Do I have to retrain the livestock every season
No, Once the herd has learnt virtual fencing they seem to be trained. If you introduce more untrained livestock to the herd we recommend training them to reduce any unnecessary learning difficulties and disrupting your already trained herd. If you see a member of the herd that after initial training is testing the virtual boundary too much, consider moving them back into the training pasture if they are disruptive, rarely we find animals may be deaf, although rare this can also reduce the effectiveness of the system.