The summer nutrition of weaner sheep: The relative roles of available energy and protein when fed as supplements to sheep grazing mature pasture herbage

In a field experiment of factorial design, concentrate supplements of cereal origin were fed to 16 groups each of eight young Merino wethers. These sheep grazed the mature herbage of Phalaris tuberosa L.-subterranean clover (Trifolium subterraneum L.) sown pastures during summer in the Mediterranean environment of Adelaide, South Australia; and under conditions of common grazing, sheep in the different treatment groups were each day individually fed rations which varied in content of available energy and of digestible nitrogen (protein). The daily allowances of energy were 48, 113, 227, and 340 g total digestible nutrients (T.D.N.), and of digestible nitrogen either 2.5, 5.0, 7.5, or 10.0 g. In addition there was a control group which received no supplement. No interaction was observed between the energy and the nitrogen portion of the supplement, each constituent having its independent effect in terms of both body weight gain and wool growth. Responses to changing energy were linear for both wool production and body weight gain. The effect on wool production of increasing protein was also linear, but the gain in body weight with each increment of protein progressively decreased, which suggested an approach to an asymptotic value. It is concluded that for body weight gain mature herbage may be deficient in both available energy and protein, but once a small need for protein is satisfied energy becomes a limiting factor. In terms of wool production summer pastures are deficient in both protein and energy, the latter constituent making a notable contribution in influencing nitrogen retention.