EFFECTS OF SUPPLEMENTAL ASCORBIC-ACID ON THE ENERGY-CONVERSION OF BROILER CHICKS DURING HEAT-STRESS AND FEED WITHDRAWAL

The objectives of this study were 1) to determine the effects of suppl emental ascorbic acid (AA) on the energy conversion of broiler chicks maintained at thermoneutral and potential heat stress temperatures usi ng indirect convective calorimetry; and 2) to determine whether change s in energy conversion are reflected in changes in lipid metabolism. I n Experiment 1, 120 2-d-old cockerels, housed in two identical environ mental chambers, were maintained under constant light (2.0 +/- 0.2 fc) and recommended thermal conditions (29.6 +/- 0.8 C; 33.4 +/- 8.0% RH) and consumed water and feed ad libitum. Beginning on Day 8 posthatch, one-half of the birds inside each chamber were randomly assigned and received feed supplemented with AA. Beginning on Day 9 posthatch, the temperature inside one chamber was increased to 34 C whereas the other chamber remained thermoneutral. This design resulted in four treatmen ts: 1) thermoneutral (TN: 27.7 +/- 0.8 C; 40.9 +/- 9.4% RH) and 0 mg A A/kg feed (ppm); 2) TN and 150 ppm AA; 3) heat stress (H: 33.8 +/- 0.5 C; 43.3 +/- 7.4% RH) and 0 ppm AA; or 4) H and 150 ppm AA. Also begin ning on Day 9 posthatch, birds were randomly assigned to one of three identical, indirect convective calorimeters designed to accommodate TN or H. Oxygen consumption, carbon dioxide production, respiratory quot ient, and heat production were evaluated daily for 8 h, through Day 17 posthatch. Following calorimetric measurement, birds were returned to their respective caging unit/chamber for the remainder of the study. Weight gain, feed intake, and gain:feed were also measured over the 9- d study. Heat exposure depressed (P < 0.05) weight gain, feed intake, and gain:feed. Ascorbic acid increased (P < 0.10) weight gain. Oxygen consumption and carbon dioxide and heat production per kilogram(0.75) decreased (P < 0.05) with age with no change in the respiratory quotie nt. Heat exposure lowered (P < 0.001) the respiratory quotient. A temp erature by AA interaction was detected in which heat-exposed birds exp ressed lower (P < 0.10) respiratory quotients when consuming the AA-su pplemented diet. In Experiment 2, 18 2-d-old cockerels, housed in an e nvironmental chamber, were maintained under constant light and recomme nded thermal conditions (29.3 +/- 0.4 C; 41.4 +/- 3.3% RH) and consume d water and feed ad libitum. On Day 9 posthatch, birds were deprived o f feed for 24 h with ad libitum access to water supplemented with eith er 0 or 400 mg AA/L. Blood samples were obtained from each bird before and after feed withdrawal and supplementation. Supplemented birds exh ibited elevated (P < 0.01) plasma AA, levels that were not affected by feed deprivation. Feed deprivation increased (P < 0.0001) plasma beta -hydroxybutyrate with no effect of AA, and decreased (P < 0.05) plasma triglycerides in the unsupplemented birds. A feed withdrawal by AA in teraction was detected in which plasma triglycerides remained elevated in birds supplemented with AA. These data suggest that supplemental A A influences body energy stores that are used for energy purposes duri ng periods of reduced energy intake.