| Summary: | A series of experiments have been conducted which examined the function of roughage in the diet of the early-weaned calf and determined the optimum specifications relating to roughage inclusion in the diet. Evaluation of the conventional voluntary roughage feeding system indi?cated an unacceptable degree of variation in performance and intake characteristics which was decreased by the feeding of a complete pellet diet containing chopped roughage (20 mm). Feeding a complete diet also encouraged the earlier development of rumen function, resulting in enhanced feed intakes with associated increased liveweight gains. The principal advantage of complete diet feeding was associated with its physiological effect in the rumen, where the feeding of a predeter?mined roughage:concentrate ratio encouraged an expeditious rise in rumen pH status. Rumen buffering capacity of individual diets accounted for as much of the variation in feed intake as the energy content of the diet. The presence of roughage also enhanced the efficiency of feed utilisation (protein and energy) of the concentrate fraction. Slaughter studies indicated that complete diets, which gave rise to higher ruminal levels of butyric acid, inspired a rumen fermentation pattern more conducive to rumen papillary growth. The optimum dietary crude fibre specification for maximising liveweight gain was 150 g per kg, which detailed levels of roughage inclusion of 220 g per kg straw and 300 g per kg hay; there was only a slight advantage to the use of hay as a roughage source rather than straw. The results suggest that the particle size of roughage should not be less than 8 mm when fed in a complete pellet diet to calves.
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