I think I might have find a reason of why garlic is bad for guinea pigs, according to
this page oxalic acid is poisonous for guinea pigs, I quote "oxalic acid poisoning following ingestion of oxalate containing plants, such as beetroot, spinach, and rhubarb" the question is, does garlic contains enough oxalic acid to be poisonous?
I searched for a food chart exposing the contents of oxalic acid in sevearl fruits/vegetables;
Fruit/vegetable--------oxalic acid content according to 3 different sources
name.........¦source1¦ source2¦ source3¦
Amaranth.........¦1.09¦ ---- ¦ ---- ¦
Chives.............¦1.48¦ ---- ¦ 0.00 ¦
Parsley............¦1.70¦ ---- ¦ 0.10 ¦
Rhubarb (stems)¦----¦ 1.34¦ 0.86 ¦
Spinach...........¦0.97¦ 0.66 ¦ 0.68 ¦
Apples.............¦----¦ ----¦ 0.00 ¦
Broccoli...........¦0.19¦ ----¦ 0.00 ¦
Garlic..............¦0.36¦ ----¦ ---- ¦
So not very clear, if garlic is bad because of its content in oxolic acid, toxic for guinea pigs, according to the chart Parsley would be worse than garlic but I haven't read anywhere that Parsley is bad for guinea pigs, to the contrary actually.
Original chart is found
here.