The object of the present investigation, was to compare the supplemental value of the microbial proteins in dried brewers' yeast with the animal proteins in white-fish meal when incorporated in a barley meal-fine bran diet containing a little lucerne meal and suitably adjusted as regards minerals. The investigation was carried out both by the nitrogen-balance method and by the statistically controlled growth method, using the individual-feeding technique. The main problem was to find the minimum percentage of yeast that had to be included in the cereal diet so as to sustain the same rate of growth and economy of food conversion, and to enable retention of nitrogen to take place at the same rate, as is obtained on a standard diet containing 7% of white-fish meal, the total digestible nutrients in all diets being kept approximately constant.
In the first nitrogen-balance trial a comparison was made between the standard diet containing 7% of white-fish meal and a similar basal diet supplemented with 15% of dried brewers' yeast. The percentage crude protein in the yeast diet was 15·39 as compared with 15·12 in the standard treatment, the total digestible crude protein being 12·34 and 12·29, respectively. The supply of total digestible nutrients was almost alike on both treatments.
After about 6 weeks on experiment, the two hogs on the yeast diet became subject to vomiting at rather frequent intervals, although they were always eager for their food and their faeces remained normal. It is unlikely that vomiting was caused by the presence in the yeast of any toxic substance and the possibility that it was due to the presence of live yeast cells is also remote for reasons given. The most likely hypothesis is that it was due to the animals ‘bolting’ their food with great avidity, since the pigs found the yeast diet to be highly palatable and they always appeared to be hungry.
The two hogs on the yeast diet made average daily live-weight gains of 0·85 and 0·80 lb., the figures over a corresponding period for the hogs on the standard treatment being 0·67 and 0·71 lb. Economy of food conversion was also better on the yeast treatment. The average daily retention of nitrogen was also higher on the yeast treatment the figures being 9·28 and 9·01 g. as compared with 8·07 and 8·90 g. on the standard diet. It is evident therefore that when yeast is incorporated in a cereal diet so as to bring the digestible crude protein content to the same level as in the standard treatment containing white-fish meal, maximum storage of nitrogen takes place, even with young pigs.
In the second nitrogen-balance trial the amount of yeast in the diet was reduced to 12% so as to investigate whether this amount would also support maximum retention of nitrogen and maximum growth. The percentage of digestible crude protein in the diet was 12·56 on the standard treatment and 11·29 on the yeast treatment, the total digestible nutrients being 64·47 and 64·18%, respectively. Over the 63 days that the pigs were in the metabolism crates the two hogs on the standard treatment made slightly higher live-weight gains and retained rather more protein in their tissues, the average retention of nitrogen being 9·43 g. on the standard diet and 8·3 g. on the yeast diet.
It appeared from these results that a cereal diet supplemented with dried brewers' yeast so as to bring the crude protein content to 14·18% (15·95% on the basis of dry matter) does not supply quite enough protein to support maximum retention of nitrogen. In the first experiment, however, when the yeast diet contained 15·39% of crude protein (17·37% on the basis of dry matter) the results were fully equal to the standard treatment containing 15·12% of crude protein (17·21% on the basis of dry matter). These conclusions were tested further by means of a growth trial with ten individually-fedpigs on each treatment.
In the growth trial the ten pigs on treatment A received a supplement of 9% of dried brewers' yeast, those on treatment B were on the standard diet containing 7% of white-fish meal and those on treatment C received a supplement of 12% yeast. The percentage of crude protein on the mean moisture basis was 14·62 in treatment A, 16·15 in treatment B and 15·4% in treatment C. It was concluded from the results of this growth trial that when approximately one-third of the total crude protein in the diet comes from either dried brewers' yeast or white-fish meal, complete supplementation of the proteins in the basal diet of barley meal and fine bran occurs, so as to enable maximum retention of nitrogen or maximum growth to take place without wastage of aminoacids. The results for treatment A suggest that the proteins in yeast may even be superior in supplemental value to the proteins in white-fish meal.
The deductions made from both methods of experimentation are therefore in close agreement and the results show that a basal diet of 2 parts of barley meal and 1 part of fine bran with a little lucerne meal and minerals when supplemented with dried brewers' yeast so as to bring the crude protein content of the mixture to about 15·5% enables the young pig to store the maximum amount of protein in its tissues and to grow at a maximum rate in conformity with the net energy content of the diet. The distribution of essential amino -acids in the proteins of yeast and in the proteins of whitefish meal, when based on equal weight of crude protein, is not significantly different, any differences being slightly in favour of the yeast proteins.
A comparison is also made between the results obtained with dried brewers' yeast and those obtained with extracted decorticated ground-nut meal used in previous trials.
