Caloric value of cardiac response to hot environments

Broiler survivors of intravenous micro-particle injections: evaluation of growth, livability, meat quality, and arterial blood gas values during a cyclic heat challenge

When broilers are exposed to high ambient temperatures, their cardiac output can increase by 20 to 50%. Previously, we developed an intravenous micro-particle injection technique to select broilers having a cardiopulmonary capacity capable of accommodating increases in cardiac output associated with fast growth and cool temperatures. In the present study, male broilers were injected at 18 to 20 d of age with cellulose micro-particles sufficient to trigger >50% mortality by 35 d of age. The survivors of micro-particle injections (cellulose survivors) and saline-injected flock mates (control group) were exposed to a moderate cyclic heat challenge (peak temperature of 35 to 37 degrees C) beginning on d 36 and continuing through d 57. We tested the hypothesis that if the cellulose survivors represent the population cohort having the most robust cardiopulmonary capacity, then during a subsequent heat challenge these survivorspotentially may perform better than their unselected flock mates. Based on data combined from two independent experiments, the cellulose survivors exhibited improved livability and body weight gain when compared with their unselected flock mates during the cyclic heat challenge. Meat quality characteristics did not differ between the groups. In a third experiment, cellulose survivors and saline-injected male broilers were reared at thermoneutral temperatures or were exposed to a cyclic heat challenge beginning on d 36. Arterial blood samples were collected from unanesthetized birds at 47 to 49 d of age and were analyzed for pH, partial pressure of CO2, bicarbonate, partial pressure of O2, and saturation of hemoglobin with O2. The blood gas values for the cellulose survivors and saline-injected broilers did not differ within a temperature regimen, regardless of whether the broilers were chronically acclimated to heat or were exposed to an acute heat challenge at the end of the experiment. The cellulose survivors did not differ in susceptibility to panting-induced respiratory alkalosis or hypoxemia when compared with their saline-injected flock mates. Overall, these observations indicate that selection for a robust cardiopulmonary capacity can confer advantages in growth and livability without affecting meat quality when broilers are exposed to a moderate heat challenge.

Heart rate, blood pressure, cardiac output, and total peripheral resistance of single comb White Leghorn hens during an acute exposure to 35 C ambient temperature

Two experiments were undertaken to determine selected cardiovascular responses of laying hens to a thermal challenge at 35 C ambient temperature. In the first, six Single Comb White Leghorn (SCWL) hens were exposed to 35 C and changes in systolic and diastolic blood pressure, heart rate, respiratory rate, and rectal temperatures were determined. In the second, seven SCWL hens were exposed to 35 C and changes in cardiac output, total peripheral resistance, blood pressure, heart rate, respiratory rate, and rectal temperature were monitored. In the first experiment heart rate and blood pressure decreased. In the second experiment there was a 58% increase in cardiac output and a 50% decrease in total peripheral resistance accompanied by a 9% decrease in mean blood pressure. Changes in cardiovascular tone preceded respiratory adjustments during acute heat challenges. This may represent one of the most efficient means of heat dissipation for the bird.

The effect of heat on the intestinal and pancreatic levels of amylase and maltase of laying hens and broilers

The effects of exposing laying hens and broilers daily to intermittent periods of 4 hr heating at 42 degrees C on the intestinal and pancreatic levels of amylase and maltase were investigated. The initial exposure to heat, characterized by heat stress, brought about a significant increase in the duodenum and jejunum parts of the tetra breed hen and only in the duodenum of broilers. The levels of amylase in the distal parts of the intestine of both breeds sharply decreased. The increase in amylase levels in the proximal parts of the intestine under the conditions of initial heating vanished after 3 days of heating; its levels continued to fall in the distal parts. In heat acclimatized laying hens the levels of amylase were lower than those of the control hens both in the intestine and pancreas. The pancreatic level of amylase was reversely related to the levels in the intestine. It is assumed that the intestinal level of amylase is regulated by the pancreas. These findings indicate that the pancreas plays an important role during the adaptation of chickens to heat, through the regulation of intestinal level of amylase. The response in maltase level to heat stress and heat acclimatization was insignificant.

The effects of thiouracil and thyroxine on resistance to heat shock

Adult White Plymouth Rock hens were submitted to a heat shock of 42.2 C. It was demonstrated that .1% thiouracil fed for 4 weeks prior to heat stress significantly increased survival time. Administration of DL-thyroxine administered at the rate of 4 micrograms/100 g body weight injected on day of testing did not alter survival time. However, survival under heat shock was significantly reduced if DL-thyroxine was administered daily for 3 days prior to testing. White Rocks exhibited significantly longer survival time under heat shock than Single Comb White Leghorns, and females of both breeds exhibited significantly longer survival time their male counterparts. Treatment with DL-thyroxine greatly reduced the survival time differences between breeds and sex. The implications of these findings for applied breeding programs are discussed.