Table Column Box Headings: Center Horizontally and Vertically

Question

I am trying to use "tabu" (specifically, longtabu) to create tables. I am attempting to follow Government Printing Office Style Manual guidance, 13.21: "Boxheads [i.e., table column headers] are centered horizontally and vertically." Here is an example from the manual itself:

Here is a pared-down example of the table I am working on. I haven't been able to figure out how to center the headers either vertically nor even horizontally:

\documentclass{article}
\usepackage{ctable}
\usepackage{longtable}
\usepackage{tabu}

\begin{document}
\begin{longtabu} to \textwidth{ X[24] X[106] X[16] X[46] }

\toprule
\rowfont[c]\normalsize Symbol & Common Name \strut\newline\strut    Scientific Name & ODA List/Target & Mapping Protocol \\
\midrule\endhead

\scshape{aecy}  &   jointed goatgrass   \strut\newline\strut\quad   \emph{Aegilops cylindrica}  &   B   &   Mandatory   \\
\scshape{aetr}  &   barbed goatgrass    \strut\newline\strut\quad   \emph{Aegilops triuncialis \emph{var.} triuncialis} &   A/T &   Mandatory   \\
\scshape{brte}  &   cheatgrass  \strut\newline\strut\quad   \emph{Bromus tectorum}  &   &   Discretionary   \\

\bottomrule
\end{longtabu}
\end{document}

I get as follows:

The headings are neither vertically nor horizontally centered. I attempted to use \rowfont[c], but this seems ineffective. I do not know how to vertically center. Other questions along the same line are not clear to me. Can anyone help? Thanks!

Answer 1

with help of makecell package, changing columns width ratios, size of \tabcolsep and vertical spice in cells:

\documentclass{article}
\usepackage{booktabs, makecell, longtable, tabu}

\begin{document}
{
\tabulinesep=1.5pt
\begin{longtabu} to \textwidth{ X[12,c,m] X[50,c,m] X[15,c,m] X[18,c,m] }
\toprule
%\rowfont[c]\normalsize
Symbol          &   \makecell{Common Name\\
                    Scientific Name}            &   \makecell{ODA\\
                                                    List/Target}
                                                        & Mapping Protocol \\
\midrule\endhead
\scshape{aecy}  &   \makecell{jointed goatgrass\\
                    \emph{Aegilops cylindrica}} &   B   &   Mandatory       \\
\scshape{aetr}  &   \makecell{barbed goatgrass\\
                    \emph{Aegilops triuncialis
                    \emph{var.} triuncialis}}   &   A/T &   Mandatory       \\
\scshape{brte}  &   \makecell{cheatgrass\\
                    \emph{Bromus tectorum}}     &       &   Discretionary   \\

\bottomrule
\end{longtabu}
}
\end{document}