Abstract

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.