1. The main experiment described was a 4 × 2 factorial designed to test levels of 18, 21, 25 and 28% crude protein in diets fed from 25–50 lb. live weight and to compare an all-vegetable diet with one containing white fish meal for pigs of 50–100 lb. live weight.

Forty-eight pigs were used in the experiment. They were weaned at an average weight of 9 lb. and were fed a standard 29% protein diet until they each weighed 25 lb. Between 100 lb. and slaughter at 200 lb. they were fed a standard all-vegetable diet containing 14% protein.

Live weights and food-consumption figures for each pig were recorded throughout the experiment and carcass-quality measurements were taken.

2. Forpigs of 25–50 lb. there was no advantage in raising the level of crude protein above 18% in a diet based on white fish meal, dried skim milk, rolled oat groats, ground barley and fine millers' offals. Level of protein fed during this stage of the experiment did not affect performance at any subsequent stage, nor did it affect carcass quality.

3. For pigs of 50–100 lb. rates of live-weight increase and food-conversion efficiency were poorer by 13 and 14%, respectively, on an all-vegetable diet containing 17% crude protein than on a diet including 7·5% white fish meal and containing 15% crude protein. This difference was not affected by level of protein fed before 50 lb., and in its turn it did not affect either growth performance from 100–200 lb. or carcass quality measurements.

4. The following notes relate to the overall performance of early weaned pigs during this experiment: After being weaned at an average of 9 lb. when 10 days old, the pigs reached 25 lb. at 36 days old and 50 lb. at 58 days old. They each ate an average of 29 lb. of the 29% protein diet 1A and 59 lb. of the diets 2 A–D fed from 25–50 lb. Between 50 and 200 lb. food-conversion efficiencies were lower than is usual for individually fed pigs at the Rowett Institute, but rates of live-weight increase were satisfactory when considered in relation to the plane of feeding followed, and the average age at 200 lb. was 185 days. Of the carcasses 85% graded A.

5. In a pilot trial pigs were changed over from the 29% protein diet 1A to the 18% protein diet 2D at 13, 17, 21 or 25 lb. live weight.

There was a linear trend towards slower rates of live-weight increase as the weight at change-over was reduced, and pigs changed at 13 lb. took 7 days longer than those changed at 25 lb. to reach 40 lb. live weight. Overall food-conversion efficiency was best when the change was made at 25 lb., but because of the high cost of diet 1 A in relation to diet 2 D there was a linear trend for the total cost of food per pig to be lower as the weight at change-over was reduced.

6. The results of the pilot trial were discussed in relation to the effects on the lightest pigs in a group of changing to diet 2 D as the average weight per pig reached 25 lb.