What type of license do you need to drive a tractor and silage trailor?

Depends on the weight of the combination, but most silage trailers I've seen are the type which attach to a Class 7 or Class 8 truck, either using a kingpin and fifth wheel, or else using a pintle and clevice. If these are 'for hire' vehicles, they would require a Class A CDL in the United States, Class 1 licence in Canada, and EC licence in South Africa, and I believe it's now called a C+E licence in the United Kingdom (bear in mind we entertain questions from a worldwide audience).

In the United States, if it's registered as a farm vehicle, and not used on a 'for hire' basis, individual states have jurisdiction over licencing requirements for farm use vehicles, as they are exempt from CDL requirements. Other countries do not differentiate between vehicles of that class which are used on a 'for hire' or private usage basis.