The Oxford Companion to Beer definition of
First Runnings, the heavy wort extracted from the mash at the start of the run-off, before any sparging has commenced. See sparging. Brewers will typically check the gravity of the first runnings as a rough indicator of the potential extract of the grains in the mash tun or lautering vessel. Gravities for first runnings vary widely depending on the water-to-grain ratio of the original mash and the gravity of the wort the brewer intends to derive from it. It is not unusual for the first runnings of the mash to exceed gravities of 20° Plato (specific gravity 1,080) and above. As the mash is sparged and the sugars are rinsed away, the gravity of the runnings gradually drops and the wort collected in the kettle becomes diluted to its intended gravity.
Very strong beers are sometimes made from concentrated worts that are made up of 90%–100% first runnings and may have original gravities exceeding 25° Plato (specific gravity 1,100). Taking only the first runnings prevents unwanted dilution of the wort but also leaves a high percentage of the potential sugar extract behind in the unrinsed spent grains. The parti-gyle system is sometimes used to extract those sugars into a second wort, thus putting them to use. See parti-gyle. In part-gyling, the first runnings may go to one kettle as a concentrated wort, perhaps to make a barley wine or imperial stout. When sparging commences, the run-off is diverted to a second kettle to make a second, weaker wort, which will be boiled and fermented separately into a lighter beer. See small beer.
When brewers wish to make very strong beers from the first runnings, but have no use for any weaker wort that would be gained by sparging, the grains may be discharged from the mash or lauter vessel having been thoroughly drained, but never rinsed. Discarding this much malt extract is expensive and somewhat wasteful, but any farm animals fed such sugar-laden spent grain are in for a rare treat.