RUMINAL MICROBIAL AND FERMENTATIVE CHANGES ASSOCIATED WITH EXPERIMENTALLY-INDUCED SUBACUTE ACIDOSIS IN STEERS

We used six ruminally cannulated steers in a two-period crossover desi gn to study ruminal fermentative and microbial changes associated with induced subacute acidosis. Steers were adapted to either an 80% alfal fa hay (hay-adapted)- or corn grain (grain-adapted)-based concentrate diet. After feed was withheld for 24 h, steers were overfed with an al l-grain diet at 3.5 x NEm daily for 3 d. Ruminal contents and jugular blood samples were collected before withholding feed and at 0 and 12 h daily for 3 d during the overfeeding period. Ruminal samples were ana lyzed for pH, lactate, VFA concentrations, and counts of total anaerob ic, amylolytic, lactic acid-producing and -fermenting bacteria, and ci liated protozoa. Blood samples were analyzed to assess acid-base statu s. Ruminal pH declined to a range of 5.5 to 5.0 with increased VFA con centrations, but normal lactate concentrations (<5 mM) were indicative of subacute acidosis. Total viable and amylolytic bacterial counts we re higher (P < .05) in grain-adapted than hay-adapted steers. Anaerobi c lactobacilli counts increased over time (P < .01) in both groups and were generally higher in grain-adapted than hay-adapted steers. Lacta te-utilizing bacteria were initially greater in grain-adapted than hay -adapted steers and increased over time in both groups following grain challenge. Total ciliates were initially higher (P < .05) in grain-ad apted than hay-adapted steers and decreased after 48 h in both groups. Blood acid-base changes were minimal. Bacterial changes associated wi th subacute acidosis resemble those reported during adaptation to grai n feeding, and the decline in ciliated protozoa may be the only microb ial indicator of a potentially acidotic condition in the rumen.