Each variety has its own cycle. Willian D.Gann also wrote in his book that, like soybean, its cycle is about four months. Most agricultural products have a long cycle because it takes time to grow. Domestic soybeans take about 140 days from planting to harvesting, and a cycle is nearly 5 months. The growth cycle of American soybeans is about 100 days, and it takes nearly 4 months to harvest and market. Metals, on the other hand, are not so particular. As soon as the machine is started, it can be shipped.
One more thing, the psychological expectation of the market will also have an impact on the choice of the main contract.