Hypothermia in newborn lambs induced by experimental immersion in a water bath and by natural exposure outdoors.

A progressively cooling water bath technique was developed to measure resistance to body cooling in newborn lambs. Cold resistance, defined as the time taken to reduce rectal temperature to 35 degrees C, was measured in 429 lambs of 12 different breeds individually immersed in a water bath. Two alternative procedures were used with water temperature falling from 37 degrees C to 12 degrees C or from 25 degrees C to 10 degrees C. In lambs tested twice the repeatability of cold resistance was high: 0.95 and 0.81 respectively in the two types of test. There were clear breed differences in cold resistance, some breeds being up to three times more resistant than others. Health was unimpaired by treatment and preweaning mortality was not affected. Ten of the breeds represented in water bath tests were also used for measurements of rectal temperature 1 h after birth in the field. There was some similarity in the breed rank order for ability to resist hypothermia in the field and in the water bath. Water bath tests of hypothermia in lambs could facilitate genetic selection for improved perinatal survival in the field.