Putting "of the world" at the end is more correct, but also probably not necessary. If you just said, "Brazil is an agricultural center." I would assume you meant "of the world." Where such a qualifier is necessary is if you were talking about a smaller area, especially using a superlative (-est). For example, if you said "Brazil is the largest agricultural center in South America," defining the area is necessary. If you just said, "Brazil is the largest agricultural center," the listener would assume you meant that there is no other agricultural center, anywhere that we know about, that is larger, which is more or less "in the world" since we don't know about any agricultural centers on other planets. So, I would say: Brazil is an agricultural center. France is an economic power.

Acho que deve ser a primeira frase