PS1-23. Performance and Carcass Responses to Monensin, Narasin, and Their Alternation in Feedlot Heifers Fed High-concentrate Diets.

Abstract Improving nutrient utilization remains a key challenge in feedlot systems, particularly under high-concentrate diets. Ionophores modify ruminal fermentation by reducing methane production and amino acid degradation while increasing propionate production, a more efficient energy source for the animal. However, responses in dry matter intake (DMI), average daily gain (ADG), and feed efficiency vary depending on dose, animal category, and diet composition. In addition, the magnitude of monensin responses has declined over time, likely due to advancements in feedlot management. Narasin has been associated with increased feed intake, whereas monensin typically improves feed efficiency. Therefore, we hypothesized that alternating narasin and monensin during the feedlot period could combine these complementary effects and optimize overall animal performance. At the beginning of the experiment, heifers averaged 210 ± 16.95 kg of body weight and were approximately 8 months of age. Animals were assigned to a randomized complete block design based on initial body weight. Treatments consisted of: (1) CON = basal diet without additive; (2) MON = CON + 25 ppm monensin; (3) NARA = CON + 13 ppm narasin; and (4) N+M = alternation between narasin (13 ppm) and monensin (25 ppm) across experimental periods. The study lasted 154 days, including 14 days of adaptation followed by 140 days of data collection, divided into five 28-day periods. Data were analyzed using the MIXED procedure of SAS, with significance declared at P ≤ 0.05. Narasin increased DMI (kg/day, P = 0.0053; % of body weight, P = 0.0147; g/kg BW^0.75, P = 0.0113) compared to MON and N+M. No treatment effects were observed for ADG (P = 0.1113) or feed efficiency (P = 0.1272). Body weight did not differ among treatments during the initial periods; however, heifers fed NARA had greater body weight in the fourth period and tended to be greater in the third (P = 0.0963) and fifth (P = 0.0538) periods compared to N+M. Carcass characteristics were not affected by treatments. Overall, narasin consistently increased feed intake compared to monensin-containing strategies, whereas alternating ionophores did not improve performance or carcass traits. These results indicate that narasin may be a viable strategy to enhance intake in feedlot systems without compromising performance.