Environmental effects on mineral supplement intake in grazing Nellore heifers under tropical conditions

Environmental factors can influence nutrient intake in grazing cattle, yet few studies have evaluated how such conditions affect mineral supplement intake. This study assessed the effects of ambient temperature, rainfall, and seasonal variation on mineral supplement intake in Nellore heifers under tropical grazing conditions. Thirty-six heifers (initial age: 9 ± 1 months.; final age: 28 months; initial and final average body weights: 170 and 386 kg, respectively) were managed in a continuous grazing system on a 24 ha pasture of Brachiaria brizantha cv. MG4. Individual supplement intake was recorded via electronic smart feeders (Intergado®), and environmental data were collected hourly from 09:00 to 18:00 h during a 21-month experimental period. Days were categorized based on the thermal comfort zone (TCZ) for zebu cattle (10 °C to 27 °C) as: within TCZ, below 10 °C (TL10), or above 27 °C (TH27). Supplement intake was evaluated in relation to hours spent outside the TCZ, rainfall occurrence, and season (dry or wet). Average intake on days within the TCZ did not differ from TL10 or TH27 (P = 0.07). However, as the duration of thermal stress increased, supplement intake and feeder visitation rates declined (P < 0.001). Heifers exposed to more than three hours below or two hours above the TCZ showed the lowest intakes. Rainfall and season had no effects on mineral intake. Prolonged exposure to environmental temperatures outside the TCZ negatively affects mineral supplement intake, highlighting the importance of considering climatic stressors in pasture-based nutrition programs.