A Multi-Component Phytogenic Feed Additive Improves Thermoregulation, Behavior, and Early Growth Performance of Feedlot Cattle During the Summer-to-Autumn Transition in Southern Brazil

Heat stress compromises performance, physiological homeostasis, and welfare in feedlot cattle, stimulating the search for nutritional strategies capable of improving thermal adaptation. This study evaluated the effects of a multi-component phytogenic feed additive, provided alone or in combination with monensin and virginiamycin, on growth performance, hematological and biochemical variables, oxidative status, thermoregulatory responses, and behavior of young Holstein cattle raised during the summer-to-autumn transition in southern Brazil conditions. Thirty-three castrated Holstein steers (10 months of age; 307.1 ± 8.58 kg) were allocated to one of three dietary treatments (n = 11 per treatment): a control diet without additives (GCON), a diet containing the phytogenic additive (GPHY), or a diet containing the phytogenic additive combined with antibiotic growth promoters (GPHY+ANT). The experimental period lasted 80 days, including 15 days of adaptation. The GPHY+ANT group exhibited greater body weight gain than the control during the adaptation period (days 1–15 p = 0.050), whereas cattle receiving the phytogenic additive alone showed greater body weight gain and average daily gain than the control between days 15 and 46 (p = 0.050). Feed intake, feed efficiency, and final body weight were not affected by treatment. Hematological variables remained unchanged, while serum albumin, cholesterol, and triglyceride concentrations were greater in both phytogenic-treated groups than in the control (p ≥ 0.05). Although no overall treatment effect was observed for serum TBARSs or reactive oxygen species (ROS), treatment × day interactions were detected (TBARSs, p = 0.050; ROS, p = 0.010). On day 15, both phytogenic-treated groups exhibited lower TBARS and ROS concentrations than the control group, whereas on day 46, TBARS concentrations were higher in the GPHY+ANT group than in the control group. Cattle receiving the phytogenic additive, either alone or in combination with antibiotic growth promoters, exhibited lower surface temperature, reduced nighttime respiratory rate, and a greater proportion of rumination than control animals (p ≥ 0.05). For standing and lying behaviors, only GPHY+ANT differed from the control, spending less time standing and more time lying (p ≤ 0.05), whereas GPHY showed intermediate values. Additionally, cattle in the GPHY+ANT group spent less time standing and more time lying than control animals (p ≤ 0.05). These findings demonstrate that the phytogenic feed additive improved behavioral and thermoregulatory responses under summer-to-autumn conditions and promoted transient improvements in growth performance during specific stages of the feeding period, without affecting final body weight, overall growth performance, feed intake, or feed efficiency.