The plough only turns the surface of the ground (grass) under. The grass will then take time to rot down. The farmer will just just harrow the top surface of the soil and sow his seed above the rotting grass.
The gardener normally likes a nice deep layer of clean soil but you will not get this in a freshly ploughed grass field.
I would rotovate the ground and hand remove any grass. This would be stacked in a corner and allowed to compost down. The traditional thing to do in this situation is to plant lots of potatoes. You clean the ground as you are digging the trenches; as you keep ridging them up you kill any weeds that start growing then, when you lift the potatoes you clean the ground again.
I would not be using any weedkiller at all. There are two types. One is a contact weedkiller that needs to fall on green leaves to have any effect whilst the other acts by poisoning the ground so that you will not be able to grow anything at all this year.
Why not join GoY, Guest, then you can continue to discuss your options with us.

I agree with Bulbaholic no weedkiller if you want to grow any food crops on the ground.

In a field, unless you are very lucky, a lot of the grass will be couch grass which has persistent roots and rotavating will just spread this around.
I'm an organic no weedkiller person too. It depends on the size of your plot what is practical, but I'd go for supressing weeds a heavy deep mulch for the first season, and gradually digging out the persistent perennial weeds like couch grass as you go over a number of years. The deep mulch method works well for me, and there's lot of information about it on the Internet. See the 'Ruth Stout' method as it may appeal.

Hmm...my garden - 4 years old- is on an old potato field and there were huge amounts of couch grass clumps in the soil as I started gardening. The plot was grassed when we moved in so I am constantly pulling up couch grass, and the 'lawn' is full of it! It is really annoying stuff but I don't think it is possible to totally eradicate it and cutting of any kind just makes it worse. I guess what I am saying is I fear you will be pulling up couch grass for a few years like me, but it could be worse, it could be ground elder or japanese knot weed!! Enjoy! I agree with the comments about weedkiller, but I do spray the stuff with a weedgun from time to time, especially at the fences where it is impossible to pull up the roots. I'm not sure how much effect it has, I'm still pulling it up all the time!

'Pulling' couch grass is a guaranteed way to make sure you are never rid of it. All it needs is for one small piece of root to be left in the ground and hey presto it is off again. You either do as Bertie has suggested and cover to suppress growth or you dig, and dig, and dig... We have a field at the end of our garden so couch comes in all the time. We are able to keep it under control t oa certain extent but not completely.

OK well, there's pulling and then there's my 'pulling' which involves digging as well! I don't think I'll ever be rid and so i tend to just accept that.

As we do too Karensusan, it is more of a pain when growing veggies...

Thanks for the advice. Couch grass is that thick stuff, isn't it? It's not that - just grass that's been in a field that sheep graze on. I've now decided against the weedkiller! Will rotavate it and weed it when it needs.

Good luck Peaches keep us informed.