Nutritional status and intake regulation in sheep. VIII.* Relationships between voluntary intake of herbage by sheep and the protein/energy ratio in the digestion products

Abstract

In data from two separate experiments in which the same herbage diets were fed to sheep, a relationship was observed between the protein/energy ratio in digestion products and the level of voluntary feed intake: I = 0.16P—0.16 (SEb = 0.015; r2 = 0.85), where I is the voluntary intake of digestible energy (DE) (MJ/W0.75), P the protein digested in the intestine (g/MJ DE) and W the body weight (kg). When supplementary casein was infused into the duodenum of sheep fed on 15 basal diets, intake changes were greatest (up to 15% increase) with six roughage diets, in which estimated truly digestible protein contributed 5.5 g digested protein (DP) per MJ DE (about 10% of DE as protein) or less. No responses were observed with two other roughages in the same range or with seven roughages for which the estimated truly digestible protein contributed more than 6 g per MJ DE (about 13% of DE as protein). The change in voluntary intake was not found to be simply linked to the protein input, in that a consistent overall estimated protein/energy ratio in digestion products was not established as voluntary intake changed in response to protein infusion. The estimated resultant protein/energy ratios established were always high (7.4–9.4 g DP/MJ DE) relative to those observed on the basal diets (3.4–8.4 g DP/MJ DE). In a further experiment with a wheat hay–straw diet, voluntary intake was measured during periods of infusion of acetic acid per rumen, and/or protein (casein) infusion per duodenum. Energy infusion and protein infusion could be shown qualitatively to have opposed effects on oral intake. However, oral intake adjustments did not appear to act to preserve or re-establish any specific Protein/energy ratio in the total nutrients absorbed. The observations are discussed in relation to factors controlling energy intake, and the effect of protein inadequacy upon level of energy intake in the sheep. *Part VII, Aust. J. Agric. Res., 23: 247 (1972).