Glucose appears to be a preferred systemic fuel during heat stress (HS) in a variety of species. Increasing the dietary grain content can enhance the post-absorptive carbohydrate status, but providing excessive fermentable starch can cause rumen disorders and this is especially true during HS. Current study objectives were to evaluate the effects of a glycerol based supplemental product on growth and metabolic variables in Holstein bull calves during controlled HS. Before the start of the experiment, bull calves (n¼14; 163.6730.1 kg body weight) were subjected to thermal neutral conditions [26.573.41C and a temperature–humidity index (THI) of 70.472.8] for 7 d (period 1; P1). During this period, productive parameters as well as blood metabolites were measured and used as covariates for the subsequent HS period. Following P1, a cyclical HS pattern was implemented for 21 d (P2) where daily ambient temperatures ranged from 29.1 to 39.71C and the THI was474 for 24 h/d and483 for at least 14 h/d. During P2, half of the HS calves (n¼7) received a control diet (CON) and the other half received the control diet supplemented with a product (300 g/d) containing gluconeogenic precursors (GLU). Throughout each period respiration rate, rectal temperature and skin temperature at the shoulder and rump were recorded at 0600, 1100 and 1500 h daily. Blood samples were obtained prior to and 4 h post the a.m. feeding during both periods. Although HS markedly reduced DMI (18%) and growth as expected, supplemental GLU did not affect body weight gain. Supplemental GLU decreased the shoulder temperature at 0600 and 1500 h (Po0.01), and decreased respiratory rate at 1500 h (Po0.02). Feeding GLU did not affect blood urea nitrogen (BUN), glucose or nonesterified fatty acids (NEFA) concentrations, but increased circulating insulin prior to the a.m. feeding (Po0.03) and this demonstrates that GLU was effective at enhancing the post-absorptive carbohydrate status. Our results suggest that feeding supplemental GLU improves some body temperature indices but did not enhance growth performance in Holstein bull calves during HS.

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