This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
In a feeding test lasting forty-two days, four pigs were fed on sweet potatoes only, during which time the pigs lost in weight instead of making a gain. The pigs in this test were rather small, averaging only twenty-two pounds. In another experiment with older pigs, averaging about one hundred pounds, the results were a little more favorable; yet the gains in weight were not large enough to make it a paying investment to feed sweet potatoes alone. When some other feed was used in combination with sweet potatoes, the results were quite different. In one test lasting for a period of twenty-eight days pigs averaging 150 pounds were fed sweet potatoes and shorts in the proportion of one part shorts to six parts sweet potatoes by weight. The pigs made good daily gains, giving the sweet potatoes a value of about 50 cents per hundred (when pork was worth 5 cents per pound) for pork production.
This may perhaps seem a very small price for the farmer to receive for his sweet potatoes, but it certainly gives him an idea as to the feeding value of his potato crop. If the market price of potatoes should fall as low as 65 cents or 70 cents per hundred, it would be a question whether or not the farmer could best afford to put his crop on the market or feed it to his hogs."
In "Farmers' Bulletin 410," the analysis of potatoes, potato skins, and potato slop is given as follows:
Protein (Nx 6.25)
Sugar as dextrose
Nitrogen free extract
This table shows that the dry substance in the slop is very different in composition from the potato itself, being a more highly nitrogenous food. The great increase in the amount of protein as compared with the total dry substance in the slop is, of course, due to the fermentation of the starch and sugar, resulting in a concentration of the nitrogenous material.
Any scheme for the operation of agricultural distilleries, whether small or large, should provide for the utilization of the by-product known as 'slop.' This is the residuum remaining after the alcohol and a small amount of water have been boiled off from the fermented distillery mash; and it contains, dissolved or suspended in the remaining liquid of the mash, all of the constituents of the materials employed except that portion of the sugars and starch which was converted into alcohol during the fermentation. This slop has been found, both in this country and abroad, to be a feeding stuff of high value and should be fed to the stock on the farm that furnishes the raw materials used in the distillery. In this way its full utilization can be secured. First, through the production of flesh, milk, or energy in the stock to which it is fed; and, second, by returning to the soil in the form of manure those necessary elements of plant food which were abstracted during the production of the potatoes or other raw materials.
The large proportion of water contained in all slop has an important bearing in determining the amount of slop solids which can be fed to any animal in one day. It has been customary in this country, where cattle have been fed with slop in sheds on the grounds of large whiskey and alcohol distilleries and not on the farm, to allow each bullock daily the volume of slop corresponding to a bushel of the grain mashed. In other words, a distillery mashing 1,000 bushels daily will distribute its slop among 1,000 head of cattle. Reduced to volume, this would be equivalent to about thirty gallons per head per day. This amount is excessive, even when fed with considerable quantities of hay and other roughage, as is shown by the flabbiness of the stock and the liquid character of their manure. The injurious effect of the slop when fed excessively, as heretofore in this country, is liable in the case of milch cows to result in dangerous contamination of their milk through the great difficulty of keeping their hindquarters clean.
In Germany, where slop feeding has been practised very successfully on the basis of careful investigations at the agricultural experiment stations, it is customary to feed much smaller volumes. According to Maercker, it is allowable to give from eighteen to twenty gallons per head per day in fattening oxen weighing from 1,300 to 1,400 pounds. More than this amount has been found injurious. Milch cows should not receive more than sixteen gallons daily. It is necessary to feed the slop as hot as possible, and since it is especially susceptible to bacterial decomposition it should also be fed when fresh.
Investigations are needed in this country to determine the composition of rations, suited to American conditions, in which potato slop takes its proper place."