THE EFFECT OF LATE PREGNANCY ON THE DIET SELECTIONS MADE BY EWES

To test the hypothesis that sheep select a diet that meets their physiological needs (imposed by late pregnancy), four foods were formulated of either a high metabolisable energy (ME) concentration (A, B 12 MJ/kg dry matter (DM)) or a low concentration (C, D 9 MJ/kg DM). The crude protein (CP) concentrations of A and C were high (279 and 191 g/kg DM) relative to B and D (105 and 95 g/kg DM); the ratios of metabolisable protein: ME were 12 g/MJ for foods A and C and 4 g/MJ for foods B and D, Twin bearing and non pregnant Greyface ewes of similar body condition were offered ad Iib., A, B and C as single foods and paired choices (A with B) and (C with D). Replication was a minimum of four ewes for single foods and six for the choices. Food intakes by pregnant and non pregnant ewes were similar until parturition was imminent, when the intakes by the pregnant ewes declined rapidly. The majority of pregnant ewes carried twin foetuses to term without problems. When the ME concentration of the food choice was high (A/B) pregnant ewes selected a greater proportion (P<0.05) of the food of high CP concentration than non pregnant ewes: pregnant 0.57 (se 0.074); non pregnant 0.28 (se 0.105). By contrast when ewes were offered a choice between foods of low ME concentration diet selection was unaffected by state: proportion of the higher CP food, C selected were: 0.70 (se 0.070) and 0.79 (se 0.069) for pregnant and non pregnant ewes respectively. These results show that when the dietary ME concentration of foods given as a choice is high, pregnant ewes select a diet that reflects their enhanced demand for protein in contrast with non pregnant contemporaries. However, when the ME of the foods is low priorities other than physiological state may motivate their diet selection. It is suggested that such a priority is the maintenance of a balanced rumen environment.