Can I apply crop on export/publish?

or... for completeness you could impose a particular crop shape on the fly, in the Print module, and export as JPG from there. This option is not so great when you want the output to reflect each photo's filename etc, also does not so intelligently deal with portrait / landscape orientation. It can auto-rotate photos to fit into the desired output specifications but then some of these photos will show rotated on-screen by default. 

Personally I'd recommend the option of making Virtual Copies and cropping those. This does create additional image versions as seen in the Catalog, intermingled with your starting photos. A good way to manage that is to highlight the photos you want in your batch, create a new  "Collection" suitably named, and in the dialog for making this Collection check the option to include highlighted images - also check the option to have these included as new virtual copies. Now you can work within this Collection as a batch, to e.g. crop them all to 16:9 or to do whatever else you want. That works in one place despite the photos actually living in various folders or whatever. And these changes stay independent of your prior editing, which remains unaffected on the starting images.

It's then easy to select these cropped versions for Export.

The Collection also provides an easy way to remove these virtual copies (only) from your Catalog, when you are finished with them. VCs made for such a purpose are fully disposable and may be seen to "clutter" your Library since they will be visible when you go to the Folder view inside LrC. But if you did want to keep them, they occupy more or less no extra storage space - just some previews - because they share in the use of the same source file that's already saved to disk.

This gives you the option also, of applying over the top any specific "look" or adjustment that you want for just this particular output batch. That works best when you overlay different adjustments that you are not already applying. So, if you have individualised the Exposure of  each photo, there is no single Exposure value you can now apply onto all these photos to make them a bit darker. But you can apply the same Tone Curve based darkening onto all these photos, and it will then have the same relative effect onto each.