Low protein diets have been proposed as a way to enhance fat reserves and reduce liveweight gain in breeding animals of very lean genotypes. The objective of this study was to examine the effect of different protein levels on daily gain, body composition and reproductive performance of gilts.

At 118 (sem=0.28) days old, 54 genetically lean gilts ((Landrace x Large White) x Large White) were allocated, considering firstly age and secondly weight, between three treatments with different dietary lysine:energy (g/MJ DE) ratios: High (0.9), Medium (0.6) and Low (0.3), fed twice daily at 2.9 x maintenance energy. Animals were weighed weekly and backfat thickness (P2) and muscle depth values were also taken. Eye muscle area measurements were taken by real time ultrasonography (Aloka 500) at the end of the experiment At 160 days of age, puberty was induced by administration of exogenous gonadotropin (PG600).