Effects of virginiamycin and monensin administered alone or together with Megasphaera elsdenii strain NCIMB 41125 on in vitro production of lactate and VFA and the effects of monensin and M. elsdenii strain NCIMB 41125 on health and performance of feedlot steers

Virginiamycin (VIR) and monensin (MON) are included in high concentrate diets to mitigate digestive disturbances and to increase performance of ruminants in intensive systems. Megasphaera elsdenii strain NCIMB 41125 (Me) rapidly utilises lactate in the rumen, thereby controlling ruminal pH and digestive disturbances. Me promotes butyrate and valerate production whereas VIR and MON promote propionate that is gluconeogenic and perceived to be more favourable to performance. The objective of the present study was to determine whether the ratio between VFA proportions is more favourable when VIR or MON is administered together with Me and whether their respective effects on health and performance are additive. Two studies of factorial design were conducted, one in vitro fermentation trial where VIR or MON was incubated in the absence of Me (Me-) or in the presence of Me (Me+), and one in vivo trial with steers where MON or Me was administered alone or together. In the in vitro trial VIR and MON were administered to the fermentation vessels in two concentrations, a low dose (0.125µg/mL) and a high dose (0.375mg/mL). Me was administered at 100µL/40mL, supplying 2.5×105cfu/mL. In the in vivo trial 224 Bonsmara steers were allocated to a finishing diet with 80kg/ton roughage and 224 were allocated to a finishing diet with 20kg/ton roughage. Within each diet 112 steers received MON (MONY) and 112 not (MONN) and 112 steers within each diet were dosed with Me (MeY) and 112 not (MeN). In the in vitro trial VIRMe- and MONMe- decreased gas production, whereas VIRMe+ and MONMe+ increased fermentation. VIRMe- and to a lesser extent MONMe- increased total bacterial counts, a result which was apparently due to stimulation of M. elsdenii numbers as determined by 16R DNA sequencing, although other lactate utilising bacteria could have contributed. Me+ reduced lactate and the reduction was more in VIRMe+ and MONMe+. Fermentation end products in Me+ favoured butyrate and valerate and VIRMe- and especially MONMe-, propionate. VIRMe+ and MONMe+ displayed the median. In the steer trial MONY decreased intake and improved feed conversion efficiency. MONY increased carcass gain and dressing percentage on the 80kg/ton roughage diet but not the 20kg/ton roughage diet. Rumen score on both diets was improved by MONY and liver score by MONYMeY on the 20kg/ton roughage diet. Carcass weight was improved by MONNMeY on the 20kg/ton roughage diet. Morbidity and mortality were not affected by MONY but were improved in the presence of Me (MeY). In general, the results suggest synergy between MON and Me.