Reproductive failure, especially in young sows, is the major contributor to the steady rise in sow culling and mortality rates in recent years. There is now considerable interest in the nutrition of the gilt and its effect on production characteristics and subsequent reproductive performance. Both fat, and more recently lean tissue, have been postulated to play singularly important roles. The objective of the current study was to use production performance criteria of both primiparous and multiparous animals to estimate the proportions of fat and lean tissue mobilised during lactation.

Ninety two animals, forty five gilts and forty seven third parity sows, based at the University commercial pig unit, were used in this study. All animals were weighed and monitored for P2 backfat depth on entry to the farrowing house, during lactation and, finally, at weaning. The data for each individual animal was then used to calculate, by linear regression, the daily rate of loss of both body weight and P2 backfat depth during lactation. These responses were then utilized to calculate the weight and P2 backfat level of each animal on days 1 and 28 of lactation.