The Addition of Narasin into a Mineral Mixture Improves Performance of Grazing Nellore Steers

Previous studies have demonstrated that narasin, a coccidiostatic and antibacterial agent, may improve performance of forage-fed cattle due to rumen fermentation changes. The objective of this trial was to determine the effects of narasin inclusion into a mineral mixture (MM) on performance of grazing steers. Two hundred and forty Nellore steers (12 ± 1 mo of age) were blocked by initial BW (177 ± 0.21 kg), and assigned to 30 lots (8 steers/lot and 10 lots/treatment). Sixty paddocks of Brachiaria brizantha with 1 ha each were used and each paddock was continuously grazed by 28 d, followed by 28 d of rest. A paddock rotation among treatments was designed every 28 d to minimize the effect of variation among pastures. Paddocks were assigned to receive 1 of 3 treatments: a control (no narasin; 0N), and an intake of 71.5 (13N) and 110 (20N) mg of narasin (Zimprova^®; Elanco, São Paulo, Brazil) per day, corresponding to 13 and 20 ppm considering 2.5% the DMI of steers. Narasin was added in the MM by adjusting their concentration in each period, according to MM intake (MMI) in the previous period. The MMI was evaluated on a weekly basis, starting 28 d before the beginning of the experiment. Forage mass in the paddocks was measured by choosing two points upon entry and exit of the lot from each pasture. All data were analyzed using the PROC MIXED procedure of SAS and evaluated by period (every 28-d) and overall (d 0 to 84). No treatment × period (P = 0.85) or treatment (P = 0.23) effects were observed for daily MMI (64, 61, and 58 g for 0, 13, and 20N, respectively; SEM = 2.19). Conversely, a treatment × period interaction was observed (P = 0.002) for ADG, because ADG was greater (P ≤ 0.01) for 13N and 20N in the 1^st and 2^nd periods and also greater (P = 0.03) for 20N vs 13N in the first 28 d of the study, whereas no differences (P ≥ 0.18) were observed in the last 28 d. Moreover, overall ADG was also greater (P < 0.01) for 13N and 20N when compared with cohorts receiving 0N, without differences between 13 and 20N (P = 0.65; 0.493, 0.575, and 0.585 kg/d, respectively; SEM = 0.0150). In summary, narasin inclusion into the mineral mixture did not impact mineral intake and improved overall steers performance during an 84-day period.