The effect of body condition and previous nutrition on voluntary intake and feeding behaviour in sheep
There is evidence from a number of species that body condition or fatness plays a role in the regulation of energy balance, through a direct effect on voluntary food intake (VFI). However, since differences in body condition are frequently confounded with differences in previous nutrition, this study investigated the role of both factors in the control of VFI in sheep. In ewes grazing autumn pastures, there was a negative effect of body condition on VFI, but no independent effect of the previous level of nutrition. However, there was an interaction between the effects of body condition and herbage availability on intake, since VFIs of fat ewes differed with sward height while VFIs of thin ewes did not. The effect of early nutrition on VFI in growing lambs was studied in an experiment where ewes were given either a restricted or adequate level of nutrition in late pregnancy and early lactation. Mean live weight and gut development at weaning were reduced in lambs whose nutrition was restricted during both pregnancy and lactation, but subsequent VFI was not affected. Feeding behaviour was investigated in fat and thin sheep eating a pelleted diet. Thin ewes had higher intakes and spent more time feeding that fat ewes, but the number of meals and rate of ingestion during meals was the same. Both fat and thin ewes increased meal frequency and ingestion rate when food access time was reduced. Differences in VFI were found to persist longer than differences in body condition. Basal plasma insulin concentrations and those measured following an exogenous insulin challenge were both higher in sheep that had remained fat, than in sheep that had previously been thin. This is consistent with the hypothesis that insulin may provide a long-term negative feedback signal to the brain in fat animals, resulting in lower VFIs.