PS11-10. Effect of a Blend of Essential Oils Alone or in Combination with Monensin on the Growth of Grass-fed Beef Cattle.

Abstract This study evaluated the effects of essential oils and monensin on the growth of grass-fed beef cattle. The experiment was conducted in São Paulo state, Brazil, in collaboration with FZEA–USP, from July 2022 to March 2023, totalling 231 days (91 days in the dry season and 111 days in the wet season). Sixty-four male calves (48 Nellore and 16 Tabapuã), averaging 280 days of age, were allocated to four treatments based on breed and initial weight: control (CTRL), monensin (MON), essential oils (EO, Oleobiotec Ruminant, Phodé) and the combination of both (MON+EO). All animals received a protein-mineral supplement at 0.1% of average body weight. It was offered 3 times per week and leftovers were recorded weekly. The experimental area consisted of four 6.7ha paddocks of Urochloa brizantha cv. Marandu, equipped with water troughs and feeders. The average stocking rate was 1.68 AU/ha, with a mean forage availability of 6,450 kg DM/ha. Treatments rotated weekly among paddocks so that all animals passed through each paddock every 28 days, minimizing paddock effect. Animals were weighed every 28 days. Data were analysed using linear and mixed models in R v4.5.2. Initial body weight was similar across treatments (P = 0.96), averaging 256.51 ± 9.1 kg. After 231 days, animals receiving the monensin-essential oil combination achieved heavier final body weights compared to control (MON+EO 383.47 kg vs CTRL, 367.31 kg, P = 0.04), while monensin alone (MON, 377.72 kg) and essential oils alone (EO, 380.25 kg) showed intermediate values. Overall average daily gain reflected the same pattern (CTRL 0.48 kg/d; MON 0.52 kg/d; EO 0.54 kg/d; MON+EO 0.55 kg/d P = 0.04). Mixed model analysis confirmed a significant treatment effect on both body weight and ADG. Time point and season were significant for both outcomes (P < 0.001), reflecting temporal growth trends and changes in forage quality across seasons. No treatment × season interactions were detected, indicating that the treatment effect was consistent regardless of season. This study suggests that essential oils, monensin and particularly their combination improved growth performance in grass-fed beef cattle compared to non-supplemented animals, supporting further investigation into their potential synergistic mechanisms. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.