Let $p$ be a prime number. $k=\bar{\mathbb{F}}_p$. Let $f=x+t\in \mathbb{F}_q$, which is irreducible in $x$ and separable in $t$. Here, $x$ is substituted as $x=\sum_{i=0}^{n}a_it^i\in \mathbb{F}_p[t][a_0,...,a_n]$, so that, $f(a_0,...a_n,t)=\sum_{i=0}^{n}a_it^i$ be separable over $k(a_0,...,a_n)$, and $a_i$ are algebraically independent variables over $\mathbb{F}_p$.
Does this mean, that $Disc_{t}f(\alpha)$ is not zero for some $\alpha$ in k. Any hint or method is of great help.