INTRODUCTION The potential for growth and high health status in the newborn calf is largely influenced by the health and metabolic status of their dam. Much of the focus of cow management has been on the perinatal period as the calf prepares for delivery into a totally foreign environment in which placentally derived nutrition is replaced by the initial lacteal secretion from the mammary gland, colostrum. The composition of this secretion is extremely important in establishing the growth potential and life-long productivity of the calf. This then reverts to normal milk which acts as a source of dietary energy and protein through to the point when the calf is able to be weaned. This initial phase is most often termed the pre-ruminant period during which milk is passed directly into the abomasum through the reflex closure of the oesophageal groove. This initial period of development is termed the pre-ruminant phase and varies in duration from 14-21 days depending on the animal's ability to initiate the intake of dry feed. During the subsequent 3-6 weeks ruminal function develops and the calf derives more of its nutrient substrate from this source than from milk. At this point the animal is weaned and derives its nutrient substrate solely from dry feeds through the activity of the newly established ruminal microbial population: this describes the ruminant phase of calf development.