If you plant more than 1 variety per field or more than 1 crop per field, you can set this via the Workflow | Farm Performance Page or under the field configuration under Farm Planning.
Secondary crops in Agworld are cover crops and under sown crops. When set as a secondary crop, they will contribute the seeding costs to the field expense on your fields.
From the Planning Page:
From Farm Performance:
Creating a seeding activity
When a seeding activity is created it will not overwrite the existing crops on the field. Instead it will add it to your crops.
Crop ratios will updated in planning to reflect the activity area on seeding plans / actuals