Effect of virginiamycin on ruminal fermentation in cattle during adaptation to a high concentrate diet and during an induced acidosis

The objective of Exp. 1 was to compare the effects of virginiamycin (VM; 0, 175, or 250 mg.animal(-1).d(-1)) and monensin/tylosin (MT; 250/ 90 mg.anim al(-1) .d(-1)) on ruminal, fermentation products and microbial populations in cattle during adaptation to an all-concentrate diet. Four ruminally cann ulated, Holstein steers were used in a 4 x 4 Williams square design with 21 -d periods. Steers were stepped up to an all-concentrate diet fed at 2.5% o f BW once daily. Ruminal pH, protozoal counts, and NH3-N and VFA concentrat ions generally were unaffected by VM or MT. Mean counts of Lactobacillus an d Streptococcus bovis were lower (P < .05) for VM-treated compared with con trol or MT-treated steers. Both VM and MT prevented the increase in Fusobac terium necrophorum counts associated with increasing intake of the high-con centrate diet observed in the control. The objective of Exp. 2 was to compa re the effects of VM and MT on ruminal pH, L(+) lactate and VFA concentrati ons, and F. necrophorum numbers during carbohydrate overload. Six ruminally cannulated Holstein steers were assigned randomly to either the control, V M (175 mg/d), or MT (250 + 90 mg/d) treatments. Acidosis was induced with i ntraruminal administration of a slurry of ground corn and corn starch. The VM and MT premixes were added directly to the slurry before administration. Carbohydrate challenge induced acute ruminal acidosis (pH was 4.36 and L() lactate was 19.4 mM) in controls by 36 h. Compared with the controls, ste ers receiving VM or MT had higher (P < .05) ruminal pH, and the VM group ha d a lower(P < .05) L (+) lactate concentration. Fusobacterium necrophorum n umbers initially increased in VM- and MT-administered steers. In the contro l steers, F. necrophorum was undetectable by 36 h. Virginiamycin seemed to control the growth of ruminal lactic acid-producing bacteria and, therefore , has the potential to moderate ruminal fermentation in situations that cou ld lead to rapid production of lactic acid.