Effect of heat stress on ingestive, digestive, ruminal and physiological parameters of Nellore cattle feeding low- or high-energy diets

This study aimed to verify the effects of heat stress and dietary energy level on dry matter intake (DMI), water intake, total and partial ruminal and intestinal digestibilities, ruminal characteristics, physiological and digestive behavioral parameters in beef cattle. Six ruminally cannulated Nellore heifers (280 ± 12 kg) were used in a 6 × 6 Latin square experiment with six treatments and six periods of 21 days. The treatments consisted of a 2 × 2 + 2 factorial arrangement, with two temperatures during the day (thermoneutrality, TN: 24°C; and heat stress, HS: 34°C), two dietary energy levels (low- or high-energy concentration) and two additional treatments of animals maintained in TN conditions but pair-fed with the heifers in HS (PFTN). The HS decreased DM intake by about 16% and increase water intake by 25.5% respectively (P ≤ 0.01). Heifers maintained in TN and fed high-energy diet showed 17.3 and 14.3% greater ruminal dry matter (DM) and organic matter (OM) digestibilities compared to their HS counterparts. The DM intake rate (ki, h⁻¹) and digestion rates (kd, h⁻¹) were affected by environment and dietary energy level (P ≤ 0.01), which heifers in HS having lower ki and kd. Heifers in HS presented lower pH and greater ruminal temperature values (P ≤ 0.01). When PFTN treatments were compared vs. HS, there was a significant effect of temperature on acetic acid. Heifers that underwent HS showed 14 % less acetic acid. The ocular temperature, body temperature, heart rate, and respiratory rate were greater in HS heifers (P ≤ 0.01) than in TN. Heifers with low energy level diets showed 33% greater values of rumination time (P = 0.05) when compared to high energy levels independent of the temperatures. We concluded that adaptive response to heat seems important factors responsible for the alteration of DMI, water, and other nutritional fractions like OM, CP, and energy intake. On the other hand, increments in intestinal digestibility in zebu heifers suggest an adaptive response of the digestive tract to heat stress conditions.