If you do not harvest your fields in time, they go to waste.
The time you have to harvest your fields is equal to the time it takes for a seed to be ready for harvest.
Note that a "day" on Town Farm is 20 hours. The pineapples that grow in 3 "days" are actually ready after 60 hours, or 2 ½ days. They therefore rot after two and a half days, so you have 5 days instead of 6 for harvest From the moment you have sown!
When this happens you have to plow the fields to get rid of the wasted crops so you can replant.
Or if you don't like where the fields are, delete them and plow new fields.