PS1-22. Monensin, Narasin, and Their Alternation in High-concentrate Diets Do Not Accelerate Puberty in F1 (Angus × Nellore) Heifers.

Abstract Age at puberty and age at first calving are key determinants of efficiency in beef production systems. Earlier reproductive maturity allows heifers to calve sooner and remain productive for a longer period, increasing lifetime productivity. Puberty onset is influenced by the interaction between genetic background and nutritional management during the pre-pubertal growth phase. Nutritional strategies that enhance body development during this stage may contribute to earlier reproductive maturity. In this context, feedlot systems combined with feed additives that improve dietary energy utilization could potentially promote earlier puberty. Therefore, we hypothesized that feedlotting pre-pubertal heifers associated with ionophore supplementation would enhance body development and advance the onset of puberty. Forty F1 (Angus × Nellore) heifers were used in this study. At the beginning of the experiment, animals averaged 210 ± 16.95 kg of body weight and were approximately 8 months of age. Heifers were assigned to a randomized complete block design according to initial body weight. Treatments consisted of: (1) CON = basal diet without additive; (2) MON = CON diet plus 25 ppm monensin; (3) NARA = CON diet plus 13 ppm narasin; and (4) N+M = alternation between narasin (13 ppm) and monensin (25 ppm) at each experimental period. The experimental period lasted 154 days, including 14 days of dietary adaptation followed by 140 days of evaluation, divided into five 28-day experimental periods. Heifers were monitored for reproductive development by transrectal ultrasonography to detect corpus luteum (CL) formation. Evaluations began during the second experimental period and were conducted weekly, totaling 14 evaluations per animal throughout the study. The presence of a CL was used as an indicator of luteal activity, and heifers were considered pubertal on the day the structure was first detected. Growth performance was also monitored during the experimental period. Data were analyzed using the MIXED procedure of SAS, and treatment effects were considered significant at P ≤ 0.05. No treatment effects were observed for age at puberty (P = 0.75), body weight at puberty (P = 0.87), average daily gain until puberty (P = 0.73), or corpus luteum diameter (P = 0.19). At the end of the experimental period, all heifers in the control treatment reached puberty, whereas approximately 80% of heifers receiving ionophores (MON, NARA, and N+M) presented a corpus luteum. These results indicate that ionophore supplementation during feedlot development did not accelerate puberty in F1 Angus × Nellore heifers. However, the feedlot system allowed early identification of heifers that reached puberty, which may support management decisions for replacement selection. Heifers that fail to reach puberty during this period may be redirected to finishing systems, whereas pubertal females may be retained as replacements, improving efficiency in beef production systems.