Producers should avoid removing milk from a calf’s diet until the calf is at least 10 weeks old, when the rumen becomes functional. Powdered milk or dry milk replacers can be used where a calf doesn’t have access to its mother (refer to Weaner management in northern beef herds).
Table 1 below indicates the categories for weaning based on age, weight and reason for transitioning the calf to an adult diet.
Development of the rumen
The rumen and reticulum start developing within a few weeks after birth as the calf begins to eat fibrous feed, with the rumen microorganisms (‘bugs’) being transferred from adult cattle.
Under normal paddock conditions, the rumen will be functional by the time the calf is 10 weeks old (see Figure 1). Milk remains the best source of energy, protein, calcium and phosphorus for the unweaned calf, but at this stage can be completely substituted.
Following weaning, the weaner must obtain all of its nutrients from a pasture-based diet typically high in fibre. In grazing cattle, the rumen microbes primarily break down this plant fibre through fermentation, which becomes the energy source for the animal.
Changes in any diet throughout the animal’s life must be made gradually so that the rumen microbes can adjust. A rapid change from a fibre diet to one high in starch (grain) results in the rumen contents becoming too acidic, causing severe metabolic disorders or even sudden death.
The age at which a calf is weaned determines its diet for the next few months, especially if weaning occurs in the dry season. Urea can be used as a non-protein based nitrogen supplement, but calves without a functioning rumen cannot utilise this. If they consume it without a fully functioning rumen, they may die.
The nutrition of a growing calf has a significant impact on its performance as it reaches maturity. Providing the correct diet following weaning, regardless of age, ensures optimum performance of the animal.