Effects of narasin or virginiamycin on growth performance and carcass characteristics of growing and finishing pigs

The objective of this experiment was to determine the effects of narasin (NAR; Skycis®; Elanco Animal Health, Greenfield, IN) or virginiamycin (VIR; Stafac®; Phibro Animal Health Corporation, Teaneck, NJ) on finishing pig growth performance and carcass characteristics. Two separate experiments were conducted at the same site in 2013 and 2014. A total of 576 pigs (initial BW = 23.2 ± 0.19 kg) were housed in 24 pens with 8 pigs per pen in Exp. 1. In Exp. 2, a total of 888 pigs (initial BW = 26.2 ± 0.12 kg) were housed in 39 pens with 8 pigs per pen. Treatments consisted of a series of unmedicated corn–soybean meal diets (CON), CON + NAR (15 mg/kg), or CON + VIR (11 mg/kg) fed for 108 d (Exp. 1) or 109 d (Exp. 2). Pen was the experimental unit in both studies. Data were analyzed as a randomized complete block design with the main effects of block and treatment (Exp. 1) and as an incomplete block design with the fixed effect of treatment and the random effects of barn and barn within block (Exp. 2). In Exp.1, NAR and VIR increased (P < 0.05) ADG and ADFI from days 0 to 28, and BW on days 28, 56, 76, and 97 as compared to pigs fed CON. During days 0–28, pigs fed NAR had a greater (P < 0.05) G:F than those fed CON or VIR. Also, during days 28–56 pigs fed VIR had a greater (P < 0.05) ADFI than pigs fed CON. Pigs fed NAR or VIR had greater (P < 0.05) carcass yield than those fed CON. In Exp.2, feeding NAR increased (P < 0.05) pig BW from days 54 through 96 compared to pigs fed CON or VIR. No differences (P > 0.05) in ADG were detected between pigs fed VIR and CON through the first 74 day, but ADG of pigs fed VIR was similar to (P > 0.05) those fed NAR from days 26 to 54. From day 0 to 109, NAR improved ADG compared to pigs fed VIR, which also had similar gain to those consuming CON (P = 0.04). Feed efficiency was similar between pigs fed NAR and VIR with pigs fed CON intermediate (P = 0.05). Pigs fed NAR had a greater (P < 0.05) HCW and loin depth than those fed CON or VIR. A subtherapeutic dose of VIR showed improvements in growth performance that were similar to NAR in one experiment. Although there were differences in the magnitude of growth and carcass effects of NAR between the two studies, pigs fed NAR showed at least a tendency to have greater G:F and in some cases increased carcass weight and yield compared to pigs consuming nonmedicated feed.