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Ninety young crossbreed bulls (approximately 25% Zebu breeding with the remainder represented by continental and British breeds in various proportions, initial weight=228.04±7.06 kg) were used in a 84-day feeding trial to assess the effects of treatments in feedlot cattle fed transition diets during high ambient temperatures. Treatments consisted of a steam-flaked corn-based diets (average 1.95 Mcal ENm/kg) supplemented with: 1) 20 mg monensin/kg diet (MON, Rumensin, Elanco Animal Health, Indianapolis, IN], and 2) 100 mg of blend of essential oils/kg diet plus 0.1 mg 25-hydroxy-vitamin-D3/kg diet (EO+HyD; CRINA® Ruminants and HyD®, DSM Nutritional Products, Basel, Switzerland). Both dry matter intake (DMI) and climatic variables were measured daily and the temperature humidity index (THI) was estimated. Daily maximal THI remained over 80 during the whole trial (avg. THI = 82.67). Cattle supplemented with EO+HyD increased average daily gain in 8.7% (1.446 vs. 1.320 kg/day, P < 0.01), gain-to-fed ratio in 4.5% (0.199 vs. 0.190; P = 0.03), final weight in 10.05 kg (349.48 vs. 339.43; P = 0.04) and tended (P = 0.07) to shown greater dietary net energy (2.5%) and observed-toexpected dietary NE ratio (3%). Even when EO+HyD tended to increase (4.3%; P = 0.06) DM intake, intake pattern variation were not different (P = 0.38) between MON and EO+HyD (Figure 1). Then, difference in ADG and final BW between MON and EO+HyD was not only a reflection of difference in energy intake, was also caused by difference in efficiency of energy utilization during conditions of high ambient temperature (a reduction of 7% in the estimated increase of coefficient of maintenance by heat load). Results indicate that supplementation with a combination of essential oil blend plus 25-hydroxy-vitamin-D3 can have greater beneficial effects than supplemental monensin on daily weight gain, final weight and feed intake during initiation-transition phase of cattle raised under high ambient temperature. [ABSTRACT FROM AUTHOR]