Effects of prenatal shearing of ewes and calf genotype on cold tolerance of newborn ruminants

Abstract

Weather-related losses are the second leading cause of death for both calves and lambs. One management practice used to reduce lamb losses due to cold exposure is shearing pregnant ewes. Research has shown that lambs born to shorn ewes have enhanced brown adipose tissue (BAT) thermogenesis and increased birth weights and survival rates. The first study was designed to determine the effects of late-gestation shearing on lamb birth weights and survival in West Texas conditions. Ramboillet ewes were either shorn or remained unshorn 20-d prior to lambing. Male lambs from shorn ewes had 12% lower survival rates at one day of age, whereas, survival rates of female lambs were not affected by shearing treatment. Lamb birth weights were not affected by shearing treatment. The results of this experiment indicate that prenatal shearing of ewes in West Texas conditions did not increase birth weights or improve survival of newborn lambs. Breedtype also affects cold tolerance. Brahman calves generate less heat from BAT when stimulated with norepinephrine than Angus calves. A second study was conducted that examined the effects of cold exposure on BAT composition and iodothyronine 5'-deiodinase (5'D) activity in Angus and Brahman calves. Calves were slaughtered as newborns, or after 48-h exposure to either warm (20° C) or cold (4° C). Rectal temperatures were higher in Angus calves at 0-h, but were similar to Brahman calves thereafter. Brahman calves had higher plasma T₃ and T₄ concentrations than Angus calves, but T₃ increased more due to cold exposure in Brahman calves. Plasma T₄ concentrations were higher in cold than warm calves. Brahman calves had less BAT lipid at birth and after cold exposure, but similar amounts after warm exposure. Ctytochrome c oxidase activity was not affected by breed, and was higher in cold than warm calves. BAT 5'D was higher in Brahman calves at birth, but not after warm or cold treatment. BAT 5'D-II activity was higher in Brahman calves, but was not affected by postnatal treatment. Brahman calves appeared to mobilize more lipid during cold exposure, but not use it effectively to support BAT thermogenesis.