PSIX-2 - Residual Effect of Narasin on Ruminal Fermentation Characteristics in Steers

Abstract: This study aimed to evaluate the effect of narasin withdrawal on steers ruminal fermentation characteristics. Nellore steers [n = 30; initial body weight (BW) = 281 ± 21 kg] cannulated at the rumen were assigned to individual pens in a randomized complete block design according to their initial shrunk BW. The trial lasted 156 days, divided into 2 periods. The first period lasted 140 days and consisted of the daily supply of narasin. In the second period (last 16 days) the animals were not supplemented with narasin, when the residual effect of the additive was evaluated. The diet was composed by 99% haylage Tifton-85, and 1% concentrate used as a delivery vehicle for narasin. The experimental diets were: 1) no feed additives (CON); 2) 13 mg of narasin/kg of DM (N13); 3) 20 mg of narasin/kg of DM (N20). The dry matter intake (DMI) was evaluated daily and the ruminal fluid was obtained on day 0, 1, 3, 5, 8 and 16 after narasin withdrawal. The data were analyzed using the MIXED procedure, and the effect was considered significant when P ≤ 0.05. The DMI was not affected after narasin withdrawal (P ≥ 0.27). There was a treatment and day interaction after narasin removal for the molar ratio of acetate (P < 0.01), propionate (P < 0.01), ac:prop ratio (P < 0.01) and ammonia nitrogen (P = 0.03). The inclusion of narasin linearly reduced acetate (P < 0.01), increased propionate (P ≤ 0.04), and linearly decreased the ac:prop ratio (P < 0.01) up to 5 days after narasin withdrawal. Narasin linearly decreased ammonia nitrogen up to 1 day after withdrawal (P < 0.01). It is concluded that use of narasin results in a residual effect on the rumen fermentation process after removal of the additive from the diets.