Forage plants such as corn, legumes, and grasses that have been harvested at early maturity, finely chopped, packed tightly to exclude air, and stored in tower silos, pits, or trenches. Properly stored silage ferments slightly and keeps for several months. It is used as animal feed.
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In various parts of Germany a method of preserving green fodder precisely similar to that used in the case of Sauerkraut has prevailed for upwards of a century. Special attention was first directed to the practice of ensilage by a French agriculturist, Auguste Goffart of the district of Sologne, near Orleans, who in 1877 published a book detailing the experiences of many years in preserving green crops in silos. Goffart's experience attracted considerable attention. The conditions of dairy farming in the United States proved eminently suitable for the ensiling of green maize fodder; and the success of the method was soon indisputably demonstrated among the New England farmers. The favorable results obtained in America led to much discussion and to the introduction of the system in the United Kingdom, where, with different conditions, success has been more qualified.
It has been abundantly proved that ensilage forms a wholesome and nutritious food for livestock. It can be substituted for root crops with advantage, because it is succulent and digestible; milk resulting from it is good in quality and taste; it can be secured largely irrespective of weather; it carries over grass from the period of great abundance and waste to times when none would otherwise be available; and a larger number of livestock can be supported on a given area by the use of ensilage than is possible by the use of green crops.
Early silos were made of stone or concrete either above or below ground, but it is recognized that air may be sufficiently excluded in a tightly pressed stack, though in this case a few inches of the fodder round the sides is generally useless owing to mildew. In America round structures made of wood and 35 or 40 ft. in depth are most commonly used. The crops suitable for ensilage are the ordinary grasses, clovers, alfalfa, vetches, oats, rye and maize, the latter being the most important silage crop in America; various weeds may also be stored in silos with good results, notably spurrey, Spergula arvensis, a most troublesome plant in poor light soils. As a rule the crop should be mown when in full flower, and deposited in the silo on the day of its cutting. Maize is cut a few days before it is ripe and is shredded before being elevated into the silo. Fair, dry weather is not essential; but it is found that when moisture, natural and extraneous, exceeds 75% of the whole, good results are not obtained. The material is spread in uniform layers over the floor of the silo, and closely packed and trodden down. If possible, not more than a foot or two should be, added daily, so as to allow the mass to settle down closely, and to heat uniformly throughout. When the silo is filled or the stack built, a layer of straw or some other dry porous substance may be spread over the surface. In the silo the pressure of the material, when chaffed, excludes air from all but the top layer; in the case of the stack extra pressure is applied by means of planks or other weighty objects in order to prevent excessive heating.
The closeness with which the fodder is packed determines the nature of the resulting silage by regulating the chemical reactions that occur in the stack. When closely packed, the supply of oxygen is limited; and the attendant acid fermentation brings about the decomposition of the carbohydrates present into acetic, butyric and lactic acids. This product is named sour silage. If, on the other hand, the fodder be unchaffed and loosely packed, or the silo is built gradually, oxidation proceeds more rapidly and the temperature rises; if the mass is compressed when the temperature is 140 to 160 Fahrenheit, the action ceases and sweet silage results. The nitrogenous ingredients of the fodder also suffer change: in making sour silage as much as one-third of the albuminoids may be converted into amino and ammonium compounds; while in making sweet silage a less proportion is changed, but they become less digestible. In extreme cases, sour silage acquires a most disagreeable odor. On the other hand it keeps better than sweet silage when removed from the silo.