Efeito do estresse por calor sobre parâmetros ingestivos, digestivo e fisiológico de bovinos alimentados com dietas de alto ou baixo nível energético

Abstract

The aim of this study was to verify the effect of a temperature of heat stress and the energetic level of the diet on the feed intake, water intake, dry matter, nutritional fractions digestibility, physiological and digestive behavioral parameters in beef cattle. The experiment was carried out in the beef cattle facilities of the Federal University of Lavras. Six heifers with an average initial weight of 280 kg were used in a 6 × 6 Latin square experiment with six treatments and six periods of 21 days. The 6 treatments consisted of a 2x2 + 2 factorial arrangement, with two temperatures during the day (24 °C and 34 °C), two diets (high and low energy level) and two additional treatments with restricted intake at the same level of the animals in heat stress to compare the variables to the condition of thermoneutrality. The data were analyzed using SAS. The animals in the treatments of heat stress had a decrease of 15% in the total dry matter intake (TDMI), which represented a reduction on average of 1.33 kg of DMI. However, there was an increase in water intake by 38.5% (P<0001). There was no difference in the total apparent digestibility coefficient of DM, OM, PB, EE and NDT (P> 0.05). Animals with free intake and fed with high energy level presented higher values of pH (pH = 5.62). The animals that underwent heat stress had higher values of ruminal temperature (39.6 °C vs 39.4 °C). For the variable N-NH3, there was a significant effect (P = <.0001) of the diet, with the highest values presented by the animals with low energy diets. No effects of the interaction between the evaluated factors and the variables related to the ocular and corporal temperature were observed. The ocular temperature at 18 hours was influenced by heat stress (P <0.001), being 1.7 °C higher than the temperature measured during the first hours of the day. There was a significant effect of heat stress on body temperature (P <0.001). The heart rate was influenced by the daytime ambient temperature (P= 0.007) with a 16% increase in beats per minute. The respiratory rate was affected as a function of the heat stress treatment, both in the day and at night (P<0.001). Animals with low energy level diets presented higher values of rumination time (33%) (P = 0.052). Heat stress increases water intake and reduces intake of dry matter, nutritional fractions and energy, but does not alter digestibility in zebu heifers. Ruminal temperature is marginally elevated under conditions of heat stress in cattle. Increased respiratory rate is the main way of controlling body temperature presented by zebu cattle in heat stress.