Too much grass?

[Ester]

Hi everyone
Since the weather turned nicer I have been putting my guinea pigs on the lawn. They seem to stuff their faces with grass and then don't seem to eat much hay (usually in the morning their hay rack in the hutch is empty - now since they have had grass time it is only half eaten, maybe). Is this bad for them or will they find their own balance? Should I limit time on the lawn? They seem to have a lot of fun there.
Thanks.

 

[Piggies&buns]

Hay and grass is the best thing for them to eat but as piggies eat constantly, the fact that they eat grass during the day should not stop them from eating hay overnight. My boys are out on the lawn for 12 or so hours a day during the height of summer but they still eat the same amount of hay.
Do you put hay out in the run during the day?
How long are they out for?
Are you also giving them veg daily? Have you reduced down their veg portion to take account of the fact that they are eating more grass?
Did you build up their grass time slowly to get their tummies used to it?
How has their weight been?