Combined use of monensin and virginiamycin to improve rumen and liver health and performance of feedlot-finished steers

Lay Summary Dietary inclusion of significant proportions of rapidly degradable grains may represent a challenge for ruminal and liver health, thus reducing the growth performance of beef cattle. Monensin and virginiamycin have been shown to reduce or inhibit the growth of bacteria responsible for the development of ruminal acidosis and liver abscesses. The present study evaluated the effects of concurrently including monensin and virginiamycin in a dry-rolled corn-based diet on ruminal pH, the occurrence of ruminal lesions and liver abscesses, and the growth and development of feedlot-finished cattle in comparison with including each additive alone. Results indicated that the simultaneous inclusion of monensin and virginiamycin did not affect the growth and development of the animals or their carcass weight, though it reduced their dry matter intake. Thus, feed efficiency improvement was observed for body development. Treatments did not affect carcass characteristics, ruminal pH, and the occurrence of ruminal lesions and liver abscesses. Monensin and virginiamycin are included in beef cattle finishing diets as prophylaxis to minimize the incidence of ruminal acidosis and liver abscesses. Due to different and probably complementary modes of action, this study aimed to determine the effects of a combination of monensin and virginiamycin, both included in the diet at recommended doses, on ruminal health, the occurrence of liver abscesses, and growth performance of feedlot-finished cattle. One hundred and forty-four steers (6 animals/pen) were fed 1 of 3 corn-based finishing diets containing 30 mg of monensin (MN), 25 mg of virginiamycin (VM), or 30 and 25 mg of monensin and virginiamycin (MN + VM), respectively, per kilogram of dry matter. Ruminal pH probes were inserted into two animals per pen and set to record pH every 10 min. On d 100, animals were slaughtered, and rumens and livers were recovered, on which occurrence and degree of ruminal damage, prevalence and number of liver abscesses, and liver scores (A-: livers with no more than two small abscesses; A+: livers with at least one large abscess or more than four medium abscesses; A: any other abscessed liver) were determined. Simultaneous inclusion of monensin and virginiamycin resulted in a 4.3% decrease (P < 0.04) in dry matter intake (DMI; 8.8, 9.2, and 9.2 +/- 0.19 kg/d for MN + VM, MN, and VM-fed animals, respectively) and similar (P > 0.13) average daily body weight gain (ADG; 1.49 +/- 0.021 kg/d) and hot carcass weight (HCW; 269 +/- 1.7 kg), compared with feeding diets containing one additive or the other. Therefore, in terms of ADG, a 9.4% improvement (P < 0.01) in feed efficiency was observed in MN + VM-fed animals. Backfat thickness (5.6 +/- 0.08 mm) and ribeye area (69.9 +/- 0.53 cm(2)) remained unaffected (P >= 0.74), as well as the minimum (4.98 +/- 0.047), mean (6.11 +/- 0.037), and maximum ruminal pH (7.23 +/- 0.033) values and the time (125 +/- 22.3 min/d), area (57.67 +/- 12.383 pH x h), and episodes (22 +/- 3.8 bouts) of pH below 5.6 (P >= 0.12). Overall, prevalence (24 +/- 3.4%) and the number of liver abscesses (1.6 +/- 0.14 abscesses/abscessed liver), liver scores (20 +/- 3.1% of A- and 4 +/- 1.8% of A livers), and prevalence (67 +/- 3.5%) and degree of damage to the ruminal epithelium (2.5 +/- 0.22% affected surface) were similar (P >= 0.18) across treatments; however, the occurrence of ruminal lesions tended (P <= 0.07) to be associated with that of liver abscesses and reduced ADG when feeding monensin alone. Compared with the dietary inclusion of monensin or virginiamycin, the combined use of these feed additives reduced dry matter intake and improved feed efficiency of feedlot-finished cattle without altering variables associated with rumen and liver health.