Genetic variation of rectal temperature in cows and its relationship to fertility

Abstract

ABSTRACT Rectal temperatures of breeding cows at the National Cattle Breeding Station, ‘Belmont’, Rockhampton were recorded in each of 2 years. The data analysed represented two Bos taurus (Hereford × Shorthorn) and four B. indicus × B. taurus half-bred lines, and 800 to 900 cows in each year. Rectal temperatures averaged 39·8°C, and were 0·5°C higher in British breed than in zebu-cross cows and 0·3°C higher in lactating than in dry British cows (no effect in zebu cross). Fertility, measured as success or failure in producing a calf at term, was affected by rectal temperature (P<0·0001 in both years). The relationship was curvilinear, the reduction in calving rate due to 01°C increment in temperature being 0009 at 39°C, 002 at 40°C and 0035 at 41°C. Response of fertility to a given change in rectal temperature was the same in different breeds. The average depression of fertility due to heat susceptibility was 015 to 0·25 in British-breed and approximately 010 in zebu-cross herds. Even in relatively heat-tolerant zebu crossbreds and in a relatively mild environment on the Tropic, both the mean rectal temperature and its genetic variability within a herd had quite large effects on reproduction. The heritability of rectal temperature was 0·25 (s.e. 012) and its genetic correlation with fertility was −0·76 (s.e. 0·35). There was a parallel effect of rectal temperatures of cows on the birth weights of their calves.