1. Gestation diets formulated to contain A, 9; B, 11; C, 13 or D, 15% protein, and lactation diets with C, 13 or E, 17% protein provided eight treatment combinations. They were given at seven centres at 2 kg/day in gestation and approximately 5·7 kg/day in lactation to sows which remained on experiment for three to five parities and produced in total 468 litters. Mean results for parities one to four are given in this summary.
2. For gestation diets A to D mean litter size at birth was 9·7, 10·1, 10·3 and 10·4 live pigs, and at weaning (at 6 weeks), 8·2, 8·7, 8·8 and 8·8. Sows which received lactation diets C and E subsequently produced 10·4 and 10·3 pigs at birth and 8·9 and 8·3 at weaning.
3. Gestation diets had no consistent effects on piglet weights. During lactation, however, diet E gave heavier pigs than C, by 0·2 kg at 3 weeks and 0·5 kg at weaning.
4. Extra protein for either gestation or lactation had a positive effect on sow weight at that stage, but a negative effect in the following stage. There were no direct treatment effects on the time required for re-conception.
5. Twenty experimental sows at the Rowett Institute, together with an additional group given diet A for both gestation and lactation, showed strong positive relationships between lactation protein intake and milk yield or composition.
6. The treatment combination AC (i.e. 9% protein in gestation and 13% in lactation) gave the lowest weight of weaned pigs per litter, and CC gave the highest. The remaining treatments gave similar weights; of these BC (nominally 11 with 13, but actually 11·5 with 13·5% protein) was concluded to provide the lowest protein intake consistent with satisfactory performance.