We study the class of planar polynomial vector fields admitting Darboux first integrals of the type ∏i=1 rfi αi, where the αi's are positive real numbers and the fi's are polynomials defining curves with only one place at infinity. We show that these vector fields have an extended reduction procedure and give an algorithm which, from a part of the extended reduction of the vector field, computes a Darboux first integral for generic exponents.