What type of trailers used in agriculture and forestry do not have to have their own registration number?

You are correct. In German traffic law, trailers used in agriculture and forestry that have a "25" speed limit sign attached to the rear—that is, trailers that are only allowed to travel up to 25 km/h—do not require their own registration number. This rule exists because these slow-moving, specialized trailers are primarily operated on fields or short stretches on public roads, and they are easily identifiable by the "25" sign.

Other trailers, such as those that are allowed to go faster than 25 km/h or those just used to transport agricultural goods regardless of their speed, must have their own registration number. Make sure to always check for the "25" sign when answering similar questions, as that is the key detail German law uses for this exception.