In this paper, we use the generalisation of Mason’s inequality due to Brownawell and Masser (cf. [8]) to prove effective upper bounds for the zeros of a linear recurring sequence defined over a field of functions in one variable.Moreover, we study similar problems in this context as the equation $G_n\left(x\right)=G_m\left(P\left(x\right)\right),(m,n)\in {\mathbb{N}}^2$, where $\left(G_n\left(x\right)\right)$ is a linear recurring sequence of polynomials and $P\left(x\right)$ is a fixed polynomial. This problem was studied earlier in [14,15,16,17,32].