Removing Wild Onions?

I'm not sure that cutting it low will get rid of it. What that does is to stop it seeding and spreading so quickly. There's not much in the way of selective herbicides that won't kill the grass as well.

Wild onions and garlic don't like a high pH so you could try increasing it by adding lime to the lawn, but I don't know how long it's residual in the lawn and what effect it may have on guinea pigs. You'd need to ask a vet.

One old countryside cure for it is to let your pigs (not guinea pigs) graze on your lawn. They love onions and garlic and will dig out all the roots. Of course, your lawn will then need replacing!

The peak time for weed killing them is autumn/winter but you need to run the mower over them first. Don't cut them, just crush them a bit.

So not too hopeful a situation. The only consolation is that the guinea pigs are unlikely to eat it if given the chance to avoid it and just eat the growing grass.

The other option, apart from killing the whole lawn and starting again, is to find a friend who has onion/garlic free grass and ask them for some of their clippings. The guinea pigs will eat those first if spread on the run area. We have a friend who has some of our clipping for their guinea pigs.

 

Unfortunately that kills the grass as well. Anyway, we don't have frosty nights anymore