Previous studies indicated that exposure of broiler cockerels to acute heat for 24 hr at five days of age increased their survivability when exposed to high ambient temperature prior to market. Since they were conducted at low relative humidity, the aims of the present study were to determine if higher relative humidity affected the response and to investigate the physiological basis of the response. The early, neonatal, heat stress consisted of increasing ambient temperature to between 35.0 to 37.8 C for 24 hr at five days of age in half of the pens (early heat stress) while the remaining pens were kept at 29.4 C (early control). At 6 wk of age a second, late, juvenile, heat challenge was administered. Ambient temperature in half of the early heat stressed pens and early control pens was gradually increased to between 35.0 to 37.8 C for 8 hr on two consecutive days. Significantly lower mortality during the second heat challenge was observed in the early heat stressed birds. This reduction in mortality ranged from 75 to 90% of that seen in the early control birds. Additionally, there were no deleterious effects on body weight, body weight gain or feed efficiency caused by exposure of birds to early heat stress. Feed restriction or administration of a commercially available electrolyte package to the water had no effect on the ability of the birds to withstand high ambient temperature. Water consumption, core and surface body temperature were increased upon exposure to late heat; however, there were no significant differences between the early heat stressed and early control groups. There was a significant reduction in plasma T₃ concentration in the late heat stressed birds. No significant differences in plasma glucose were observed among the heat treatment groups. A significant increase in total plasma protein occurred during the first sampling period during late heat stress, with values returning to control levels during the second sampling period. No significant differences between the early heat stressed and early control groups were observed in plasma T₄ and total plasma protein during late heat. Heterophil to lymphocyte ratio was lower in the early heat stressed group than in the early control group during the second day of late heat exposure These results indicate that thermotolerance can be induced by exposing broiler chicks to 35.0 to 37.8 C for 24 hr at 5 days of age with no adverse effects on performance. Although the mechanism by which Early, neonatal, heat exposure induces thermotolerance is unknown, it is clear that it does not resemble acclimation. / Master of Science

A procedure was developed to examine factors contributing to skin strength of meat-type chickens. Skin breaking strength was initially measured in lines of chickens divergently selected for high (H) and low (L) juvenile body weight, their reciprocal crosses (HL and LH), and an F₂ generation derived from HL and LH matings. Skin of chicks from the LL line was weakest, that from the HH and LH matings strongest, and that from HL and F₂ matings was intermediate. Percentage heterosis for breaking strength was significantly positive while percentage recombination was not significant. Skin breaking strength and protein, fat, moisture, and total collagen concentrations of skin from the breast, thigh, and back of male and female commercial broilers were then examined in response to diets containing relatively wide or narrow ratios of calories to protein (C:P). Comparisons were made at 28, 42, and 56 days of age. Chickens fed diets containing wider C:P ratios had weaker skin than those fed diets with narrower C:P ratios, with the differences being greater at older than at younger ages. Males had stronger skin than females. Among body sites, breast skin was stronger than thigh skin with that from the back being intermediate in strength. Skin breaking strength did not appear to be consistently associated with either the protein, fat, or collagen concentrations in the skin. Skin from broiler chicks fed diets with differing C:P ratios was also examined histologically. Males had stronger and thinner skin than females, with the differences in thickness due primarily to differences in the thickness of the hypodermis. Regardless of sex, feeding diets containing wider C:P ratios resulted in weaker and thicker skin. The thicker skin was associated with an increase in the thickness of the hypodermis and a decrease in the thickness of the dermis and epidermis. With the exception of differences between back and thigh skin, increases in breaking strength occurring between skin from different sites were associated with a reduction in total skin thickness and in the thickness of the hypodermis. It was concluded that increased hypodermis thickness and/or decreased thickness of the dermis and epidermis reduced skin strength. Two experiments were conducted in which skin strength was examined in relation to heat stress and supplemental ascorbic acid. In the first experiment, chicks fed diets containing 0 or 100 mg of ascorbic acid per kg of diet were subjected to either a constant ambient temperature of 23C, a constant ambient temperature of 32C, or a treatment consisting of 32C from 1600 to 1200 hr and 400 from 1200 to 1600 hr (32/400). Both heat stress and supplemental ascorbic acid tended to increase collagen concentrations in the skin, but differences in breaking strength were inconsistent in response to these treatments. Supplemental ascorbic acid, however, partially alleviated reductions in body weight due to heat stress but did not decrease body temperatures of heat-stressed chicks. In the final experiment, diets containing either 0, 200, or 400 mg/kg of supplemental ascorbic acid were fed to chicks maintained at ambient temperatures of 23 and 34C. Contrary to previous observations, ascorbic acid failed to ameliorate the depressive effect of heat stress on body weight. Supplemental ascorbic acid tended to reduce adrenal, bursa of Fabricius, and spleen weights but had no effect on heterophil and lymphocyte numbers or ratios. Adrenal ascorbic acid concentrations were not significantly affected by supplemental ascorbic acid and plasma ascorbic acid levels were inconsistent among treatment groups. Neither supplemental ascorbic acid nor heat stress significantly influenced skin strength. It was concluded that supplemental ascorbic acid was not beneficial in improving skin strength in commercial broiler chicks. / Ph. D.