Residual effect of narasin on ruminal fermentation characteristics in lambs

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This study aimed to evaluate the effect of narasin withdrawal on wethers ruminal fermentation characteristics. Four wethers cannulated at the rumen were subjected to a 4 × 4 Latin Square experimental design. The trial lasted 144 days, divided into 4 periods of 36 days each. The diet was composed by 950 g/kg of coastcross hay and 50 g/kg of ground corn, used as a carrier for the narasin. The experimental diets were defined by frequency of narasin offer: 1) no feed additives (N0); 2) 13 mg of narasin/kg of DM every 24 h (N13); 3) 26 mg of narasin/kg of DM every 48 h (N26), and 4) 39 mg of narasin/kg of DM every 72 h (N39). The narasin was offered for 25 days, and at the last 11 days of each period, all the animals received the same proportion of forage and concentrate, but without narasin inclusion (withdrawal period). Last day of narasin offering was denominated day 0 and the first day of withdrawal day 1. The fed offer and orts were daily recorded to determine de DMI and nutrient intake. Ruminal fluid was collected 6 h after concentrate offering from d0 to d11 to determine ruminal pH, SCFA and ammonia nitrogen. There was no difference for dry matter intake among treatments after narasin withdrawal. There was no treatment and day interaction for total SCFA concentration, molar proportion of acetate, butyrate, isobutyrate, valerate, isovalerate and rumen pH. Treatment and day interaction after narasin withdrawal interactions was observed for molar proportion of propionate (P < 0.01), Ac:Prop ratio (P = 0.03) and ammonia nitrogen concentration (P = 0.02). The increase in molar proportions of propionate for all narasin-containing treatments compared with the control was no significant by day 5 after narasin withdrawal (P ≥ 0.13). After narasin withdrawal, all doses of narasin inclusion decreased Ac:Prop ratio until day 3 after withdrawal (P ≤ 0.05) when compared with control diet. Effect of narasin on ruminal ammonia nitrogen concentration remained until day 3 after withdrawal (P ≤ 0.04), from day 4 there was no treatments effect (P ≥ 0.11) on ammonia nitrogen concentration. The data obtained from the present study demonstrate that narasin has a prolonged effect after withdrawal from the diet, being observed an effect on ruminal fermentation characteristics within 4 days after narasin withdrawal. These data suggest that the residual effect of narasin can be used as strategic feeding in situations where daily supplementation is not possible. Additionally, concerning scientific research using reversal designs, there is a need for a minimum of 5 days of washout between the experimental periods.