Yarding  In poultry keeping, yarding is the practice of providing the poultry with a fenced yard in addition to a poultry house. Movable yarding is a form of managed intensive grazing. Yarding is often confused with free range. The distinction is that free-range poultry are either totally unfenced, or the fence is so distant that it has little influence on their freedom of movement. Before the discovery of vitamins A and D in the 1920s, green feed and sunshine were essential to the health of poultry. Vitamin D was synthesized from sunlight on the skin (as with humans), while Vitamin A was obtained through green forage plants such as grass. Yards small enough to be fenced economically were soon stripped of palatable green forage and become barren. This is followed by a build-up of manure, parasites, and other pathogens. Free range husbandry was the most common method in these early days. Most farms had only a small free-range barnyard flock. Larger flocks were kept in small houses build on skids, which were dragged periodically to a fresh piece of ground. This method is similar to the modern practice of pastured poultry. Experts of the day estimated the sustainable level to be about fifty hens per acre (80 m² per hen), with one hundred hens per acre (40 m² per hen) as an absolute upper limit if special care was taken. These levels are sustainable in the sense that the turf can make use of the nutrients in the manure left behind by the chickens, and in the sense that, at this stocking density, the chickens will not completely destroy the turf through scratching. At the Oregon Station on clay soil it was found that the day droppings from 200 laying hens on an acre [20 m² per hen] in four years made the soil too rich for the successful growth of cereal crops where cropping the ground was done every other year. The night droppings were put on other land. If the soil contains too much manure for the crops it is safe to assume that it is not in the best condition for poultry. Sooner or later it is bound to show not only in a failure of grain crops but in failure of poultry crops. For a permanent system under average conditions of soil and climate the following points are suggested for consideration.   ... It is not assumed that as many as 500 hens may not be profitably kept on an acre [8 m² per hen] for a few years under favorable conditions. It has been done, but it is a different matter when it is planned to make a permanent business of it.