Before the feeding of antibiotics to pigs can be confidently recommended, more evidence is required as to their effect on growth and efficiency of food conversion. The present series of trials were designed with this object in view. The investigations were carried out by statistically designed growth trials as well as by nitrogen-balance determinations in the metabolism crates.

The main point under investigation in the first growth trial was whether the addition of penicillin to a well-balanced diet supplemented with whitefish meal would result in an increased rate of growth and efficiency of food conversion. It was concluded that, although the trend both as regards live-weight gain and efficiency of food conversion was in favour of the penicillin-fed pigs, the differences were too small to reach statistical significance. Nitrogenbalance determinations were also carried out over a period of 63 days on the white-fish meal diet with and without penicillin. The mean daily retention of nitrogen by the two hogs on the control treatment was 11·59 g., and by their two litter-mates on the penicillin treatment 11·42 g.

In the second growth trial penicillin was added to a diet composed of barley meal and fine bran together with a little lucerne meal and minerals and supplemented with extracted-decorticated groundnut meal. In both the group-feeding and in the individual-feeding trial there were ten pigs receiving this basal diet and twenty pigs on the same diet supplemented with 18 mg. of procaine penicillin per lb. of meal fed. This amount of penicillin is more generous than normally used by food manufacturers. There was a striking contrast in the eagerness with which the pigs fed. The penicillin-fed pigs licked their troughs quickly and cleanly in contrast to the other pigs, and it was quite apparent that the antibiotic had a tonic effect in this respect. Although this difference diminished as the trial proceeded, it was still evident when the pigs averaged 100 lb. live weight. It was also quite obvious that the pigs receiving penicillin drank more water than the control pigs. After 10 weeks on experiment the forty pigs receiving penicillin averaged 87·5 lb. in live weight as compared with a mean of 81·7 lb. for the twenty pigs on the control diet. This compares with mean live weights of 186·6 lb. (penicillin) and 177·9 lb. (control) over the whole 20 weeks that the trial lasted. The differences for the group-fed pigs were too small to reach statistical significance, owing to the wider variations within the groups. Highly significant differences, in the period from 36 to 90 lb. live weight, were obtained with the individually fed pigs. The mean live-weight increase per day was 0·95 lb. on the penicillin treatment as compared with 0·88 lb. on the control diet. Correspondingly significant differences characterized the figures for efficiency of food conversion over this period, the mean requirements being 2·97 lb. on the penicillin treatment and 3·21 lb. on the control diet. In the period from 90 to 200 lb. live weight the differences observed were very small and not statistically significant. Over the whole period from 36 to 190 lb. live weight the control pigs required on an average 5·3 more days and consumed approximately 19 lb. more meal per pig, an increase of 3·4%.

The same diet, supplemented with ex. dec. groundnut meal, was used in the second nitrogen-balance trial. The addition of penicillin failed to bring about any significant improvement in the retention of nitrogen or in the utilization of the dietary protein.

The effect of adding penicillin alone and also in conjunction with a vitamin B12 supplement (Distafeed), to the diet supplemented with ex. dec. groundnut meal, was investigated in the third growth trial. The penicillin-fed pigs were better feeders from the start than the control pigs, but in this trial the Distafeed seemed to have an adverse effect on palatability. From a common initial live weight of 40 lb. for each pig up to 90 lb. live weight, the pigs on the control treatment required on an average 3·6 days longer and consumed 10·54 lb. more meal than the pigs receiving procaine penicillin. The liveweight increase per day averaged 0·92 lb. on the control diet in comparison with 0·98 lb. on the penicillin treatment. The figures for efficiency of food conversion were also in favour of the penicillinfed pigs, being 3·05 and 3·25 lb., respectively. To increase 155 lb. in live weight the penicillin-fed pigs required on an average 4 days less with a saving of about 13 lb. of meal per pig, in comparison with the control pigs. The improvement was entirely confined to the period immediately after weaning. No improvement resulted from the simultaneous addition of vitamin B12 with the antibiotic.

The nitrogen retention of pigs was also investigated with and without penicillin and vitamin B12, using diets supplemented with ex. dec. ground-nut meal and extracted soya-bean meal. A summary is given of the results of nitrogen-balance studies with twenty pigs to investigate the effect of antibiotics on protein metabolism.

It was concluded from the above trials that the main effect of procaine penicillin was to stimulate the appetite of the pigs on all-vegetable diets. It seemed therefore that unless the pigs were allowed to benefit fully from their improved appetites, by being allowed full-feed, the maximum effect from the feeding of penicillin would not be attained. In the fourth growth trial this point was tested, using the group-feeding lay-out and adjusting the ration daily so that food was available almost continuously. With the individually fed pigs in this trial the opportunity was taken to test the statement that the magnitude of the response to the feeding of antibiotics is greater with diets supplemented with soya-bean meal than with other vegetable-protein concentrates.

The group-fed pigs under conditions approaching ad lib. feeding were able to consume without wastage distinctly higher amounts of meal than we usually feed in these growth trials, when the meal is restricted according to the feeding chart. The pigs receiving penicillin displayed at first better appetites than the control pigs, and made correspondingly better live-weight gains, but as the trial proceeded the appetite of the control pigs gradually improved and eventually surpassed that of the penicillin-fed pigs. It seemed as if the latter had been overeating in the initial period and were suffering from surfeit. After 11 weeks on experiment the mean live weight of the control pigs was slightly higher than that of the group that received penicillin. It is also interesting to note that with ad lib. feeding, no outstanding difference in water consumption was observed between the treatments, the water consumption being mainly influenced by food consumption.

The results for the individually fed pigs in the fourth growth trial showed that the effect of adding penicillin to a cereal diet supplemented with ex. soya-bean meal was even less pronounced than with ex. dec. ground-nut meal. No significant effect was shown, even in the period immediately after weaning, on the rate of gain or on the efficiency of food conversion as a result of adding 18 mg. of procaine penicillin per lb. of meal. In this trial the pigs were exceptionally good feeders and the soyabean meal diet proved to be highly palatable.

It may be concluded from the results of these trials that with healthy pigs and using normally balanced palatable rations, the improvement in the rate of growth, efficiency of food conversion and rate of nitrogen retention to be expected from the inclusion of penicillin in the diet is generally too small to make it economically worth while.