16) housed in commercial feedlot pens with 227 ±13 and 249 ± 6 hd/pen cohort steers and heifers, respectively. Cattle were transitioned from a diet containing 46.5% forage and 53.5% concentrate to a diet containing 9.5% forage and 90.5% concentrate dry matter (DM) basis) over 40 d. In addition, wheat replaced barley as the grain source during the dietary transition. Both mean and minimum RRpH decreased as the proportion of concentrate in the diet increased. The area (duration severity) that RRpH was < 5.6, duration that RRpH was <5.6, and the number of cattle experiencing a bout of low RRpH (pH < 5.6 for > 180 min), increased with increasing concentrate. Despite having a high risk for low RRpH, most cattle had only 1-3 bouts of low RRpH during the diet transition, and extent was mild. Steers had greater dry matter intake (DMI), lower RRpH, and greater standard deviation of RRpH than heifers, suggesting that susceptibility to RA may differ between steers and heifers. In experiment 2, ruminal pH, short-chain fatty acid concentrations and serum acute phase proteins were measured in 28 cannulated steers during the final 5 wk of finishing when fed a diet containing 5% forage and 95% concentrate (DM basis). Rumen and livers were examined and pathology scores were determined at slaughter. There was no difference in minimum pH, mean pH, or duration that ruminal pH was < 5.5 between steers with or without pathology. However, steers with pathology spent more time with ruminal pH < 5.2 and tended to spend more time with ruminal pH < 5.8. Acetate concentration tended to be greater in steers with pathology than without pathology. Serum amyloid A was greater and haptoglobin tended to be greater in steers with pathology than those without. Overall, liver and rumen pathology was associated with a greater duration that ruminal pH is < 5.2 and a chronic systemic acute phase protein response. In summary, feedlot cattle experience low RRpH during dietary transition and that the risk increases with increasing levels of concentrate. However, during the dietary transition the extent of low RRpH was mild. During the last 5 wk of finishing, the duration that ruminal pH was < 5.2 and the plasma concentration of serum amyloid A, were associated with greater rumen and liver pathology scores, suggesting that low ruminal pH occurring during the latter part of finishing may have an impact on risk for rumenitis and liver abscesses