crabgrass

Question
can i give my guinea pig crabgrass???

Answer
If you are talking about crabgrass from your lawn I would say no only because of the amount of chemicals, urine and other pollutants that grass is exposed to.

You can call a pet store and ask what they feed but I think most of them use timothy hay that you can buy in a bag at the store.
If your grass is organic and such I guess a small amount won't hurt them. Be careful because unusual foods like greens can give them diarrhea. And that can lead to many disasters.

Here is a warning about this from a site on guineas:
When feeding "wild" greens, such as grass and dandelion greens, make sure they have not been sprayed with chemicals, or contaminated by droppings or urine from other animals, such as cats, dogs and birds. The dangers of pesticides are obvious, and feces can carry any number of parasites which can be transmitted to cavies eating contaminated greens. Even if dogs and cats are dewormed regularly, their feces can contain protozoa/bacteria which can cause debilitating diarrhea in a guinea pig.

http://www.aracnet.com/~seagull/Guineas/feeding.html

So as I said, be very careful about it. Since their main food is hay, so you must be buying that now.