Cow pasture rabbit hunting

They are not living in the pasture just eating there. Like ZacAttack said, turn out on the edges and briar heads surrounding the pasture.

You want to wait and shoot rabbits when the sun goes down

The grass is not tall enough. Find a piece of property that is thick with briars. Get someone on a tractor with a bush hog to start cutting it. Start cutting the outside working your their towards the middle. Get enough people to cover the outside perimeter of the field. You will slaughter the rabbits. Drag a pipe behind the tractor instead of a bushog, and you can do this multiple times.

You can shoot them but they might have worms

The parasite they're talking about are the larvae of a fly called Cuterebra.

It drops eggs, they hatch, and the small larvae are able to get into the body (through a number of ways) and they go develop in an area just underneath the skin. This is the when the rabbits are said to "have worms."

The worm (larvae) devleop into adult flies, escape the skin, and fly away.

Couple things:

First frost is just a good idea of when the flies won't be out.

This whole process takes 30-70 days depending on temps. So if a larvae gets into a rabbit a month before the flies are gone for the winter the process will still be going on and the rabbit still has worms despite it being cold. So, the rate in which rabbits become infected obviously decreases as flies disappear but once they got it, they got it until the cycle is complete.

Proper cooking should eliminate any issues you'd have whehter it has these fly larvae or not (even though it would be kinda gross.)

Parasites are interesting creatures and it is helpful as outdoorsmen to understand them so you know what to avoid (and what you shouldn't worry about).

Knowing about various parasites like ricebreast in ducks, worms in various game animals/fish,etc.. are all helpful to know about and really don't take very long to understand.

You need 2 people if you don't have dogs. Split up on two sides of a fence, briar patch etc.

Kick the edges and try to flush anything out.