Thirty six individually-fed pigs were used in a within-litter comparison of the 6 treatments of a 3 × 2 factorial experiment. The pigs were about 9 weeks old at the start and were slaughtered as their individual weights reached about 200 lb.

Three types of basal diet were given with and without a supplement of CuSO4.5H2O included at 0·1% (250 p.p.m. added Cu).The diets were based on (1) barley and fish meal, with 17% crude protein for pigs up to 103 lb. live-weight and 15·4 % from then on, (2) maize and soya (extracted soya bean meal), with about the same levels of protein and (3) maize and soya, but with only 13·4% crude protein for pigs up to 103 lb. and 11·9% for pigs of 103 lb. to 200 lb. Daily feed allowances were determined from a fixed scale based on live-weight.

Pigs given the high-protein maize-soya diets grew more rapidly, required less feed per lb. gain and had fatter carcasses than those given the barley-fish meal diets. This was probably because of the higher TDN content of the maize-soya mixture.

Pigs given the low-protein maize-soya diets grew less rapidly, required more feed per lb. gain and had fatter carcasses with smaller ‘eye’ muscles than those receiving the high-protein maize-soya diets, these being the usual effects of an inadequate supply of total protein or of an essential amino acid.

CuSO4.5H2O4 added as 0·1% of each of the three types of diet improved growth rate before 103 lb. live-weight but had no consistent effect on feed conversion efficiency. Between 103 lb. and slaughter the copper sulphate did not affect performance on the barley-fish meal or high-protein maize-soya diet, but in the low-protein maize-soya diet it reduced feed conversion efficiency and rate of gain by 5%. Although this interaction was not statistically significant it agrees with an observation made in Florida and warrants further investigation.