The occurrence of ovulation in the sow after parturition is usually inhibited for the duration of suckling. A system of management in which lactating sows are mated and conceive, therefore routinely commencing a successful pregnancy concurrent with lactation, would obviate the need for early weaning of piglets and promote adoption of economic systems incorporating more extensive lactation housing. Previous studies have shown that first parity sows rarely show ovulation during lactation when group-housed in multisuckling systems with multiparous sows, and suggested that reduced food intake through social competition and/or higher suckling intensity were responsible for this (Hulten, 1997). The object of this experiment was to investigate, in a more controlled way, the factors contributing to this parity effect by removing the possibility of social interactions between primiparous and multiparous sows, and investigating possible dietary influences within parity.