Nutrition of the bacon pig XIV. The determination of the relative supplemental values of vegetable protein (extracted, decorticated ground-nut meal) and animal protein (white-fish meal)

Extract

In the carrying out of comparative determinations of the supplemental values in pig feeding of the proteins in different protein concentrates, the writers consider the following conditions to be fundamental, and must be observed if reliable results are to be obtained:

(1) The comparison must be conducted over the period from weaning to about 90 lb. live weight. Measurements made in the later stages of growth can have no meaning, since supplementation of the basal mixture (cereal plus middlings or fine bran) with the protein-rich concentrates under comparison has no significant effect at tins stage on the rate of growth or nitrogen retention. Examples of the misleading results that are obtained by prolonging the period of comparison to 150 or 200 lb. live weight, instead of terminating the trial before the attainment of 90 lb. live weight, were encountered during the course of the present investigation, and are cited in the text.

(2) The diets to be compared must be so designed as to contain, as nearly as possible, equal percentages of ‘total digestible nutrients’, as based on computations from digestion coefficients derived from digestion trials with pigs.

(3) The daily allowances of meal must be scaled to live weight, and the diet must be sufficiently palatable to ensure clean food consumption.

(4) The diets must make adequate provision for the mineral and vitamin requirements of young growing pigs.

(5) The comparison in the case of growth experiments must be made by the method of individual feeding, and in accordance with the requirements of the modern statistical lay-out, so that the records of live-weight gain and food consumption may be submitted to statistical analysis.

(6) The diet forming the standard for comparison should contain the minimum amount of protein needed for the maximum rate of growth compatible with the available supply of net energy. In the case of white-fish meal, which was the standard protein supplement in the present comparisons, this condition has been found in earlier work to be realized when a diet composed substantially of barley meal and middlings contains 7% of this animal-protein food.

(7) In the initial tests, the experimental diet should contain, as nearly as possible, the same percentage of digestible crude protein as the standard diet, and the percentage of the protein supplement under test should be such as to provide as much digestible crude protein as is contained in the 7% of white-fish meal in the standard diet. Under such conditions of feeding, differences of protein quality in the two feeding treatments will be the factor responsible for any observed differences in the rate of live-weight increase or of nitrogen retention.

Should the pigs on the experimental treatment display a poorer rate of growth, or of nitrogen retention, than those on the standard treatment, then further trials should be undertaken in which the percentage of protein supplement under test is progressively increased, whilst still maintaining equality between the experimental and standard diets in respect of ‘total digestible nutrients’, until a level is reached at which the experimental pigs show a rate of live-weight increase, or of nitrogen retention, equal to that of the pigs on the standard treatment. The final stage of the comparison, therefore, is based on the results of feeding tests in which the maximum possible rate of growth, or of nitrogen retention, is obtained, both on the standard and experimental treatments, with diets supplying, respectively, the minimum amounts of the two types of protein supply.

The object of the present investigation has been to compare the supplemental values of a typical animal-protein concentrate (white-fish meal) and vegetable-protein concentrate (ex. dec. ground-nut meal). The aim has been to determine what percentage of ex. dec. ground-nut meal must be incorporated with a basal diet, composed approximately of 2 parts by weight of barley meal, 1 part of middlings (or fine bran) and a small allowance of lucerne meal and minerals, to promote the same rate of growth, or of nitrogen retention, in young pigs as is obtained with a standard diet containing roughly the same proportions of the basal foods in conjunction with 7% white-fish meal, the diets under comparison being made up so as to contain equal percentages of ‘total digestible nutrients’.

On the basis of pig digestion trials, 8 parts by weight of ex. dec. ground-nut meal contains as much digestible crude protein as 7 parts of whitefish meal. The diets under comparison in the initial growth and balance trials were made up, therefore, to contain, respectively, 7% of white-fish meal and 8% of ex. dec. ground-nut meal. Their contents of crude protein, digestible crude protein and ‘total digestible nutrients’ were approximately equal. Over a range of live weight from 36 to 90 lb., the pigs receiving the white-fish meal averaged 0·92 lb. of live-weight increase per day, with a mean efficiency of food conversion of 3·08 lb. per lb. of live-weight gain. The corresponding figures for the pigs receiving the ex. dec. ground-nut meal were 0·73 and 3·87 lb. These differences were strongly significant and pointed clearly to the poorer supplementa. value of the vegetable protein supplement. Confirmation of this finding was given by the results of nitrogen-balance trials on young pigs. At all stages, pig (1) receiving the standard diet containing 7% of white-fish meal was retaining nitrogen at a distinctly higher rate than pig (2) subsisting on the diet containing 8% of ex. dec. ground-nut meal. Over the 43 days of continuous measurement, pig (1) consumed 1719·5 g. of nitrogen, and retained 720·5 g., or 41·9% of the total nitrogen intake; whereas pig (2), which had steadily fallen behind in live weight during this period, consumed a total of 1591·9 g. of nitrogen, and retained 566·6 g., or only 35·6% of the ingested nitrogen. It was clear from the results that the protein assimilated from the diet containing ground-nut meal is not so suitably balanced in respect of amino-acids as the digestible protein in the standard diet, and is subject on this account to a higher degree of de-amination, with a correspondingly greater wastage in terms of urinary nitrogen.

This conclusion was substantiated by re-calculating the results for the same range of live weight, namely, 62·92 lb., a range which falls within the limits that have been shown to be necessary for a sensitive comparison. For this live-weight gain, pig (1), receiving white-fish meal, required 22 days, over which period the total consumption of nitrogen amounted to 771·5 g., of which 327·6 g., representing 42·5% of the ingested nitrogen, was retained, and 226·9 g. (29·4% of the nitrogen intake) eliminated in the urine. The corresponding results for pig (2) receiving groundnut meal were distinctly inferior. The time needed for the same gain in live weight averaged 25·3 days; the necessary nitrogen consumption amounted to 859·3g., and of this, 301·8g.(35·1% of the intake) was voided in the urine and 309·5 g. (36·0%) was retained in the body of the animal.

A succession of growth trials was next carried out in which the effect of increasing the level of ex. dec. ground-nut meal to 12, 14, 15, 16, 17 and 20%on the rate of live-weight increase between weaning and 90 lb. live weight was investigated. Although the successive increases led to progressive improvement of the results in relation to those obtained on the standard diet containing 7% of white-fish meal, was not until the 20% level of feeding was reached that the diet containing ex. dec. ground-nut meal gave as good results in respect of rate of growth and efficiency of food conversion as were obtained by the use of the standard diet.

The same result was obtained in the investigation of the problem from the standpoint of nitrogen retention, the inclusion in the experimental diet of as much as 20% of ex. dec. ground-nut meal being found necessary to so rectify the ill-balance of the amino-acids in the cereal part of the diet as to make possible, between weaning and 90 lb. live weight, the maximum rate of nitrogen retention such as was obtained in the case of the pigs receiving the control diet containing 7% of white-fish meal.

In view of the variation from consignment to consignment of the protein content of such proteinrich concentrates as white-fish meal and ex. dec. ground-nut meal, it is necessary to define the different diets under test in terms of their protein content rather than their content of protein supplement. Examined from this standpoint, the results of the growth trial and the balance trial, in which ex. dec. ground-nut meal formed 20% of the diet under test, showed a substantial measure of agreement. Averaging the results of these trials, it is found that the standard diet supplying 7% of white-fish meal contained 14·2% of crude protein on the air-dry basis (16·4% on the basis of dry matter), and of this, 32%, or roughly one-third, came from the white-fish meal. In the case of the diet supplying 20% of ex. dec. ground-nut meal, the crude protein content on the air-dry basis rose to 18·5% (21·2% on the basis of dry matter), and as much as 54%, or rather more than half, of the total protein was derived from the vegetable-protein supplement. The two diets contained, respectively, 11·6 and 15·6% of digestible crude protein, and of ‘total digestible nutrients’, 62·4 and 62·7% respectively (air-dry basis). On the two diets as so made up, equality in respect of rate of growth and nitrogen retention in young pigs was made possible, only, however, at the expense of a less efficient and more wasteful utilization, as evidenced by the greater degree of de-amination and a correspondingly higher loss of nitrogen in the urine, of the protein in the ration containing ex. dec. ground-nut meal.

The feeding of as much as 20% of ex. dec. groundnut meal in the rations of young bacon pigs for the purpose of ensuring the maximum rate of growth compatible with the available supply of net energy may seem an extravagant usage of the protein concentrate, but this is really not so if it is kept in mind that, in accordance with the writers’ earlier findings, the protein supplement may be omitted altogether at about 90 lb. live weight without occasioning any depression of the rate of live-weight gain or the efficiency of food conversion. Thus, in the growth trial in which the diet containing 20% of ground-nut meal was compared against the standard diet, no more than 35·5 lb. of the vegetable-protein concentrate was required, on an average per pig, between the live weights of 36 and 90 lb., and following the attainment of 90 lb., the use of the ground-nut meal could, if desired, have been discontinued.

Perhaps the best way of economizing in the use of ground-nut meal, however, is to feed it in conjunction with a small allowance of an animal-protein food, such as white-fish meal. In growth trial no. 4, for example, the diet containing 6% of ex. dec. groundnut meal and 2% of white-fish meal gave as good results as the diet containing 15% of ex. dec. ground-nut meal. Thus, the introduction of as little as 2% of white-fish meal into the diet enabled the percentage of ground-nut meal to be reduced from 15 to 6%, with a consequential reduction of the total digestible crude protein from 13·9 to 11·2%, without affecting the rate of live weight gain or the efficiency of food conversion.