Volume 27, 238

Tropical Grasslands (1993) Volume 27, 238–249

Rearing heifers in the subtropics and tropics:
nutrient requirements and supplementation

R.J. MOSS

Queensland Department of Primary Industries, Mutdapilly Research Station, Ipswich, Queensland, Australia

Abstract

Dairy herd productivity is influenced by the age and live weight of heifers at calving, yet the importance of adequate nutrition and management of replacement heifers is often overlooked, and production losses associated with slow growth are not fully recognised. In northern Australia, few heifers calve by 2 years of age, and very few would achieve what is considered to be optimal size at calving. The relatively slow growth of heifers is associated with the use of tropical pastures, which support growth rates in the order of 0.25 kg/d in unsupplemented calves. These low growth rates result from the high fibre content, low dry matter digestibility and low protein : energy ratio of tropical grasses. Supplementary feeding is necessary to increase this growth rate above 0.6 kg/d. Environmental factors can also affect performance, and growth rates are consistently less during the warm, wet season than the dry season. Heat stress, high humidity and internal and external parasites reduce intake and hence growth rate.
If heifers are to achieve their potential within the herd, they need to calve at around 85% of their mature size. Live weight of cows at maturity increases with their level of feeding, so growth targets will increase with herd production, requiring higher concentrate inputs and improvement of the basal forage. Too rapid growth in pre-pubertal heifers may cause excessive fattening and impair milk production but this is unlikely to occur with tropical pastures. Heifers compete with the milking herd for feed resources and optimum live weight of heifers on a particular farm will be dependent on the level of milk production, and hence live weight of cows on that farm.