In general, one is required to take t'rumos and maasros from grain only once the pile of grain is been smoothed (מירוח הכרי, meruach hak'ri). (I think other halachos also kick in then.) What exactly is this act of smoothing, and why would a farmer do it?
Double AA answered the question already: The obligation for terumos and ma'aseros begins when the harvest ends. The harvest ends when the bales are topped off.