Multiple ovulation and embryo transfer techniques increase the possibility of improving the reproductive efficiency of beef cows by twinning. However, the gestation length of cows carrying twin calves is approximately 10 days shorter than cows carrying a single foetus and twin calves at birth are lighter (0.85 of controls) and growth rate to weaning is less (0.85 of controls) compared with single born calves (Dickerson and Anderson 1988). Changes in gestation length and the low birth weight of the twin calf may partly be associated with changes in the nutritional status of the dam and the increased nutrient requirement. It is important to define the nutritional requirements of twin bearing cows if twin calves are to be successfully produced and reared.

The cows were a mixture of Hereford and Charolais cross Friesian suckler cows and heifers. Animals used for the measurement of heat production were paired where possible according to parity and live weight, one member of each pair carrying a single calf and the other twins.