Vetch is a nitrogen fixing leguminous plant, often grown as green manure.
From the 1881 Household Cyclopedia
The tare (also known as common vetch) is a plant of a hardy growth, and when sown upon rich land will return a large supply of green fodder for the consumption of horses or for fattening cattle. When intended for this use, the seed ought to be sown tolerably thick, perhaps to the extent of four bushels per acre, though when intended to stand for seed a less quantity is required, because otherwise the thickness of the crop will prevent the plants from blossoming and podding in a sufficient way. When meant for seed early sowing ought to be studied, otherwise the return will be imperfect; but when for green food any time betwixt the first of April and the latter end of May will answer well, provided crops in succession from the first to the last mentioned period be regularly cultivated. Instances are not wanting of a full crop being obtained even when the seed was sown so late as the middle of June, though sowing so late is a practice not to be recommended. After the seed is sown and the land carefully harrowed, a light roller ought to be drawn across, so that the surface may be smoothed, and the scythe permitted to work without interruption. It is proper also to guard the field for several days against the depredations of pigeons, who are remarkably fond of tares, and will pick up a great part of the seed unless constantly watched.
Horses thrive very well upon tares, even better than they do upon clover and rye-grass; and the same remark is applicable to fattening cattle, who feed faster upon this article of green fodder than upon any kind of grass or esculent with which we are acquainted. Danger often arises from their eating too many, especially when podded; as colics and other stomach disorders are apt to be produced by the excessive loads which they devour.