Degree bounds for modular covariants

Let $V,W$ be representations of a cyclic group $G$ of prime order $p$ over a field $k$ of characteristic $p$. The module of covariants $k[V,W]^G$ is the set of $G$-equivariant polynomial maps $V \rightarrow W$, and is a module over $k[V]^G$. We give a formula for the Noether bound $\beta(k[V,W]^G,k[V]^G)$, i.e. the minimal degree $d$ such that $k[V,W]^G$ is generated over $k[V]^G$ by elements of degree at most $d$.