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In agriculture, crop yield (also known as "agricultural output") refers to both the measure of the yield of a crop per unit area of land cultivation, and the seed generation of the plant itself (e.g. if three grains are harvested for each grain seeded, the resulting yield is 1:3). The figure, 1:3 is considered by agronomists as the minimum required to sustain human life.
One of the three seeds must be set aside for the next planting season, the remaining two either consumed by the grower, or one for human consumption and the other for livestock feed. The higher the surplus, the more livestock can be established and maintained, thereby increasing the physical and economic well-being of the farmer and his family. This, in turn, resulted in better stamina, better over-all health, and better, more efficient work. In addition, the more the surplus the more draft animals — horse and oxen — could be supported and harnessed to work, and manure, the soil thereby easing the farmer's burden. Increased crop yields meant few hands were needed on farm, freeing them for industry and commerce. This, in turn, led to the formation and growth of cities.
Formation and growth of cities meant an increased demand for food stuffs by non-farmers, and their willingness to pay for it. This, in turn, led the farmer to (further) innovation, more intensive farming, the demand/creation of new and/or improved farming implements, and a quest for improved seed which improved crop yield. Thus allowing the farmer to raise his income by bringing more food to non-farming (city) markets.
Historically speaking, a major increase in crop yield took place in the early eighteenth century with the end of the ancient, wasteful cycle of the three-course system of crop rotation whereby a third of the land lay fallow every year and hence taken out of human food, and animal feed, production.
It was to be replaced by the four-course system of crop rotation, devised in England in 1730 by Charles Townshend, 2nd Viscount Townshend or "Turnip" Townshend during the British Agricultural Revolution, as he was called by early detractors.
In the first year wheat or oats were planted; in the second year barley or oats; in the third year clover, rye, rutabaga and/or kale were planted; in the fourth year turnips were planted but not harvested. Instead, sheep were driven on to the turnip fields to eat the crop, trample the leavings under their feet into the soil, and by doing all this, fertilize the land with their droppings. In the fifth year (or first year of the new rotation), the cycle began once more with a planting of wheat or oats, in an average, a thirty percent increased yield.