Guinea pigs and grass

Hi all,
My grass needs a bit of help....it's sparse and weedy.
I was considering using some evergreen on it but am a bit worried about whether it will be bad for the guineapigs, don't want to give them a tummyache or worse!
Would it hurt them, or can you suggest anything else I can use to improve it that would be safe.
Many thanks

The time off grass should be on the packet.  Water in well and most procucts are around 7 days or so for grasing.  If no details like that on packet ring thier customer care line which is somewhere on the back of pack.

Thanks folk!

Fertiliser helps grass outcompete the weeds.  Just keep the GPs off it until it's been rained in well.