Effects of high ambient temperature on respiration rate, rectal temperature, fetal development and thyrold gland activity in tropical and temperate breeds of sheep

Abstract

The effects of high ambient temperatures on rectal temperature (RT), respiration rate (RR), fetal development and serum thyroxine (T 4) concentrations were stuaied in two experiments involving 35 ewes and 26 lambs from the following ewe groups: 1) Barbados Blackbelly (B), a tropical breed; 2) Dorset (D), a temperate breed; and 3) Blackbelly × Dorset crosses (B×D). Data were obtained on four B, five D and five B×D ewes exhibiting estrus during the summer (Exp. 1). In Exp. 2, eight B, seven D and six B×D ewes were maintained in two environmental chambers (cool, 22.2C; hot, 33.8C) from day 125 of gestation to seven days before the expected lambing date for each breed group (D and B×D, 140±1; B, 144±1 day of gestation). The B and B×D ewes were more heat-tolerant than D ewes as measured by significantly lower RT and RR in each experiment. Mean lamb birth weight, crown-rump length, number of functional uterine caruncles and caruncle weight and size did not vary significantly among breed groups or temperature chamber (Exp. 2), and there was no indication that the high temperature imposed caused fetal dwarfing in lambs removed from the uterus at a standard age of seven days before expected parturition. Serum T 4 varied markedly among breed groups (P<0.05) in each experiment with B ewes having the lowest and B×D ewes the highest concentration. In Exp. 1, follicular stage T 4 concentrations in B and B×D ewes were lower (P<0.02) than those during the luteal stage of the estrous cycle. The decrease in D ewes was not significant. High ambient temperature (Exp. 2) depressed T 4 levels in D ewes (P<0.05) and also depressed the pituitary-thyroid response to thyrotropin releasing hormone in D lambs. Such was not the case in B and B×D ewes and their lambs.