Effects of a supplement containing multiple types of gluconeogenic precursors on production and metabolism in Holstein bull calves during heat stress

• We evaluated the effects of a commercial glucose precursors during heat stress in bull calves.
• We assessed the effects of this product on productive performance of bull calves during heat stress.
• We examined changes in the level of blood parameters during heat stress and in combination with a dietary treatment.
• This product changed insulin level but not productive performance and body temperature indices.

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 (n=14; 163.6±30.1 kg body weight) were subjected to thermal neutral conditions [26.5±3.4 °C and a temperature–humidity index (THI) of 70.4±2.8] 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 39.7 °C and the THI was >74 for 24 h/d and >83 for at least 14 h/d. During P2, half of the HS calves (n=7) 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 (P<0.01), and decreased respiratory rate at 1500 h (P<0.02). 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 (P<0.03) 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.