Drying in the sun

Why are rice straw bundles stacked upside down and hung on racks in the fall rice field?

Because they need to dry in the sun. The grain is traditionally dried in the sun in order to reduce the moisture content for long term storage. When the grains are spread out in the field, the amount of moisture that evaporates is maximized.There are a few different economical methods of sun drying. Field drying, which is pre- drying a hand harvested crop before threshing, and panicle drying, which  is a smaller scale method for farmers. Large scale farms require rice dried on pavement and out of the fields. Farmers can also dry the grains mechanically but it is more expensive and less environmentally friendly. This  traditional way of drying can have some disadvantages such as rain or night time humidity. Excessive respiration can lead to delays in drying or fungal growth, which could cause yellowing and grain loss.