Effects of the Combined Use of Virginiamycin and Salinomycin on Rumen Fluid Kinetics of Nellore Steers

Abstract Text: Two experiments were conducted to evaluate the combination of dietary virginiamycin and salinomycin on rumen fluid kinetics of Nellore steers fed high concentrate diets. In experiment one, 8 Nellore steers (322 ± 26 kg initial BW) were allotted to a replicated 4x4 Latin square design (21-d periods), in a 2x2 factorial arrangement of treatments, with 2 salinomycin concentrations (0 and 13 ppm) and 2 virginiamycin concentrations (0 and 15 ppm) in the diet DM, which contained 80% concentrate. In experiment two, 8 Nellore steers (434 ± 35 kg initial BW) were allotted to the same experimental design, with 2 concentrate inclusions (70C and 90C diets had 70 and 90% concentrate, respectively) and 2 virginiamycin concentrations (0 and 15 ppm) in the diet DM. Salinomycin was included in all diets (13 ppm). In both experiments, on d 20 of each period, 300 g of polyethylene glycol 4000 (PEG) were diluted in 600 mL of deionized water and infused into the rumen of each steer at 0800. Rumen fluid samples were collected before infusion and at 1.5, 3, 6, 12, and 24h after infusion, and PEG concentrations were determined by colorimetry. Ruminal fluid rate of passage (RP; %/h) and volume (FV; L) were calculated from linear regression of the natural logarithm of PEG concentration on time. Fluid retention time (RT; h) was determined as the inverse of RP, whereas flow rate (FR; L/h) was calculated as RP multiplied by FV. Statistical analyses were performed using the MIXED procedure of SAS, and the model included the random effects of animal, period, and Latin square, and the fixed effects of salinomycin or concentrate, virginiamycin, and the two-way interaction. No interactions were observed for any variable. In experiment one, rumen fluid kinetics variables were not affected by treatments. In experiment two, steers receiving the 70C diet showed greater (P≤0.02) RP (9.90 and 7.92 ± 0.48 %/h for 70C and 90C, respectively), FV (39.07 and 31.47 ± 1.89 L for 70C and 90C, respectively), and FR (3.69 and 2.55 ± 0.23 L/h for 70C and 90C, respectively), whereas RT was greater (P<0.01) for steers fed the 90C diet (9.77 and 13.24 ± 1.15 h for 70C and 90C, respectively). No effects of virginiamycin were observed for any variable. In conclusion, rumen fluid kinetics of Nellore steers is affected by dietary concentrate inclusion, but not by the combined use of virginiamycin and salinomycin.

Keywords: antibiotic; ionophore; Zebu