Recent work in the United States (Reese, Moser, Peo, Lewis, Zimmerman, Kinder and Stroup 1982) and in Australia (King and Williams, 1984) has clearly shown that nutrition during lactation can affect the fertility of young sows. If first-litter sows are restricted in food intake during lactation, so that they lose body weight and backfat, they take longer to return to oestrus after weaning than their counterparts fed ad libitum. To reproduce successfully the sow must have some mechanism(s) of monitoring its nutritional status, preferably before conception takes place. It is difficult to imagine mechanisms capable of monitoring body reserves per se. It is more likely that the sow can monitor changes in its body reserves, for example, by measuring the rate of lipid mobilized from adipose tissue or the rate of protein breakdown from muscles, or both.

Crossbred gilts (Landrace x Large White) were selected at about 45 kg body weight (approx 3 months of age) and allocated to treatment according to litter and live weight. A total of 22 groups each of 24 animals was selected. Within each group gilts were fed (13.0 MJ DE/kg, 160 g/kg CP) either ad libitum (High), 2.0 kg/day (Medium) or 1.8 kg/day (Low) from selection through until mating.