There is only one herder. You can only transport two horses across the river at a time, and then you have to ride one of them back, so it takes three times to send four horses across the river. In order to save the return time, first use C to transport two horses from A Ding, and then ride C Horse to pick up B Horse. The three trips are arranged as follows:

For the first time, two horses (3 minutes),

Otsuichi rides a horse for the first time (2 minutes).

On the second trip, Jia Ding took two horses (8 minutes).

The second time, C a horse (3 minutes),

The third trip, two horses (3 minutes).

Total 3+2+8+3+3= 19 minutes.