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You can download an example video that shows how to add a field and run an example job here 

Can I the cash crop harvest overlap the cover crop planting date?

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No, the DSSAT crop model does not support overlapping dates; however, we recommend you set the planting date of the cover crop to match the date you actually planted it and move the harvest date of the cash crop to one day before the cover crop planting date so the dates do not overlap. Since the cash crop is not growing, there is little impact to the model outputs. In the future, we will update to handle this on the service side so users can enter actual dates of planting and harvest in the case there is overlap. For now, you will need to adjust the dates so they don't overlap.

Can I set cash crop to fallow?

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No, but this feature is coming soon in a future update of the software.